An electrodynamic balance (EDB) for extraterrestrial cloud formation studies
by
MASSACHUETS INSTiTE
OF TECHNOLOGY
Shaena R. Berlin
JUN 10 2014
B.S. Earth, Atmospheric & Planetary Sciences
Massachusetts Institute of Technology, 2013
LIBRARIES
SUBMITTED TO THE DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY
SCIENCES
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE IN EARTH, ATMOSPHERIC AND PLANETARY SCIENCES
AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY
JUNE, 2014
0 2014 Massachusetts Institute of Technology. All rights reserved.
Author
Signature redacted
Shaena R. Berlin
Department of Earth, Atmospheric and Planetary Sciences
May 28, 2014
redacted
Certified by_Signature
A/
Acepedby
Accepted
b
b
Daniel J. Cziczo
Thesis Supervisor
Signature redacted
Robert D. van der Hilst
Schlumberger Professor of Earth Sciences
Head, Department of Earth, Atmospheric and Planetary Sciences
1
An electrodynamic balance (EDB) for extraterrestrial cloud formation studies
by
Shaena R. Berlin
Submitted to the Department of Earth, Atmospheric & Planetary Sciences on May 28, 2014 in
Partial Fullfillment of the Requirements for the Degree of Master of Science in Earth,
Atmospheric & Planetary Sciences
Abstract
Ice clouds scatter and absorb solar radiation, affecting atmospheric and surface temperatures
(Gettelman et al., 2012). On Mars, where ice contained in clouds makes up a large portion of
total atmospheric water vapor, ice clouds also alter the planetary water budget (Maltagliati et al.,
2011; Rafkin et al., 2013). Thus, it is important for climate models to be able to accurately
predict the conditions under which ice clouds can form. Typical Martian temperatures at cloudformation height range from -150-200 K (Trainer, Toon, & Tolbert, 2009). Heterogeneous
deposition nucleation is thought to be the dominant freezing mechanism on Mars due to the
abundance of mineral dust to serve as ice nuclei (IN) (Mdittanen et al., 2005). The parameters
for such nucleation are not well characterized at such low temperatures (Trainer et al., 2009).
Previous experimental studies have investigated the relative humidity required for deposition
nucleation within the Martian temperature range. However, most studies took place on bulk
aerosol samples, did not use mineral dusts analogous to Martian dust, or were constrained by
particle lifetime and temperature limits.
In this project, we repurpose a single-particle instrument and set it up to perform
experiments for more precise ice nucleation data under Martian atmospheric conditions. We use
an electrodynamic balance (EDB) to levitate individual particles with diameters around 10 pm.
We calculate the size of the particle and changes in size based on the holding voltages. The
system can be cooled to 200 K in its current configuration, and the relative humidity and
atmospheric constituents can be controlled by adding gas. To test the EDB, we perform
validation experiments. We investigate deliquescence and efflorescence on salts at room
temperature and 0 'C. We modify the cooling system, thermocouples, and relative humidity
sensors and begin freezing experiments with Arizona Test Dust (ATD) and with Mojave Mars
Simulant (MMS) dust. We investigate water uptake on MMS particles and find it to be nonhygroscopic but wettable, uptaking monolayers of water between 65-95% relative humidity.
From 200 K to 220 K, MMS does not nucleate up to 115% RHice, suggesting that higher
supersaturations are needed for ice clouds to form; some Martian cloud modelers should revisit
the critical supersaturation parameterization. Future work will improve the EDB and use it to
examine phase functions and light scattering.
Thesis Supervisor: Daniel Cziczo
Title: Associate Professor
2
Acknowledgements
First, I would like to thank my thesis advisor Dan Cziczo. He took a chance bringing me into his
group when I had no lab experience and has been patient and supportive throughout this project.
I truly appreciate the amount of time he has spent mentoring me and his help figuring out what I
would do after graduating MIT.
Next I thank Amy Bauer, who donated the EDB and spent her own vacation time coming
to Boston and working with me in lab. Every day of her expertise was worth about a month of
me troubleshooting alone!
I am grateful for the rest of the Cziczo group: Alexandria, for taking over the EDB after I
finished and carrying on with my work; Yi-wen, who always was there helping me have
breakthroughs with equipment malfunctions; Maria, who taught me about electronics and optics;
Karin, who trained me about working in a lab; Sarvesh, who always had new ideas and solutions;
and all the visitors and UROPs.
To my family and the cycling team: You all kept me sane and happy, even when it
seemed like the leak-testing would never end.
Finally, I would like to thank the MIT Department of Earth, Atmospheric & Planetary
Science and the Praecis Presidential Fellowship program for funding: Being able to focus on
research without dividing my time working for funds made a huge difference in the amount I
could accomplish in this short year.
3
Contents
A bstract.........................................---
. ..................---................................................................
Acknow ledgem ents.........................................................................................................................
2
List of Figures & Tables
7
1 Introduction
3
..................................................................................................
...................................................................................................................
1.1 M artian ice clouds
................................................................................................
9
9
1.2 Previous studies on ice nucleation ..................................................................................
10
Theoretical approaches ..........................................................................................................
10
Experim ental techniques and findings ................................................................................
11
Observations .......................................................................................................................
12
Lim itations of clim ate m odels and previous experim ents .................................................
13
1.3 Using an electrodynamic balance (EDB) for ice nucleation studies ...............................
14
Theory....................................................................................................................................
14
O ther uses of ED B .................................................................................................................
14
M odifying and updating ED B ..........................................................................................
15
Applying ED B to more precise aerosol experim ents .........................................................
16
2 Experimental setup.....................................................17
2.1 O verview .
.....................................................................................................................
17
2.2 Electrodynam ic balance (EDB).......................................................................................
18
Electrom agnetic trap ..............................................................................................................
18
Pressure/vacuum and feedthrough system s ...........................................................................
19
Sensors near the trap..................................................20
2.3 Optics....
..............................................................................................................
21
2.4 Cooling system ....................................................................................................................
23
Cooling bath ....................................................................................................................
23
Flow path and therm al transfer..............................................................................................
23
2.5 G as flow system .... transe..............................................................................................
24
2.6 LabV IEW program ..............................................................................................................
25
2.7 Lim itations and specifications of com plete system ..........................................................
26
Tem perature limits.................................................................................................................
26
Pressure limits........................................................................................................................
26
Size range of trapped particles: ........................................................................................
26
3 Procedures..................................................................................................................................
27
3.1 Particle generation and descriptions................................................................................
4
27
Generating solid particles ..................................................................................................
27
Sa lts .......................................................................................................................................
27
Arizona Test Dust (ATD) ..................................................................................................
27
M ojave M artian Simulant (MM S) dust .............................................................................
27
3.2 Trapping and balancing particles ....................................................................................
28
3.3 Size calculations ..................................................................................................................
28
3.4 Validation experim ents ....................................................................................................
29
Theoretical deliquescence points:.......................................................................................
29
Deliquescence experim ents ...............................................................................................
30
Efflorescence and hysteresis .............................................................................................
32
Arizona Test Dust (ATD) freezing ....................................................................................
33
4 New results.................................................................................................................................
35
4.1 M artian Dust Sim ulant ....................................................................................................
35
Room Tem perature ................................................................................................................
35
Freezing .................................................................................................................................
36
5 Conclusions and future work ..................................................................................................
38
5.1 Suggestions for future studies ........................................................................................
38
Software and data acquisition hardware im provem ents .....................................................
38
Chamber feedthrough redesign ...........................................................................................
39
Further freezing experim ents.............................................................................................
39
Phase functions ......................................................................................................................
40
5.2 Sum m ary .............................................................................................................................
41
References .....................................................................................................................................
42
Appendices ....................................................................................................................................
49
A . Specifications .......................................................................................................................
49
Trap assem bly........................................................................................................................
49
Ceram ic supports ...................................................................................................................
49
Sapphire windows..................................................................................................................
50
Exterior parts and dim ensions ...........................................................................................
51
Cooling bath ..........................................................................................................................
53
Inner vacuum feedthrough lines ............................................................................................
53
B. Feeding wires........................................................................................................................
54
C . Leak testing...........................................................................................................................
57
Leak testing procedures ......................................................................................................
57
5
Vacuum feedthrough lines .....................................................................................................
58
Gaskets, O -rings, and exterior fittings ................................................................................
59
D . Therm ocouple testing ...........................................................................................................
61
E. Relative hum idity specifications and calculations ............................................................
62
Sensor specifications .............................................................................................................
62
UPSI relative hum idity measurem ent overview ................................................................
63
Calculate parasitic capacitance ...........................................................................................
64
Calculate relative hum idity from capacitance .......................................................................
64
Calibration com parison: UPSI vs Honeywell RH sensors ....................................................
65
Testing Honeyw ell sensor below its rated limits ..............................................................
66
Calculate RH w ith respect to ice ...........................................................................................
67
DRH as a function of tem perature .........................................................................................
67
F. LabV IEW program ...............................................................................................................
68
Overview of program .............................................................................................................
68
Virtual instrum ents ................................................................................................................
70
Tem perature ...........................................................................................................................
70
Relative hum idity ..................................................................................................................
71
M anual inputs ........................................................................................................................
71
D ata logging ..........................................................................................................................
72
Com m on errors ......................................................................................................................
72
W ...............................................................................................
73
Potential improvem ents ......................................................................................................
74
G . Other issues and solutions ...............................................................................................
75
Pow er .....................................................................................................................................
75
RH ..........................................................................................................................................
75
U nable to trap particle ...........................................................................................................
75
Unable to see particle ............................................................................................................
75
H . Aqueous solution droplet generation................................................................................
76
Salt droplet size calculations .............................................................................................
77
Program ming in LabV
6
List of Figures & Tables
Figure 1: Diagram of ice nucleation (Cziczo & Froyd, 2014, Fig. 2). Nucleation modes that
require liquid water (dark blue circles) will not occur on Mars. In this study, we investigate
dep o sition freezing ........................................................................................................................
10
Figure 2: EDB setup: Before (left) and after (right) ..................................................................
15
Figure 3: Supersaturation required for freezing with various studies. Boxed region shows regime
reached in this project with the EDB. Taken from Cziczo et al. (2013), Fig 4. ........................
16
Figure 4: Schem atic of setup....................................................................................................
17
Figure 5: Electrom agnetic trap setup ........................................................................................
18
Figure 6: Feedthrough system (side view). Colored paths indicate 'inner chamber' Light grey
indicates 'outer cham ber'..............................................................................................................
19
Figure 7: Sensors outside the trap .............................................................................................
21
Figure 8: O ptical setup, birds-eye view ....................................................................................
22
Figure 9: Photo: ED B table setup ............................................................................................
22
Figure 10: Flow path for cooling fluid. Left: flow between Lauda bath and chamber. Right: Flow
in sid e ch am b er. .............................................................................................................................
23
Figure 11: Gas flow system. Upper branch: Dry gas. Lower branch: Humidified gas............. 24
Figure 12: H ardware-software interface. ...................................................................................
25
Figure 13: Deliquescence curve for NaCl compared to other studies. Representative error bars on
left. Estimated from Tang, Munkelwitz, & Davis, 1977, Fig. 8, and Gysel, Weingartner, &
B altensperger, 2002 , F ig . 3 ...........................................................................................................
31
Figure 14: Efflorescence data from EDB (A and B) compared to other studies. Representative
error bars on left. Estimated from Tang, Munkelwitz, & Davis, 1977, Fig. 8, and Gysel,
W eingartner, & Baltensperger, 2002, Fig. 3.............................................................................
32
Figure 15: Hysteresis loop: Deliquescence and efflorescence of NaCl using EDB, 20 'C and
0 'C. Top panels: A dry NaCl particle deliquesces at ~75% RH. Bottom panels: An NaCl droplet
crystallizes around 45% RH ..........................................................................................................
33
Figure 16: Ice nucleation phase diagram. Red line shows homogeneous freezing curve for
droplets (Koop et al., 2000). Black line shows water saturation, i.e. 100% RH(liquid water). Blue
shaded area shows relative humidity maximums we reached in this study. Symbols show ATD
minimum Sice thresholds for 1% freezing below -40 0C given by Cziczo et al. (2009), Kanji,
Florea, & Abbatt (2008), Knopf & Koop (2006), and M6hler et al. (2006)............................
Figure 17: MMS water uptake, 25 0 C and 00C, with representative error bars. .........................
Figure 18: RH(liquid) vs RH(ice), -70'C, based on Koop & Murphy (2006)..........................
34
35
36
Figure 19: Time series of MMS experiment at -68 0C. Maximum RHice=1 15%. Particle diameter
rem ained unchanged . ....................................................................................................................
36
Figure 20: Ice nucleation phase diagram. Red line shows homogeneous freezing curve for
droplets (Koop et al., 2000). Black line shows water saturation, i.e. 100% RH(liquid water). Blue
shaded area shows relative humidity maximums we reached in this study. Symbols show MMS
minimum Sice thresholds for 1% freezing given by Cziczo et al. (2013) and Ladino & Abbatt
(2 0 13 )............................................................................................................................................
37
Figure 21: ThorLabs USB camera to replace Burle security camera. ......................................
Figure 22: Schematic of proposed hardware and software integration and automation...........
Figure 23: Block diagram of proposed single particle scatterometer/polarimeter. From Maria
Zawadowicz, M IT, M asters Thesis Proposal...........................................................................
Figure 24: ED B trap assem bly.................................................................................................
38
39
7
40
49
Figure 25: Ceram ic standoff specifications .............................................................................
49
Figure 26: Sapphire window diagram ......................................................................................
50
Figure 27: (left) Outer shroud dimensions. (right) Inner shroud dimensions. ..........................
51
Figure 28: Chamber diagram and vacuum flange specifications. CF: 6-hole ConFlat with Viton@
or copper gasket. KF: Kwik-Flange. ISO LF: Large Flange with Viton® O-ring. .................. 52
Figure 29: Cooling bath connections and specifications. Tubing is covered with ultra-flexible
foam rubber pipe insulation (M cMaster).................................................................................
53
Figure 30: Inner vacuum feedthrough line specifications. 3 x Swagelok convoluted metal tubing,
part # 32 1-8-X -6D FR ....................................................................................................................
53
Figure 31: Inner-outer wire connections..................................................................................
54
Figure 32 : W ire feed hole .............................................................................................................
55
Figure 33: Albright knot between thermocouple wire and fishing line ...................................
56
Figure 34: V acuum feedthrough line leak spots ...........................................................................
58
Figure 35: Common leak sources. (a) Feedthrough wires and VCR gaskets, (b) ConFlat fittings
and internal-external feedthroughs, (c) Inner shroud base, (d) Outer and inner shroud windows 60
Figure 36: Thermocouple scatterplot: TC1 is suspended in the air by the trap. TC2 is touching
th e m etal........................................................................................................................................
61
Figure 37: Thermocouple testing time series, cooling then warming trap. Thermocouple in air
and touching metal are within 2 0C at all times and are equal after cooling is complete. ......... 61
Figure 38: Honeywell RH sensor calibrations ...........................................................................
63
Figure 39: RH sensor com parison.............................................................................................
65
Figure 40: RH(T) Honeywell vs UPSI at low T ......................................................................
66
Figure 41: Starting the LabVIEW program ..................................................................................
68
Figure 42: User interface of Main.vi. Sampling interval is set to 2 seconds. Thermocouples agree
within 0.02'C, and RH sensors agree within 2%. RH(ice) is physically meaningless in this case,
since the screenshot was taken at room temperature. ..............................................................
69
Figure 43: D A Q A ssistant screenshot......................................................................................
70
Figure 44: National Instruments Measurement and Automation Explorer (NI-MAX) ............ 73
Figure 45: Droplet generator setup. Changeable heights given by variables. N=replaceable
nozzle. Successful conditions: A=8.125", B=8", C=16", D=8.75", E=50mL.......................... 76
8
1 Introduction
Clouds influence the atmospheric temperature profile, alter the balance of incoming and
outgoing radiation, and vertically redistribute water and dust (Colaprete & Toon, 2000). Cloud
droplets under terrestrial conditions usually form on aerosols called cloud condensation nuclei
(CCN) or ice nuclei (IN) (Seinfeld & Pandis, 2006). Previous studies have shown that a higher
IN concentration leads to faster nucleation, faster precipitation, and reduced cloud lifetime (e.g.
Chou et al., 2001, Gettelman et al., 2012). Currently, the roles of different types of aerosols on
these effects under varying atmospheric conditions requires more research (Gettelman et al.,
2012).
In this project, we study aerosol-water interactions using an electrodynamic balance
(EDB) described in detail below. The EDB suspends individual particles for up to their
atmospheric lifetimes and allows for a wide range of atmospheric compositions, water vapor
contents, and temperature conditions. We first investigate conditions found on Earth, then delve
into conditions expected on Mars. The goal of these experiments is to measure changes in
particles under various atmospheric conditions.
1.1 Martian ice clouds
The Martian atmosphere contains water ice clouds similar to Earth's cirrus clouds. The clouds
scatter and reflect incoming solar radiation and modify the atmospheric temperature profile
(Colaprete & Toon, 2000, Rafkin et al., 2013; Trainer et al., 2009). Observations from spacecraft
such as the Mars Pathfinder show that such clouds extend up to 50 km vertically and can form in
temperature regimes from 150-220 K (Trainer et al., 2009). Few laboratory studies have
examined cloud formation under these conditions since the very low temperatures and vapor
pressures present in the Martian cloud-forming region are not relevant to terrestrial conditions
(Phebus et al., 2011). Extrapolations made from classical heterogeneous nucleation theory
(CHNT) to colder temperature regimes may be inaccurate, and thus models adapted from Earth
to describe Martian conditions could lack physical meaning (Iraci et al., 2010).
Two distinct processes can cause ice crystals to form: homogenous freezing and
heterogeneous freezing. Pathways for these types of freezing are shown in Figure 1.
Homogenous nucleation, the spontaneous formation of ice without the requirement of ice nuclei
(IN), occurs at very low temperatures and high saturation (Koop et al., 2000). Heterogeneous
nucleation, in which a particle reduces the energy barrier for ice crystallization, can occur at a
warmer temperature and lower relative humidity than homogenous nucleation (Chou et al.,
2011).
Aerosols such mineral dust are ubiquitous in the Martian atmosphere (Ladino & Abbatt,
2013; Rafkin et al., 2013; Smith, M.D. et al., 2001). Due to this abundance of potential IN in the
Martian atmosphere and its low water vapor content, homogeneous freezing is not expected to be
a significant contributor to ice formation. Rather, the mineral dust particles may serve as IN that
induce heterogeneous nucleation. Since Martian temperatures are far below conditions that
support liquid water, nucleation pathways that require liquid water will not occur. Thus,
depositional nucleation will be the dominant mechanism, where water vapor deposits directly
9
onto the surface of the IN without requiring a transition through a liquid water phase. (Mdittanen
et al., 2005).
Droplet Activation
Condensation Nucleus
Droplet
(Grey = Soluble Particles take up
or Insoluble)
water
ice Nucleus (IN)
(Black
=
immersion, Condensation Freezing
Solid)
Contact Freezing
Z
Deposition Freezing
Mars+
Homogeneous Freezing
Homogeneous Freezing Nucleus (HFN)
0
Higher Temperature, Lower Humidity
-
Lower Temperature, Higher Humidity
Figure 1: Diagramof ice nucleation (Cziczo & Froyd, 2014, Fig. 2). Nucleation modes that
require liquid water (dark blue circles) will not occur on Mars. In this study, we investigate
depositionfreezing.
Martian ice clouds not only alter radiative balance but also affect the hydrological cycle.
Precipitation from Martian ice clouds moves water from the atmosphere to the surface and
consists of ice crystals around 70 pm in diameter (Whiteway et al., 2009). Crystals form on
mineral dust with a wide range of diameters that increases from ~0.1 pm in the upper boundary
layer to over 10 pm toward the ground. Ground fogs can form and mix water vapor between the
surface and the lower atmosphere (Pathak et al., 2008).
1.2 Previous studies on ice nucleation
Theoretical approaches
Classical heterogeneous nucleation theory (CHNT) is the framework for most ice nucleation
models. There are two hypotheses for freezing: The stochastic hypothesis describes a timedependent probability of forming a critical ice nucleus and results in a nucleation rate J. It
depends on the saturation ratio relating water vapor pressure S = Pulq/Pvap (Seinfeld & Pandis,
2006). The singular hypothesis asserts that nucleation takes place at a deterministic temperature
depending on the properties of the ice nucleus (Niedermeier et al., 2011). For more discussion
10
and applications of these theories, see Niedermeier et al. (2011). Both theories have in common
that the presence of IN reduces the Gibbs energy barrier to ice formation (Pruppacher & Klett,
2010).
Models should account for microphysical properties of the particles. The size and density
of particles affect their lifetimes and settling velocities (Pathak et al., 2008). The distribution of
'active sites' upon which water contact is favorable depends on surface area and particle type
(e.g. Marcolli et al., 2007). Such properties affect the freezing outcomes enough that modelers
desire further experiments and improved parameterizations (e.g. Phebus et al., 2011).
Experimental techniques and findings
Many recent studies have investigated heterogeneous nucleation. Ladino & Abbatt (2013) used a
continuous flow diffusion chamber (CFDC) to study freezing on Martian dust simulants from
203 to 223 K. The CFDC flows particles through a region of supersaturations created by wetting
and cooling two plates to different temperatures. They showed that the critical supersaturation
required to freeze 1% of particles increased with decreasing temperature within this range.
Marcolli et al. (2007) used a differential scanning calorimeter (DSC) to explore heterogeneous
ice nucleation on Arizona Test Dust (ATD); the technique determines freezing by measuring the
latent heat released by droplets upon nucleation. Niedermeier et al. (2010) compared theoretical
freezing behavior with experimental measurements of immersion freezing on supercooled ATD
of various sizes and coatings. They used the laminar flow diffusion chamber LACIS to determine
that coatings that might develop as aerosols age serve to alter particle surfaces and lower
nucleation efficiency. Knopf & Koop (2006) compared nucleation of ATD and coated ATD over
multiple freeze-thaw cycles. They found that preactivation of particles alters the nucleation
efficiency.
Cloud chambers have also been used to study nucleation with suspended particles.
Studies in the Aerosol Interactions and Dynamics in the Atmosphere (AIDA) chamber at the
Karlsruhe Institute of Technology have investigated several questions related to freezing. Cziczo
et al. (2013) tested deposition freezing of Martian simulant dusts from 189 to 215 K and
determined the critical supersaturations required to nucleate ice throughout this temperature
range; those experiments investigated regimes similar to that explored in this EDB project.
Connolly et al. (2009) studied heterogeneous freezing of mineral dust at temperatures at or above
the homogeneous freezing point; they constructed a parameterization to describe active site
density for ice nucleation for immersed mineral dust freezing and for depositional freezing on
dry mineral dust. M6hler et al. (2006) investigated the supersaturations required for deposition
nucleation on three types mineral dust particles between 196 and 223 K.
One of few techniques that has allowed for studies below ~200 K has been cooling with
bulk aerosol on substrates. Several studies investigated heterogeneous deposition nucleation
from 150 to 180 K by cooling particles on a solid silicon substrates. They found very high
critical supersaturations (in excess of 200% RHice) for nucleation at low temperatures and created
new parameterizations to use in models at such low temperatures (Iraci et al., 2010; Phebus et al.,
2011; Trainer et al., 2009).
11
Observations
Satellites and landers have provided observations that can be used to infer ice nucleation
conditions. The SPICAM (Spectroscopy for the Investigation of the Characteristics of the
Atmosphere of Mars) confirmed the presence of supersaturated water vapor in the Martian
atmosphere, a state that climate models previously assumed could not exist (Maltagliati et al.,
2011). The Mars Global Surveyor Thermal Emission Spectrometer (TES) recorded infrared
surface temperature data and atmospheric dust loading from 1997-2006. It provided temperature
profiles and geospatial data about dust storms that improved understanding of Martian
atmospheric dynamics across all latitudes (Home & Smith, 2009; Smith et al., 2001). More
recent landers such as the Phoenix investigated the presence of liquid water and polar regions
(e.g. Smith et al., 2009). The Mars Reconnaissance Orbiter (MRO) gives details about
atmospheric dust and water ice distributions (McCleese et al., 2010).
Lidar instruments obtained information about cloud and dust heights. The Mars Orbiter
Laser Altimeter (MOLA) tracked dust storms and cloud formation to create a seasonal
atmospheric activity profile (Neumann, Smith, & Zuber, 2003). Whiteway et al. (2009) described
the following inferences from LIDAR images of dust loadings and backscatter from the Phoenix
lander: Clouds form nightly around 4 km in altitude, at temperatures up to -65 'C. The clouds are
frozen water, not carbon dioxide (C02); the frost point of CO 2 is -120 'C. In the morning, these
clouds precipitate. The assumed critical supersaturation (referred to as Scit or RHice) is 110130%. These observations and investigations confirm the presence of water ice clouds on Mars
but raise further questions about their formation conditions.
12
Limitations of climate models and previous experiments
Martian models that cover the water cycle have difficulty modeling both water vapor and clouds.
With a lack of observations and realistic experiments, modelers are forced to extrapolate Earthlike parameterizations to Martian conditions or to make assumptions based on thermodynamic
equilibrium. Recent experiments at low temperatures show much higher supersaturations
required to nucleate ice than expressed in models (e.g. Iraci et al., 2010; Ladino & Abbatt, 2013;
Phebus et al., 2011). However, previous experimental setups also do not give modelers clear
answers because of limitations:
* Temperature: Cloud chamber setups cannot reach below ~190 K.
* Single-particle precision: Bulk aerosol on substrates, or many particles in chambers,
*
do not give precise data about the behavior of individual particles or possible effects
of aging and multiple freeze-thaw cycles on single particles.
Atmospheric composition: Large-volume chambers are constrained to inert or
readily-available atmospheres. Other planets may contain atmospheres with different
compositions that affect their cloud formation.
Earlier models assumed nucleation depended mostly on the contact angle (wetting parameter), a
free energy term between water vapor and aerosol contact. Varying the contact angle changed
the nucleation rate, while due to a lack of data, supersaturation was fixed around S=1 (e.g.
Michelangeli et al., 1993). Maittanen et al. (2005) used S=1.18 at 200 K for heterogeneous water
ice nucleation to model Martian cloud particle formation. Davy et al. (2008) developed a coupled
boundary layer-dust model to help interpret the results of lidar observations; however, they
assumed ice forms as soon as RHice exceeds 100%. Pathak et al. (2008) used the Mars
Microphysical Model (MMM) to investigate diurnal fog and cloud formation. They used water
data abundance from the TES observer, which does not include supersaturated conditions.
Recent improvements take into account new knowledge of nucleation mechanisms and
the importance of reasonable nucleation results on the rest of the climate (e.g. Navarro et al.,
2013). The NASA Ames Mars GCM models nucleation, dust, and water vapor saturation ratio,
but achieves vastly different saturations depending on the time step used. The saturation ratio
never exceeds 1.5 (Urata et al., 2014), which as described in the experimental section above may
not actually be saturated enough to nucleate ice at Martian temperatures. Navarro et al. (2013)
included detailed cloud microphysics of water ice clouds in updates on the LMD Mars Global
Climate Model that allow supersaturations to occur. However, they still assume that below the
hygropause, the presence of dust particles causes the water vapor to remain at or below
saturation.
13
1.3 Using an electrodynamic balance (EDB) for ice nucleation studies
Theory
The principle behind an electrodynamic balance (EDB) comes from Millikan's 1909 oil drop
experiment in which he experimentally determined the electric charge of the electron. Millikan
levitated small oil droplets using two metal electrodes to counteract the forces of gravity, drag,
and electric charge. The density and mass of the oil droplets were known, so the force needed to
counteract the electric charge could be derived (Millikan, 1913). Other geometric configurations
of electrodes were later developed, with varying stability and electric field characteristics as
summarized in Hartung & Avedisian (1992). Arnold & Folan (1987) presented the spherical void
electrodynamic levitator (SVEL), in which the electrodes surround a spherical void where the
particles are inserted. This configuration allows for adequate particle stability and also allows for
optical measurements of radiation scattered by the particle. In this project, we use a SVEL to trap
particles, and future projects will take advantage of potential optical techniques.
Other uses of EDB
After scientists developed the initial technique for suspending droplets using electromagnetic
fields, the setup could be modified and used for other types of studies. In atmospheric science,
the EDB has been used to study aerosol growth, droplet-chemical interactions, polar
stratospheric cloud formation, and ice nucleation (e.g. Carleton et al., 1997; Cohen, Flagan, &
Seinfeld, 1987; Gysel, Weingartner, & Baltensperger, 2002; Hoffman et al., 2013; Krieger et al.,
2000; Tang, Munkelwitz, & Davis, 1977).
A series of studies between 1977 and 2002 used an EDB to determine water uptake and
diameter change of various salt particles under different temperatures and relative humidities
(Cohen et al, 1987; Gysel et al., 2002; Tang et al., 1977-1978). These studies compared water
uptake measured with an EDB to that expected using microphysical theories. In this project, we
perform similar experiments and compare our results with these to validate that our setup works
properly. Equations and assumptions are elaborated on in the Validation Experiments section.
Another important use of the EDB was to investigate polar stratospheric cloud (PSC)
formation. PSCs matter because the chlorine chemistry that occurs on the cloud surfaces leads to
the yearly stratospheric ozone loss in the Antarctic (Solomon et al., 1986). Studies investigated
the conditions under which acidic aerosol droplets freeze so the chlorine surface chemistry can
occur. Droplets appear to remain metastable to extreme conditions then go through unusual
liquid-solid phase transitions to form unpredicted hydrates. Experiments with the EDB cooled
metastable acid-water droplets to stratospheric temperatures to observe the conditions of these
phase transitions (Carleton et al., 1997; Imre, Xu, & Tridico, 1997; Krieger et al., 2000).
Most recently, the EDB has been used to investigate freezing mechanisms. A group at
Karlsruhe Institute of Technology levitates droplets to investigate homogeneous freezing,
heterogeneous freezing of immersion and contact mechanisms on various aerosols, and dynamics
of droplet freezing (Duft & Leisner, 2004b; IMK-AAF, 2014). Their setup allows for rapid
repetition of single-particle experiments with sophisticated equipment and automation. One such
14
project investigated contact freezing probability by suspending single droplets and inserting
particles that may freeze the particle upon contact (Hoffman et al., 2013). The Bertram group at
U. British Columbia studies ice- and mixed-phase cloud formation with aerosol types present in
the Arctic regions using an EDB paired with light scattering optics (UBC, 2014). In this project,
we start to investigate deposition freezing, by suspending a potential IN and adding water vapor.
Modifying and updating EDB
This setup takes an EDB from the 1990s (Hunter [now Bauer] et al., 2000) and repurposes it for
ice nucleation studies. We reassemble the trap and components, set up a cooling bath and gas
flow system, fix the vacuum pressure system, and add data acquisition devices and software. We
discuss future modifications that could improve the setup further.
Figure 2: EDB setup: Before (left) and after (right)
15
Applying EDB to more precise aerosol experiments
This EDB can suspend individual aerosol particles of most types (with 10<d<50tm and low
vapor pressure) for an indefinite length of time. The temperature can be cooled to 200 K with the
current setup, and could cool even lower with a different cooling fluid. Water, nitrogen (N ), or
2
other species can be added as the atmosphere. Sensors measure the relative humidity and
temperature experienced by the particle. DC holding voltage requirements change as a particle
takes up water or crystallizes, allowing us to determine change in size and the precise conditions
under which such changes occur.
Using these abilities, we can help fill a gap in reliable data on ice nucleation at
temperatures from 200-220 K. Other ice experiments have shown heterogeneous nucleation at
very high supersaturations under these conditions, but few have used single particles. In this
project, we demonstrate the utility of the EDB and validate its procedures for previouslycharacterized conditions as marked in Figure 3. Now demonstrated, the EDB can be pushed to
lower temperatures and higher supersaturations to give new data to inform model
parameterizations.
G ATO,
racd ot *- [2010]
irai t al. 120101
Si substrate, Iaci at al 120101
Au substrate, Schng et at. [2006]
JSC-1 (1gh1), Phebus ot al, [20113
+lay.
100
IN
SJ$C-1 (unfractmiwA. Ph** Raetda. (011I
0 Si substrate. Fortin et al. [2003]
Si substrate, Tainer et al. [2009)
1 JSC-1, 1%, Ladino and Abbatt [2013)
C M4S, 1, kiL
a Ablutt
h (20131
ATO, 1% Ladme and Abbett (2013)
ATO, % MaeIw ot al. (20051 .nd (20061
JMMS, 11, this study
ME
0
0
0
0 0
M
150
160
170
1K
190
190o
200
210
220
230
Ternwoature (K)
Figure3: Supersaturationrequiredfor freezing with various studies. Boxed region shows regime
reached in this project with the EDB. Taken from Cziczo et al. (2013), Fig 4.
16
2 Experimental setup
2.1 Overview
This section describes the setup of the electrodynamic balance and its components. More
thorough descriptions with part specifications, design considerations, calculations, and
troubleshooting steps are given in the appendices. The goal of these experiments is to alter
atmospheric conditions and observe changes in particles using this instrumental setup.
The experimental setup consists of the following components, each described in detail below:
1. Electromagnetic trap
2. Pressure and vacuum system
3. Wiring feedthroughs
4. Optics
5. Cooling system
6. Gas flow system
7. Software programs
SN2, C02,
1I-
etc
Screen
with
article
Microscope
S
Shroud
Outer
Shroud
Vacuum Cha bet
Figure 4: Schematic of setup
17
2.2 Electrodynamic balance (EDB)
The EDB chamber contains the following components, described below:
* Electromagnetic trap
" Pressure and vacuum system
" Wiring feedthroughs
Electromagnetic trap
The electromagnetic trap is where particles are trapped and manipulated. The trap contains
electrodes, insulators, holes for particles to enter and leave the trap, and orifices to use for optical
access. This setup is shown below in Figure 5. Relative humidity sensors and thermocouples are
positioned just outside the trap, as described further in the wiring feedthroughs section.
1 CM
DC
AC
md
G
Figure 5: Electromagnetictrap setup
This trap is a spherical void electrodynamic levitator (SVEL) as described by Arnold & Folan
(1987), with a 1 cm 3 spherical void contained within cylindrical electrodes. A DC current is
applied to the top electrode and grounded to the bottom electrode to counteract the gravitational
force of the particle. An AC current is applied to the middle electrode to center the particle
horizontally in the trap. The electrodes are isolated from one another via sapphire washers and
ceramic screw insulators.
The electric field inside the spherical void must be consistent to ensure stable particle
trapping. To enable a uniform electric field, we must prepare the inside of the trap assembling it.
First, we clean the electrodes by sonicating them in acetone. Then, we coat all metal on the
inside of the spherical void with Aerodag G, a colloidal graphite coating used as an electrical
lubricant that adheres well to metals (Aerodag G, 2013); even coatings of Aerodag G deposit a
film of graphite that helps spread the charge uniformly across the spherical void. For low
temperature experiments, we then coat the inside of the trap with a thin layer of hydrophobic
vacuum grease to minimize the propensity of water to condense on the inside of the trap rather
than on the particle; this helps to reach higher supersaturations. Finally, we assemble the trap and
attach the wires for the power sources.
Wires are screwed into holes in the sides of the electrodes. The wires go through the
feedthroughs as described in the wiring feedthroughs section. Two power sources provide the
18
particle holding forces. Typical experiments require a direct current voltage of 1-60 V and
alternating current voltage of 300-800 V. More specifications are given in Appendix A.
Pressure/vacuum and feedthrough systems
Wires for power, relative humidity, and temperature measurement connect from the trap to their
measurement devices and sources through a set of vacuum feedthrough lines. The reason for the
feedthroughs is to enable a two-chamber vacuum system. This system minimizes thermal
transfer between the inner flow path and the outer chamber and allows for more efficient cooling
(or heating) of the particle. It also allows controlled pressure changes on the inside or outside.
The feedthrough system is shown in Figure 6. More details of the types of gaskets and
connections are given in Appendix A. The procedure that I found to work best for replacing
wires or feeding more wires through the trap is described in Appendix B.
s = 1/4" Swagelok
Thrma
duples
RetIv Iuidt
Rdg
Power
Cc
I
Figure 6: Feedthroughsystem (side view). Coloredpaths indicate 'innerchamber' Light grey
indicates 'outer chamber'.
19
The inner chamber consists of the trap, RH sensors, thermocouples, the inner shroud, and
vacuum feedthrough lines with an output flow valve. Humidified N 2 (or any gas) is flowed
through this inner chamber. Most importantly, the cooling fluid (described in the Cooling system
section) makes metal-to-metal contact with the inner chamber and thus cools the particle.
The outer chamber contains a large volume (-5 liters) of air that can be pumped down to
near-vacuum (<4 torr) and currently has a leak rate of -1 torr/hour. Leak testing procedures are
described in Appendix C. This outer chamber is isolated from the ambient atmosphere with the
outer shroud and closed outputs. Once the outer chamber is pumped down, there is little air
between the cooling feedthroughs (in physical contact with the inner chamber) and the outer
chamber walls. Thus, thermal transfer is minimized and the trap can be cooled or warmed to
more extreme temperatures.
Sensors near the trap
The purpose of the experiments is to measure changes in particles under various atmospheric
conditions. To do this, we use relative humidity sensors and thermocouples placed approximately
1 cm from the particle outside the trap as shown in Figure 7.
We use two type E thermocouples, which are rated to -200 'C and contain nickel and
chromium positive (purple) and negative (red) wires, respectively (OMEGA Engineering, 2014).
One thermocouple is screwed into the metal cold finger just below the trap, while the other is
stationed in the air next to the trap. This way, any variations due to metal versus air heat transfer
can be noted; the particle itself is suspended in the air so should experience a temperature more
like the thermocouple in the air. Fortunately, once the temperature is stable, both thermocouples
read within 2 'C of one another. (See temperature testing in Appendix D for more details).
To measure relative humidity, we use two sensors with different characteristics and
outputs. Specifications, calibration procedures, and calculations are described in more detail in
Appendix E. For experiments down to -40 'C, we use a Honeywell HIH-4000-001 sensor, which
has an accuracy of +/- 1.5% (Honeywell, 2014). Below -40 'C, we use the UPSI G-US.13 low
temperature capacitive sensor, which has a lower accuracy but confirmed performance down to 90 'C. We tested the Honeywell sensor below -40 'C and showed that it does not function under
wet conditions, as described further in Appendix E.
On the exterior of the feedthrough lines, we attach type E thermocouple extension wire to
carry the thermocouple signals to the data acquisition (DAQ) devices. Ordinary 24 AWG wire is
used for the Honeywell RH sensor. A more complex setup is used for the UPSI ultra-lowtemperature RH sensor, described in Appendix E.
We use the following National Instruments devices to send the signals to the LabVIEW
programs (described later): USB-TC01 and USB-921 1A for thermocouples, USB-6008 for the
Honeywell sensor.
20
Figure 7: Sensors outside the trap
2.3 Optics
Particles on the micrometer size range are invisible to the naked eye. The optical train
illuminates the particles and allows us to know when they are stable or growing. Future work
could expand the use of the optics to light scattering or more advanced graphical analysis to
determine more properties of the particles (e.g. Duft & Leisner, 2004a).
The optics are made possible by four viewing ports in line with the trapped particle in the
electromagnetic trap (as visible in Figure 7 above and Figure 9 below). These coincide with
windows on the inner and outer shrouds. Each shroud contains six windows, placed as shown in
Figure 8. Window specifications are described in Appendix A.
A helium-neon (HeNe) laser outputs a red beam at 632.8 nm wavelength. This beam hits
an adjustable optical mirror at a 45 degree angle, which reflects through the viewing windows on
the trap. A camera focuses on the inside of the trap and sees the illuminated particle.
To align the laser, we place a mm-diameter stick such as an Allen wrench through the
holes where the particles drop. The stick scatters light, which can be viewed on the screen to
which the camera is connected. We can maximize the light scattering from the stick by adjusting
the mirror position. When properly aligned, it is easy to see light scattered by small (-10 Ym)
particles inserted into the trap.
21
/
~-
12"
Shroud/trap
3 "5
/35
8"
16"
IE
15"t
Figure 8: Optical setup, birds-eye view
Figure 9: Photo: EDB table setup
22
2.4 Cooling system
Cooling bath
A recirculating cooling bath is used to adjust the temperature that the particle experiences. This
setup uses a Lauda Brinkmann RP1290 bath, which holds 12 liters of cooling fluid and can cool
down to -90 C (Lauda Brinkmann, 1998). The fluid used is Kryo 85 (polydimethylsiloxane), a
low-viscosity silicone oil that can be cooled to -85 C (Lauda Brinkmann, 2010).
Flow path and thermal transfer
The cooling fluid follows a flow path shown in Figure 10. The hoses to and from the Lauda bath
are insulated with ultra-flexible foam rubber pipe insulation and insulation tape (McMaster-Carr)
1cm
Figure10: Flow path for coolingfluid. Left: flow between Lauda bath and chamber.Right: Flow
inside chamber.
The coolant flows up through the hoses into a copper 'cold finger', which makes contact
with the support stand for the electromagnetic trap. Cry-con grease spread between the supports
increases thermal transfer upwards toward the trap (Torikachvili et al., 1983). Macor ceramic
supports reduce thermal loss to the metal supports.
Using this flow system and evacuating the outer chamber, the EDB can reach a
temperature of -70 'C when the bath is set to -85 'C. This system could be further improved by
pre-cooling the input gas flow into the inner shroud and by insulating the metal tubing inside.
23
2.5 Gas flow system
To control the humidity over the suspended particles, a gas manifold has been built (Figure 11).
Compressed nitrogen (N 2 ) is used for these experiments, although any gas could be used to
mimic different atmospheres. The N2 is split into two paths, each controlled by a mass flow
controller (Alicat, 2013). One path is only dry N2, which passes through a copper coil in the
cooling bath to ensure no water remains in the flow. The other line passes through a bubbler to
become humidified. These two paths rejoin and enter the top of the inner shroud down to the
particle. Altering the balance of wet versus dry N2 alters the humidity of the N2 entering the
shroud. The maximum stable flow rate for these single-particle experiments is approximately
100 sccm..
5 m
FC
10
+
FC
+02
~Lauda
bath
31\
c
Figure 11: Gasflow system. Upper branch:Dry gas. Lower branch:Humidified gas.
24
C
2.6 LabVIEW program
Data from the experiments is recorded using LabVIEW 2009 software. LabVIEW is a software
program that interfaces with hardware and data acquisition devices. The program currently reads
and stores temperature, relative humidity, and manual user inputs. Near-term improvements will
leverage the LabVIEW program to automate experiments and run feedback loops with the flow
system, cooling bath, camera, and power supplies. More details and screenshots are shown in
Appendix F.
The current iteration of the LabVIEW program does the following:
1. Creates a text file
2. Reads the external hardware (thermocouples and relative humidity sensors)
3. Calculates the relative humidity from the capacitance data, for the UPSI sensor
4. Calculates the relative humidity with respect to ice
5. Graphs the relative humidity and temperature real-time
6. Allows for manual input of DC voltage, AC voltage, and notes about particle behavior
7. Records these fields and the date/time at a user-specified interval
NI-TC-01
PC
TC1
LabVIEW
NI-USB-9211
TC2
---.
NI USB-6008
13a
H
Honeywell RH
Agilent U1701
UPSI RH
Agilent DataLogger
Figure 12: Hardware-softwareinterface.
25
-+WEData.txt
2.7 Limitations and specifications of complete system
More detailed physical specifications are described in Appendix A, and troubleshooting
procedures are described in Appendix G. A summary of limitations is as follows:
Temperature limits
*
"
*
"
"
Lauda bath: Lower: -90 0C. Upper: +200 "C.
Kryo 85 cooling fluid: Lower: -85 'C. Upper: +30 0C.
Viton gaskets (external/output seals): Lower: rated to -70 'C. Upper: +200 0C.
Honeywell RH sensor: Lower: rated to -40 'C. Upper: +85 'C (storage up to 125 0C).
USPI RH sensor: Lower: rated to -90 0C. Upper: +85 0C.
Pressure limits
* When evacuated, the outer chamber leaks at ~1 torr/hour.
" When evacuated, the inner chamber leaks at - torr/hour.
* If overpressured above ~1000 torr, the sapphire windows on the shrouds can break.
Size range of trapped particles:
The range of aerodynamic diameters that can be captured in the trap depends on the
particle density, temperature, and voltage and falls between ~5 jm-50 pm. The "spring point"
method used to calculate the particle size is described in section 3.3. The particles trapped in this
setup are larger than the average particles in the terrestrial or Martian atmospheres. The mean
diameter of dust in the Martian atmosphere is in the 1.6 jm to 3.7 jm range (Tomasko et al.,
1999), although Pathak et al. (2008) included sizes from .002-166 yim in a 1-D Martian model.
The effects of size on nucleating ability have been studied and can be accounted for (e.g.
Archuleta, DeMott, & Kreidenweis, 2005; Marcolli et al., 2007). If Martian particles were
mainly small (less than 0.1 jm), water uptake properties of the particles might depend on size
(Bahadur & Russell, 2008). However, the Kelvin effect can be neglected for low-molecularweight particles above 50 nm (Gysel et al., 2002; Seinfeld & Pandis, 2006), which encompasses
most of the volume of dust in the Martian atmosphere (Tomasko et al., 1999). Thus, any
difference in nucleation between typical Martian dust and the dust we trap will depend on
properties such as the number of active sites rather than differences in intrinsic thermodynamic
regimes.
The number of active sites where nucleation might take place depends on the surface area
of a particle (Kanji, Florea, & Abbatt, 2008; Pruppacher & Klett, 2006; Trainer et al., 2009). The
ability of a single active site to trigger nucleation is determined by a contact parameter described
by classical nucleation theory (Pruppacher & Klett, 2006). By estimating an active site density
using parameterizations such as that developed by Marcolli et al. (2007) or Connolly et al.
(2010), the nucleation ability of a larger particle can be used to calculate the nucleation ability of
smaller particles.
26
3 Procedures
3.1 Particle generation and descriptions
Generating and drying solid particles
The EDB traps particles with aerodynamic diameters between 5 pm-50 pm, as described above
in "Size range of trapped particles". To generate such particles does not require sophisticated
equipment. We use a mortar and pestle to grind samples of dusts or salts, which results in
adequate particles for this setup.
Particles are not wet-generated or allowed to saturate with water prior to use, but the
long-term history of the particles is unknown. Wet generation has been shown to redistribute
soluble material and alter the shape of particles, which may affect their nucleation properties
(Garimella et al., 2013). . It is also possible that at some point in an experimental particle's
history, water could have been trapped in ink-bottle-shaped internal pores, and the particle
therefore does not dry out completely during the experiment. On Mars in particular, this is
unrealistic, as there is insufficient liquid water in the atmosphere or near-surface that would be
able to change the particle properties this way. At the beginning of each experiment (after
trapping the particle), dry N2 is flowed into the inner shroud to ensure it begins from a dry state
(~5% RH). Internal pore structure could be analyzed using mercury porosimetry such as in
Hunter et al. (2000) to determine whether more careful particle desiccation is necessary for
studying Martian dust analogues.
Salts
Lab-quality granular sodium chloride (NaCl) is used for validation experiments, because it is
well characterized. Ammonium sulfate (NH 4 )2SO4 is another well-characterized salt with a
known deliquescence point that could also be easily trapped.
Arizona Test Dust (ATD)
Arizona Test Dust (ATD) is a mineral dust used for validation of freezing studies. Its
heterogeneous freezing properties are well-studied (e.g. Atkinson et al., 2013; Connolly et al.,
2009; Knopf & Koop, 2006; Niedermeier et al., 2010). The bulk density of the type used in this
setup is 2.35 g/cm3
Mojave Martian Simulant (MMS) dust
Mojave Martian Simulant (MMS) dust simulates the composition and spectral properties of
Martian mineral dust. Physical properties of Martian dust are inferred from the Mars Pathfinder
rover imagery and soil/rover wheel interactions (Beegle et al., 2005; Peters et al., 2008). Its bulk
density is 1.078 g/cm 3 . Recent studies have shown MMS to be non-hygroscopic, but wettable
and an effective ice nucleus (Cziczo et al., 2013; Ladino & Abbatt, 2013).
27
3.2 Trapping and balancing particles
The following steps are taken to trap and balance particles:
1. Turn on the cooling bath, set to room temperature; this removes the possibility of
dropping a trapped particle later on due to sudden internal movement of fluids.
2. Turn on the N2. Set to 50 sccm dry flow only. Set the gas line aside until after particle is
trapped.
3. Plug in all hardware and start the LabVIEW software. Start the Agilent capacitance meter
and Agilent DataLogger software (Agilent Technologies, 2012).
4. Secure the inner shroud to prevent ambient airflow from disturbing particles in the trap
5. Align the laser.
6. Start the power supplies, with initial voltages approximately DC=10-25V, AC=300600V.
7. If using the UPSI RH sensor, make sure it is not in contact with the metal of the trap;
electric fields from the trap will affect the capacitance.
8. Generate the particles.
9. Collect particles on the tip of a zip-tie.
10. Impart static charges by scraping the zip-tie on the top of the trap.
11. Watch falling particles on the screen
a. If a particle becomes trapped, attach the top of inner shroud. Adjust laser mirror to
ensure there is only one particle trapped. If there are multiple particles, remove
top and blow particles out of trap with N2.
b. If particle is not trapped, clear trap with N2 and repeat until particle is trapped.
12. After a particle is trapped, attach the top of inner shroud and attach the outer shroud.
13. Attach the N2 line and dry out the particle; gradually increase flow to 100 sccm.
14. Calculate the initial particle size using the "spring point method" described below.
15. Perform experiments, recording the spring point each time the particle size changes.
16. After the experiments are over, dry out and warm up the trap before removing shrouds to
reduce water condensation and corrosion.
3.3 Size calculations: Spring point method
To determine the aerodynamic diameter and changes in aerodynamic diameter of the
particle through the course of experiments, we use the 'spring point' method described Hunter et
al. (2000). In this method, a particle is stabilized in the trap and the DC voltage is optimized so
that the particle appears motionless. Here, the AC field and the aerodynamic drag "balance each
other and confine the particle to a small region of the trap" (Hunter et al., 2000). Then, the AC
voltage is gradually increased until the 'spring point'. The spring point occurs at the AC voltage
where the particle begins to oscillate vertically. At this point, the trapping potential overcomes
the aerodynamic drag of the particle and causes it to harmonically oscillate (Hunter et al., 2000).
The person conducting the experiment can easily see this point and record its field parameters to
calculate the particle size. As a particle grows or interacts with water vapor, its spring point
changes.
28
Bauer developed a spreadsheet with formulas based on the SVEL stability fields and
aerosol microphysical parameters to calculate the particle diameter. The user inputs the spring
point AC and DC voltages, the temperature, and particle density. The spreadsheet calculates the
aerodynamic diameter using the equations and constants below. The particle diameter can be
determined using equations of motion (1) and (2) as a function of the AC field strength E' and the
drag parameter d at the spring point. From Hunter et al. (2000):
(1) E'
2C 1 Vspg
VdcCOZOW 2
(2) d =
9ga
(1+f)pwa2
Where:
0 Vsp is the AC voltage at which oscillation begins [generally 500-1000 V]
*
Vdc is the DC balancing voltage [10-50 V]
*
*
*
*
*
*
*
g is standard gravity [980 cm/s 2]
Co and C 1 are geometric constraints for a given trap geometry [for spherical void
geometry, Co=Ci 1]
zo is the trap radius [0.635 cm]
o> is the AC driving frequency [60 Hz]
p is the viscosity of the gas [temperature-dependent, e.g. 0.00018 g/cm*s for roomtemperature air]
1 + f is the Cunningham drag correction [for particles <15 pm]
p is the particle density
*
a is the particle radius
In the current setup, the AC power source and the maximum drag parameter limit the minimum
size of the trapped particles. The power supply can reach 1000 V under typical conditions, but
low ambient humidity can reduce this maximum voltage toward 600 V due to static electric
sparks (Meech, 2008). Using the spring point calculations spreadsheet, the range of aerodynamic
diameters for NaCl and MMS that can be captured is ~5 ypm-50 ypm. The range depends on the
particle density and temperature. The minimum 'spring-able' size of a given particle type can be
calculated as shown above and in Hunter et al. (2000).
3.4 Validation experiments
To validate that the EDB setup functions properly, we perform three types of experiments and
compare our results with those from other studies. These tests and results are described below.
1. Deliquescence of sodium chloride (NaCl) at room temperature and 0 0C
2. Efflorescence of NaCl at room temperature and 0 'C
3. Freezing of Arizona Test Dust (ATD)
Theoretical deliquescence points
Salt particles spontaneously change to liquid droplets when exposed to a certain relative
humidity. This change is called deliquescence. It is an equilibrium transformation that occurs at
29
the point that will increase entropy of the system (Lamb & Verlinde, 2000). The deliquescence
relative humidity (DRH) can be calculated using thermodynamic properties. At a given relative
humidity, a salt can be either a solid or an aqueous solution; the phase that occurs is whichever
enables the salt to maintain a lower Gibbs free energy. Deliquescence occurs when the relative
humidity increases sufficiently that the Gibbs free energy of the solid salt is equal to its free
energy as a saturated solution (Seinfeld & Pandis, 2006).
A related term, water activity (a,,), can be calculated using aqueous salt solubility data
(Seinfeld & Pandis, 2006, and references therein). The relation between water activity and DRH
is shown in equation 3:
() DRH
(3)
- = aws
100
W
The DRH depends directly on the solubility of the salt in water and water activity; more soluble
salts deliquesce at lower relative humidities (Lamb & Verlinde, 2000). Thus, DRH also varies
with temperature, because the solubility of a salt and thus the vapor pressure of water over a
solution varies with temperature (Seinfeld & Pandis, 2006). Equation 4 describes the variation of
DRH with temperature, where A, B, and C are solubility constants given in Appendix E
(Seinfeld & Pandis, 2006, p. 454).
(4)
DRH(T) = DRH(298) exp
1H
[A (I-
-
B
*
in ()
-
C(T - 298)]l
Deliquescence experiments
Previous studies have measured deliquescence of single particles using an EDB and compared
the DRH and growth factors to those calculated by theory (Cohen et al., 1987; Gysel, et al.,
2002; Tang et al., 1977; Tang & Munkelwitz, 1993). Growth factors describe the expected
change in diameter of the particle after it deliquesces and are commonly plotted as D/Do. Kohler
theory describes how water vapor condenses to form droplets by combining the solute
thermodynamics described above with the Kelvin effect, which describes how vapor pressure
changes over a curved surface (Pruppaccher & Klett, 2010). For particles less than ~100nm
(those in the Aitken or nucleation mode), calculations must take into account the Kelvin effect,
since the curvature of a small droplet is significant (Hu et al., 2010; Russell & Ming, 2002;
Seinfeld & Pandis, 2006). For particles larger than 100nm, the Kelvin effect can be neglected
(Gysel et al., 2002).
In this study and in the other EDB studies, experiments are performed using - 0pim NaCl
particles. We test at 0 0C and room temperature (20-25 0C). Using Equation 2, it can be shown
that the difference in DRH for NaCl between 00C and 25'C is well below the precision of our RH
sensor. We show these calculations in Appendix E.
When performing experiments, if the particle becomes uncentered or unstable when
viewed on screen, we manually increase the DC voltage until it stabilizes; these adjustments
signify an increase in particle mass due to hygroscopic growth (Hunter et al., 2000). Since dry
NaCl particles are cubic and our size calculations are based on spherical particles, the absolute
changes in diameter shown below have errors of ±10% (Gysel et al., 2002).
30
As shown in Figure 13, experiments performed with our EDB agree well with both
experiments performed by other groups and with the theoretical DRH. The theoretical DRH at
room temperature is 75.3% (Seinfeld & Pandis, 2006, Table 10.1), and our experiments yield a
DRH of 75.0%, with RH sensor accuracy of ± 1.5%.
Hygroscopic growth of NaCI, 20 0 C
2
L
*
*
.
1.8
*
LL
L
L
L
.
L
A
B
c
Theoretical DRH
Tang, Munkelwitz, & Davis 1977
Gysel, Weingartner, & Baltensperger 2002
L
S
I
I
Best Estimate DRH, these studies = 75±0.6
1.6 F-
S
0
1.4 F
I
of
1.2K
1
0
10
20
30
40
50
60
r
r
70
80
90
100
Relative Humidity (%)
Figure 13: Deliquescence curve for NaCl compared to other studies. Representative error
bars on left. Estimatedfrom Tang, Munkelwitz, & Davis, 1977, Fig. 8, and Gysel,
Weingartner, & Baltensperger,2002, Fig. 3.
31
Efflorescence and hysteresis
Efflorescence is the reverse process from deliquescence; at a certain relative humidity (the ERH),
a concentrated electrolyte droplet loses its water and crystallizes. However, efflorescence occurs
far below the DRH because it is kinetically favorable to remain a liquid droplet until a lower
relative humidity (Martin, 2000). This results in a hysteresis loop as shown in Figure 15. The
ERH can be estimated theoretically for certain electrolytes but is not as straightforward to predict
as the DRH and depends on the droplet concentration and size (Gao, Chen, & Yu, 2007).
Experimental studies have determined the ERH for large NaCl droplets to be between
43%-48%. Using our EDB, we performed efflorescence experiments by first deliquescing an
NaCl particle, then quickly switching to dry N 2 flow (before the particle became too large and
fell) and drying the particle out. Our results, shown in Figure 15 and Figure 14, agree with theory
and other experiments. Any imprecision is likely due to slight differences in the RH experienced
by the particle and the RH experienced by the sensor (positioned 1-2 cm away).
Efflorescence of NaCl, 20 0 C
2
A
40
Ll
L
A
B
Gysel, Weingartner, & Baltensperger 2002
Tang, Munkelwitz, & Davis 1977
Theoretical ERH
N
(
-
1.8
L~
L
2
00>
Best Estimate ERH, these experiments = 40±1.4
1.61
>
K2>2
0008
0D
1.4
00@
1.21
*sssssssmsmcmeK2>
IG 010010860080"S"
r I r r
r
r
r
60
70
1
0
10
20
30
40
50
80
90
100
Relative Humidity (%)
Figure 14: Efflorescence datafrom EDB (A and B) comparedto other studies. Representative
error bars on left. Estimatedfrom Tang, Munkelwitz, & Davis, 1977, Fig. 8, and Gysel,
Weingartner, & Baltensperger,2002, Fig. 3.
32
Deliquescence of NaCl, 20 0 C
Deliquescence of NaCI, OC
1.6 -
1.6 -
1.5 -
SA
3
B
1.5 -
c
0>1.4-
1.4-
A
0A
1
B
C
Theoretical DRH
Estimated DRH,
studies = 75.4
Theoretical DRH
1.3
1.3 -
Estimated DRH,
1.3=these
-
1.2
1.2
1.1
0.9
T.
0
20
40
60
80
0.9
100
0
Efflorescence of NaCI, 20 0 C
0 A
1.6
E
1.5 -
40
60
80
100
80
100
Efflorescence, OC
*
B
Theoretical ERH
1.6
Estimated ERH,
1.5
these experiments =40 [E
1.4 --
- 1.4 -
1.3
El
A
B
Theoretical ERH
Estimated ERH,
these experiments = 45.5
1.3-
1.2
1.2-
-
1
0.9
20
[ism0
0
20
40
60
80
0.9 0
100
Relative Humidity (%)
20
40
60
Relative Humidity (%)
Figure 15: Hysteresis loop: Deliquescenceand efflorescence of NaCl using EDB, 20 0C and
0 C. Top panels: A dry NaCl particledeliquesces at ~75% RH. Bottom panels: An NaCl droplet
crystallizes around45% RH.
0
Arizona Test Dust (ATD) freezing
Other experiments on ATD have shown freezing at supersaturations and temperatures achievable
using this EDB setup. Kanji et al. (2008) showed deposition nucleation at -40 'C and 110-120%
RHice. Knopf & Koop (2006) showed ice nucleation at a wide range of RHice for ATD, with
minimums close to 105% RHice for 200<T<240 K. M6hler et al. (2006) measured freezing from 76 OC to -50 "C and 110% to 115% RHice. These results overlay the saturations we reached in this
study in Figure 16.
This indicates that ATD particles could freeze in the EDB setup; however, the thresholds
given are the minimum supersaturations shown to freeze 1% of ATD particles in their respective
33
experiments. Without testing hundreds of particles, it is uncertain that ATD would freeze in the
EDB. We are not confident that we froze ATD with our setup. At -65 'C and 112% + 5% RHie,
after flowing wet N2 for five hours, an ATD particle did fall, indicating that it perhaps froze or
gained mass. However, other variables could have caused this, such as frost from above falling
on top of the particle.
Future experiments could probe colder temperatures and higher relative humidities. For
these studies, temperature and RH were limited due to the cooling bath fluid type and the
tendency for water vapor to deposit on metal surfaces other than the particle, respectively. Using
lower-temperature fluid and coating the inner flow path with superhydrophobic substances could
enable higher supersaturations.
Ice Nucleation Phase Diagram: Minimum S crit for ATD
2
n
100 nm Homogenous Freezing Cur~e
Water Saturation
1.9-
EDB limits in these experiments
Kanji, Florea, & Abbatt (2008)
Knopf & Koop (2006)
Mohler et al. (2006)
1.8 1.7-
1.61.5 1.4-
1.31.2-
-70
-60
-50
-40
-30
-20
-10
0
Temperature (0C)
Figure 16: Ice nucleationphase diagram.Red line shows homogeneousfreezing curvefor
droplets (Koop et al., 2000). Black line shows water saturation,i.e. 100% RH(liquid water). Blue
shaded area shows relative humidity maximums we reached in this study. Symbols show ATD
minimum Sice thresholdsfor 1% freezing below -40 C given by Cziczo et al. (2009), Kanji,
Florea,& Abbatt (2008), Knopf & Koop (2006), and Mdhler et al. (2006).
34
4 New results: Martian Dust Simulant
4.1 Room Temperature
We suspended MMS particles in the EDB and investigated their hygroscopicity. As shown in
Figure 17 at room temperature and 0 0C, MMS particles did not deliquesce above 90% RH.
MMS dust is composed of inorganic material and not characterized as a hygroscopic material, so
this result is as expected (Peters et al., 2008). From 65%-85% RH, some MMS particles do
uptake some water, but the increase in mass due to this water did not alter the necessary holding
voltage by more than 1-3 volts (5-10%). This water likely adsorbed to the particle surfaces as
monolayers or was captured in internal pores (e.g. Keyser & Leu, 1993; Marcolli, 2013)
MMS water uptake, OC
2
0
1.8
0
A
B
1.6
E
1.4
1.2
1
-0
rrrr
10
20
30
40
50
60
70
80
90
100
MMS water uptake, 250C
2
1.8
-
*
0
1.6
B
C
1.4
1.2
TFm-a
1
0
10
94%*e
20
30
40
50
60
70
80
90
Relative Humidity (%)
Figure17: MMS water uptake, 25'C and & C, with representativeerror bars.
35
100
4.2 Freezing
Based on these experiments, the critical supersaturation for Martian freezing should be at
least 1.15. Model parameterizations that use values less than 1.15 should be revisited.
Dry MMS particles were cooled to -70 'C. Wet N2 was added for several hours until reaching a
maximum RH(liquid)=60%, RH(ice)=1 15%. The RH(liquid) to RH(ice) conversion is shown in
Figure 18, and a representative time series of RH from one of these trials is shown in Figure 19.
No particle freezing was detected, and the diameter remained constant.
Relative humidity: liquid vs ice, -70C
120
10080-
0
a,
C.,
60-
I
a:
40L
20
0
0
10
20
30
40
50
60
70
RHlq (%)
Figure 18: RH(liquid) vs RH(ice), -70'C, based on Koop & Murphy (2006)
Timeseries of MMS experiment, -68C
120
-----
110
5
100
90
0-
RHi, experienced b MMS particle.
D/D 0 =1 throughout
80
CD)
70
-
60-
5040-
30
20
r
0
100
200
300
400
500
600
Time [minutes]
Figure 19: Time series of MMS experiment at -68 "C. Maximum RHice=115%. Particlediameter
remained unchanged.
36
No ice nucleation occurred up to 115% RHice at -60 OC to -70 *C. At -70 0C, the lowest
temperature tested in this study, Ladino & Abbatt (2013) found a required supersaturation of
175%. At the higher temperatures of these experiments (--60 OC), the RHice is closer to 1.3
(Cziczo et al., 2013; Ladino & Abbatt, 2013). These other studies froze many particles in each
run and determined 'significant' ice nucleation at the point where 1% or 10% of particles froze.
This is shown in Figure 20. Due to the single-particle nature of the EDB, many more
experiments and slightly higher relative humidity would be needed to describe a significant
result for an upper bound on critical supersaturation.
Since these particles are larger than typical Martian aerosols, they may contain more
active sites and be more efficient ice nuclei than real aerosols present in the Martian atmosphere
(Connolly et al., 2009). Thus, any critical supersaturations determined using the EDB would give
a lower limit to the critical supersaturations to nucleate ice on the real Martian particles. Based
on these experiments, the critical supersaturation for Martian freezing should be at least 1.15.
Model parameterizations that use values less than 1.15 should be revisited.
Ice Nucleation Phase Diagram: Minimum S crit for MMS
___
2 _________cr__________t
-
1.9-
-
1.8 U
100 nm Homogenous Freezing Cure
Water Saturation
EDB limits in these experiments
Cziczo et al. (2013)
Ladino & Abbatt (2013)
1.7 -NM
1.6V5 1. 5 CUU
1.41.31.2U
1.1
-70
-60
-50
-40
Temperature
-30
-20
-10
0
(0 C)
Figure 20: Ice nucleationphase diagram.Red line shows homogeneous freezing curve for
droplets (Koop et al., 2000). Black line shows water saturation,i.e. 100% RH(liquid water). Blue
shaded area shows relative humidity maximums we reached in this study. Symbols show MMS
minimum Sice thresholdsfor 1% freezing given by Cziczo et al. (2013) and Ladino & Abbatt
(2013).
37
5 Conclusions and future work
5.1 Suggestions for future studies
Software and data acquisition hardware improvements
By upgrading hardware or connecting existing hardware to the computer, EDB experiments
could be made more autonomous and repeatable. First, the computer should be replaced with a
newer PC with USB 2.0 and USB 3.0 ports and more RAM. We already purchased a new CMOS
camera (ThorLabs, 2014) that can connect via USB to record experiments; this is shown in
Figure 21. In addition to recording video, the image from this camera (or a more high-resolution
camera) could be integrated into LabVIEW to create a holding loop for trapping particles. We
elaborate on this concept in Appendix F, but it essentially would create a grid and seek to keep
the particle centered on the grid by adjusting the DC voltage. For this, the power supplies must
also be controlled by the PC. Finally, by connecting the mass flow controllers to the PC (using
an Alicat BB9 controller, already purchased), the relative humidity could be targeted and
controlled with LabVIEW.
A schematic is shown in
Figure 22. If the LabVIEW program knew the particle density and the voltage required to
stabilize at the measured RH, and automatically adjusted the voltage as the particle uptakes or
loses water, it could record experiments with higher precision and of longer duration than we
performed previously.
Figure21: ThorLabs USB camera to replace Burle security camera.
38
E
RH
Trap
patlej
__
Particle too
low/high?
Set
DC/AC
RH too
low/high?
Set flow
controllers
Temp too
Set cooling
low/high?
bath
LabVIEW
-311
5
Power
supplies
F7
cntrollers
Cooling
bath
Figure 22: Schematic of proposed hardwareand software integrationand automation
Chamber feedthrough redesign
The chamber could be redesigned or modified to still allow for the inner-outer chamber vacuum
but to improve the wire feedthrough system. The current vacuum feedthrough lines crack easily,
limit the total number of wires to ~10 wires, and make replacing wires arduous. Since the
original chamber was designed, new high-density wire feedthrough assemblies have been created
(e.g. Douglas Electrical Components, 2010; Thermo-Electra, 2009). By de-welding the current
feedthrough lines and replacing them with a high-density feedthrough assembly, measurement
sensors could easily be added or removed from the setup and leaks would be minimized.
Further freezing experiments
A new superhydrophobic coating (Hydrobead Two-Part) will be applied to the trap and interior
of the inner shroud. The coating is good for electronics and contains a hydrophobic layer and an
anti-icing part. By improving the hydrophobic coating on the inside of the inner shroud, the
particle should be able to experience higher relative humidities. We already approached the
conditions required for freezing ATD, so ATD freezing should be pursued with the current setup.
If the improved coating allows for significant increases in RH (above -120% RHice), further
experiments should be performed on MMS.
39
Phase functions
The EDB could give insight on how particles interact with light through scattering and phase
function experiments. On Earth and also for other planets observed with satellites or telescopes,
the intensity of light reflected back to space can give information about the atmosphere; the light
scattered depends on the spatial distribution of particles, their number density, and their optical
properties (Laan et al., 2009). Radiative forcing models often assume homogeneous spherical or
spheroid particles in light scattering calculations. However, experiments have shown that
atmospheric particles are often irregularly shaped, and the light scattering measured from
particles such as mineral dust varies significantly from that predicted using Mie-Lorenz theory
(e.g. Laan et al., 2009; Mufioz et al., 2010).
A central database called the Amsterdam-Granada Light Scattering Database collects
scattering matrices and phase functions from various dust and particle laboratory experiments
(Mufioz et al., 2012). This data has been collected from ring setups where a laser beam is
transformed and polarized and passed through aerosol particles. Light intensity scattered from
the particles is detected at various angles encircling the aerosols (Laan et al., 2009; Muffoz et al.,
2010). We could set up a similar system using the EDB as the focal point, with single particles
scattering the laser beam and a detector set up at multiple angles around each window of the
inner shroud. A schematic of this setup is shown in Figure 23. The EDB already contains six
viewing windows, each of which spans several degrees and could accommodate multiple
detector measurements. Maria Zawadowicz (Cziczo Group, MIT) has designed a prototype
detector and will attempt phase function experiments using this setup.
EDB with particle
laser
rotating stage
--
EO
EOM
A
QWP
detector
Figure 23: Block diagram ofproposed single particle scatterometer/polarimeter.From Maria
Zawadowicz, MIT, Masters Thesis Proposal.
40
5.2 Summary
In this project, we successfully assembled an electrodynamic balance (EDB). We set up a gas
flow system, a cooling system, and optics. We installed and calibrated thermocouples and
relative humidity sensors and developed a software program to record their readings. To validate
the setup, we performed deliquescence and efflorescence experiments on NaCl particles at room
temperature and 0 'C and compared our results with those from other studies. We then cooled the
system to 200-220 K and tested water uptake on Arizona Test Dust and Mojave Martian
Simulant dust, with no freezing observed up to 115% RH with respect to ice. Given that our
particles are larger than typical Martian aerosols, they have more surface area and should be
more efficient ice nuclei (Connolly et al., 2009); our experiments may underestimate the critical
supersaturations. These results suggest that the critical supersaturation is too low in models such
as those described by Davy et al. (2008) and Navarro et al. (2013).
Overall, we have shown that an EDB is a viable instrument to study single particles at
very low temperatures at low supersaturations. Future studies with the EDB could improve
understanding of aerosol behavior under such regimes. Improvements in hardware and software
could increase the precision of experiments and allow for studies of particle light scattering.
41
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48
Appendices
A. Specifications
Trap assembly
Ceramic Insulators
Upper electrode - DC
Middle electrode - AC
Optical windows
Sapphire Washers
Bottom electrode - ground
Thermocouple
Figure 24: EDB trap assembly
Ceramic supports
2.756"
1/4-20
]
~ -~_~
--
0.511"
3/16-24
Figure 25: Ceramicstandoff specifications
49
Sapphire windows
Sapphire windows are of a non-standard size and thickness. Replacements can be ordered fron
Swiss Jewel.
T1
Figure 26: Sapphire window diagram
Table 1: Sapphire window specifications
Location
Inner shroud
Quantity
4
D (in)
19
T (in)
1
Inner shroud
4
15.8
1
Outer shroud
6
1
.06
50
....
.....
.
Exterior parts and dimensions
1/4"
6.2"
I
1/4" Ultratorr
1''
-32x1/4.
6-32x3/4"
A
Ll
8-32x1/2"
7"1
D
D
-
6.0"
4.75"
8-32x1/2"
Li.
I-
_
-i
>i--6-32x3/4"
Figure 27: (left) Outer shroud dimensions. (right) Inner shrouddimensions.
51
IT
8"1
3.5'
3.5"
Key:
I CF
X KF
I ISO L
s 1/4" S agelok
E Feedthrough wire
1$1.51
2.75''
18"
511
1.25"1
1.5"1
215"
3.5"
2.75"
51IE
-A
Figure 28: Chamberdiagram and vacuum flange specifications. CF: 6-hole ConFlatwith
Viton@ or copper gasket. KF: Kwik-Flange. ISO LF: Large Flange with Vitone 0-ring.
*
*
Feedthroughs: 1.3" OD ConFlat, 4 pins, ceramic insulators
o Type E thermocouple wire inside, with Type E thermocouple extension wire
outside feedthroughs
2.75" OD ConFlat fittings: each with 6 x 1.5-inch 1/-28 screws
52
Cooling bath
The cooling bath is connected to the EDB chamber via plastic tubing, with the last few inches
hose clamped over copper tubing; this is shown in Figure 29. The bath temperature limits are
-90 OC to +200 OC.
Key:
[1/2" Plastic tubing
Wiu1/2" Cu tubing
am 1/4" Cu tubing
s 1/4" Swagelok
3"2
EDB
Chamber
I
42"
S
--
24"
Cooling bath
19"1
24"
Figure 29: Cooling bath connections and specifications.Tubing is covered with ultra-flexible
foam rubberpipe insulation (McMaster).
Inner vacuum feedthrough lines
Key:
[1/4" Convoluted metal tubing
ujl/2" OD weld
i1/8" ID weld
V 1/4" VCR (female)
10"
:
6"
Figure 30: Inner vacuum feedthrough line specifications. 3 x Swagelok convoluted metal tubing,
part # 321-8-X-6DFR
53
B. Feeding wires
To modify wires or replace shorted wires, wires must be pulled from the base of the inner shroud
through the vacuum feedthrough lines to the exterior connectors. The feedthrough lines bend,
and the inner diameter of the sections with the welded female VCR fittings is only 1/8" (shown
in Appendix A). This makes it difficult to feed wires through the entire system all at once. The
following procedure seemed to optimize wire-feeding:
1. Open the outer chamber to expose the vacuum feedthrough wires. Rest the outer chamber
on a chair/box placed under the table, to minimize strain on the feedthroughs.
2. Undo the VCR fittings for the feedthrough line of interest. (Remember to replace the
VCR gaskets!)
3. Undo the exterior ConFlat fitting and separate the inner-outer wire connections
Figure 31: Inner-outer wire connections
4. Attach a thin wire to the end that was attached to the wire connector. Draw it back
through to the VCR fitting; this is to make it easier to feed the new wires through the
ConFlat fittings without requiring removal of double ConFlat fitting.
5. Remove the outer and inner shrouds.
6. Unscrew the copper stand under the trap. Feed wires back through the hole at the bottom
of the stand to minimize friction with the new wires
54
Figure 32: Wire feed hole
7. Attach new wires to the old wires. Strip the ends of both sets of wires. Twist them
together securely. Solder a strong connection on top of them, or use the Albright knot
method described in (*). It is vital to attach these wires securely and without increasing
the overall diameter of the wires. If this is done incorrectly, try the method indicated by
(**).
8.
9.
10.
11.
12.
13.
14.
15.
Pull the new wires through from the upper VCR fitting, to minimize the length of wire
pulled at once.
One the new wires are through the top fitting, sever the wire above the vacuum
feedthrough and replace the VCR gasket.
Reattach the wires and pull through the feedthrough.
Sever the wire and replace the second VCR gasket.
Reattach the wires and pull the rest of the way through the Conflat fittings.
Reattach the end connections.
Test the wires for continuity using a multimeter.
Tighten the VCR fittings and reassemble the chamber.
(*) An alternative method to soldering that can be as or more effective is to use fishing knots.
The Albright knot can join two wires of equal or unequal diameter in a strong bind without
greatly increasing the overall diameter. To tie an Albright knot, make a sideways loop out of the
thicker or less tidy wire. Push the end of the other wire through the loop. Wrap it around both
55
strands at least five times, then pull it back through the loop and pull tight (Grog LLC, 2013). An
example is shown in Figure 33
(**) If the wire connection breaks inside the feedthrough line, retrieve the wires from both
ends. Place a pump at the bottom of the line and the end of a long piece of yarn at the hole below
the trap stand (or the other end of the line). Pump the yarn through the line. Tie fishing wire to
the top end of the yarn using an Albright knot. Pull the fishing wire through the line. Tie the
fishing wire to the new wires, and pull the wires through the line.
Figure 33: Albright knot between thermocouple wire andfishing line
Other notes:
" Power wires: Ends on the trap need to be soldered to fittings that can be screwed into the
trap.
*
"
RH wires: Ends on the trap need to be soldered to sockets for the RH sensors.
Thermocouple wires: Need to be reattached to the appropriate locations.
56
C. Leak testing
Leaks presented difficulties with the experimental setup. Leaks between the inner and outer
shrouds prevent the outer chamber from being pumped out and thus diminish extreme
temperature capabilities. Leaks from the outer shroud to the ambient atmosphere allow for the
intrusion of water vapor which condenses inside during cold temperature experiments. Leaks
from the inner shroud disrupt N2 flow and introduce uncertainty about the conditions which the
particle experiences. This section discusses leak testing procedures, sources of leaks, and
solutions.
Many parts have the potential to leak. This system contains several types of gaskets and
fittings, is subjected to cold temperatures, and was not in use for over a decade. The following
are typical leak sources, some of which are predictable and others less obvious.
Leak testing procedures
Ideally, one should simplify the system and test for leaks from the innermost to the outermost
parts of the chamber. This reduces the chances of resolving outer leaks then re-creating them
when it becomes necessary to reopen the chamber to test inner parts. Test accessible places such
as tubing and outer Swagelok fittings first, in case the leak is easy to fix.
We used the following strategies to find leaks:
* Pump out and measure pressure: Test the inner chamber, the outer chamber, and the
entire system. See which can hold pressure and which can leak. Measure leak rates. After
each troubleshooting step, gasket change, or leak patch, pump out and measure pressure
again.
* Overpressure and 'Snoop': Slightly overpressure the area of interest (e.g. inner shroud,
outer chamber) using compressed N 2 . Increase the pressure from room pressure (-760
torr) to 800-900 torr. Overpressuring above 900 torr risks breaking the shroud windows.
Apply Swagelok Snoop Liquid Leak Detector to any suspect areas. If bubbles form, N2 is
escaping the system and that is a leak.
* Helium leak detector: Using a helium leak detector such as the Balzers HLT 150 allows
one to find smaller leaks; overpressure the area of interest using helium, and use the leak
detector to find any region where helium is escaping.
* Underpressure and spray: With the leak detector, another strategy can also be
implemented. Once the leak is small enough to pump out the chamber below 10 torr,
pump the chamber down then spray helium on the exterior of the interest area. If the leak
detector senses helium, helium is entering the system near the location it has been
sprayed.
57
Vacuum feedthrough lines
The vacuum feedthrough lines caused the hardest-to-detect leaks. These lines are Swagelok
Vacuum Flexible Convoluted Tubing #321-4-X-6DFR, which consist of six-inch long stainless
steel tubes with 1/4" female VCR fittings on either end. Specification are shown in Appendix A.
The lines are only rated to -40'C and are bent and moved each time one moves the chamber or
changes the internal wiring setup. These factors likely contribute to small cracks forming along
parts of the tubes. Such cracks cause major leaks between the inner and outer shroud systems.
These leaks can be detected by opening the chamber, overpressuring the inner shroud, and
'snooping' the feedthroughs. To fix the cracks, we coated the suspect sections of tubing with
Dow Corning 734 flowable sealant and allowed it to set.
Figure 34: Vacuum feedthrough line leak spots
58
Gaskets, O-rings, and exterior fittings
Starting from the interior of the system, the following can cause leaks:
" VCR gaskets: The feedthrough lines carry wires from the inner shroud to the exterior
connections. There are three lines, each with two /4" copper VCR gaskets. Each gasket
should be replaced every time the fitting is opened, else it will leak.
" Conflat gaskets: After the wires pass through the VCR gaskets, they go through two sets
of 2.75" OD Conflat fittings. Each Conflat fitting contains either a Viton (rubber) gasket
or a copper gasket. The Viton gaskets can crack with age or cold and should be checked
occasionally. The copper gaskets work by creating a metal-to-metal seal; each time the
fitting is removed or replaced, it must be tightened further than the previous time to
ensure a deeper metal cut.
* O-rings: The main chamber arms are connected with large O-rings, which need to be
properly positioned to ensure a tight seal. The inner shroud base requires an O-ring that
leaks unless tightened very well. The thermocouple and power feedthroughs from the
interior to the atmosphere have Viton gaskets that must be checked and tightened. The
windows on the inner and outer shroud contain one O-ring each, which should be greased
and positioned carefully.
59
(a)
(a)
(b)
(b)
(c)
(d)
Figure 35: Common leak sources. (a) Feedthroughwires and VCR gaskets, (b) ConFlatfittings
and internal-externalfeedthroughs, (c) Inner shroud base, (d) Outer and inner shroud windows
60
D. Thermocouple testing
Thermocouple reading: Air vs metal
3020 -
TC*
TC2 = 1.
10-
R2
004*TC 1
+ -0. 1561
1
0-10-
N-20-30-40-50-604
-70-
-80r
-60
r
r
-50
-40
-30
-20
-10
0
10
20
30
TC1
Figure 36: Thermocouple scatterplot:TCJ is suspended in the air by the trap. TC2 is touching
the metal.
Thermocouple testing over time: Cooling then heating
30
20
10
0
-_
_10_
-10
- TC1 (air)
TC2 (metal)
-30
-40
-50-60
-70
r
1000
r
2000
--- r--
3000
4000
5000
Data point (10s)
rrr
6000
7000
8000
Figure 37: Thermocouple testing time series, cooling then warming trap. Thermocouple in air
and touching metal are within 20C at all times and are equal after cooling is complete.
61
E. Relative humidity specifications and calculations
Sensor specifications
Table 2: RH sensor specifications
UPSI US-13 (UPSI, 2014)
Honeywell HIH-4000-001
Electrical leads
Power requirement (V)
4
3
1-10, 5 best
Output
Capacitance (244-30lpF)
5 (3 calibrated)
4-5.8
Voltage
Accuracy (%)*
0
3.5
Temperature (specified
operating range)
-90 OC to +85 C
-40 OC to +85 OC
Response time (sec.)
0.25
5
Transfer equation
(linear)**
C=C55 (0.8955+0.002RH)
VOUt=Vsupp1y(0.0062RH+0.16)
Best-fit line***
RH=(C-241.8)/.54
RH=(V-0.8083)/0.033
*Better if calibrated
**Given in specifications guide, for 25 0C
***Calculated using MIT lab calibrations
Honeywell sensor calibration
We calibrated the Honeywell sensor by exposing it to known proportions of wet and dry N2,
using the Alicat flow controllers. We performed a linear regression to show that the voltage
varies linearly with relative humidity. Then, we compared the Honeywell sensor to other RH
sensors in the lab which had been calibrated with respect to multiple salts with known
deliquescence points. These comparisons are show in Figure 38
62
Honeywell RH sensor calibration
4
3.5 -
Honeywell RH sensor
V = 0.03304*RH + 0.8083
9
R21
3-
1.51-
0
10
20
40
50
RH (%)
60
70
80
90
100
Honeywell vs lab RH sensor comparison
10N
8 -
30
Voltage
Lab V = 3.021*Honeywell V -2.481
R2 =0.999
06
L.0
4 -
2-
0 -rrr
0.5
1
1.5
r
2
2.5
Honeywell sensor %oItage
r
3
r
r
3.5
4
Figure38: Honeywell RH sensor calibrations
UPSI relative humidity measurement overview
The UPSI sensor outputs capacitance rather than a linear voltage reading. It thus requires the
following stages for RH measurement: Hardware connectivity, parasitic capacitance calculations,
and capacitance-to-RH conversion. These steps are described in the following sections.
1. Attach wires of measured length, known material & diameter to sensor
2. Attach wires from shroud vacuum feedthroughs to Agilent U1701B Handheld
Capacitance Meter. Meter measures capacitance with 0.1 pF resolution and 1% accuracy,
5x/sec (Agilent Technologies, 2012).
3. Use IR-to-USB cable to log data to Data Logger for Agilent Handheld Capacitance Meter
(free software, requires VEE Runtime, Agilent 10 Libraries Suite, PL-2303 Driver)
4. Save data every 1 second to csv file
5. Read most recent line of csv file into LabVIEW
63
6. Calibrate and convert capacitance to RH
7. Record RH in LabVIEW text file
Calculate parasitic capacitance
The length of wires between the UPSI RH sensor and Agilent capacitance meter add parasitic
capacitance Cp (Standard Wire & Cable Co., n.d.)
1.
[pF] _
Calculate CP IT
2.2E
iog
1.3D
where d=diameter of conductor (in), D=diameter over insulation, E = insulation dielectric
constant, f=stranding factor
Here, using 2 twisted 24 AWG wires: d = 0.02", D=0.045", E ~ 3.6
Cp~ 7.4 pF/ft
Cp x 4.5 ft = 33.3 pF
2. Compare to measured Cp=34.3 pF (using capacitance meter on wires without RH sensor
attached). This agrees well with the calculated Cp. Enter this parasitic capacitance into
LabVIEW.
3.
CRH=Ctot-Cp
Calculate relative humidity from capacitance
1. UPSI gives transfer function C=C55(0.9052+0.0017RH+310-6RH 2 +710-8 RH 3 )
2. UPSI gives linear regression C=C55(0.8955+0.002RH)
RH in %, C in pF
3. Measure C55 = capacitance at 55% RH, room T, cold - do this once per year, should be
~270pF
4. Use as input into LabVIEW
5. Subtract Cp
6. Solve for RH
64
Calibration comparison: UPSI vs Honeywell RH sensors
1. Calibrate UPSI with respect to Honeywell sensor.
2. Measure RH (Honeywell) & capacitance (UPSI) at various relative humidities with both
sensors in inner shroud.
3. Fit capacitance to RH measured by Honeywell sensor, use to determine offset or
difference between this sensor and manufacturer's transfer function.
Honeywell RH vs UPSI Capacitance, Room T
320
*
Sensor
Derived function: C = 0.5089*RH + 246.1
R2 = 0.966
300-
UPSI transfer function (linear): C=C 55(RH/500+.8955
280UC.'
0
o 260Cz
240-
220-
200
0
10
20
30
40
RH (%)
50
60
70
80
Figure 39: RH sensor comparison
The UPSI sensor generally aligned with the given transfer function. Large fluctuations occurred
if the sensor was in contact with the metal of the EDB trap, where voltages disrupted the
capacitance measurements.
65
Testing Honeywell sensor below its rated limits
The Honeywell RH sensor is only rated to -40 'C and was not tested below that (personal comm.,
02/07/14). We tested it concurrently with the UPSI RH sensor at temperatures as low at -70 0C.
Conclusions:
*
*
The Honeywell sensor only works for one procedure: If cooled under dry conditions, then
humidified, it responds to the humidity and tracks the UPSI sensor up to 60% RH(liquid).
Once the Honeywell sensor has been humidified below -40 'C, it stops working and does
not respond to further changes in the chamber humidity; this is shown in Figure 40
below. A possible explanation is that water freezes onto the sensor at such low
temperatures.
Drying out: Honeywell vs UPSI, -50 C
90
80
70
Honeywell RH
UPSIRH
t-o*
60
rA
50 -rK
X
40
r
rr
30
20
0
10
20
30
40
50
Time (min)
Figure 40: RH(T) Honeywell vs UPSI at low T
66
60
70
Calculate RH with respect to ice
Relative humidity with respect to ice for a given temperature is calculated using the following
method:
1. Calculate the vapor pressure with respect to ice. (Murphy & Koop, 2005). Valid for
T>100K
Pice = exp 9.550426 -
+ 3.53068 ln(T) - 0.00728332T)
T
2. Calculate the vapor pressure with respect to liquid. (Seinfeld & Pandis, 2006, eqn. 1.10)
a
1
373.15
T
Pliq[mbar] = 1013.25 * exp(13.3185a - 1.97a
3.
2
0.6445a 3 - 0.1299a 4 )
-
Convert pressure unit to pascals.
Pliq[Pa] = Piq * 100
4. Calculate RH with respect to ice, with measured RHiiq
RHice = RHiiq (la)
Pice
DRH as a function of temperature
(2)
DRH(T) = DRH(298) exp
{s [A(! -
- B * In (
-
C(T - 298)])
Table 3: DRH constants [Seinfeld & Pandis, 2006, Table 10.1-10.3]
Salt
NaCl
A
0.1805
B
-5.310x10- 4
C
9.965x10-7
DRH(298) [%]
75.3
AH, (kJ/mol)
1.88
Table 4: Range of theoreticaldeliquescence relative humiditiesfor study conditions
Salt
NaCl
DRH (233K)
75.3018
DRH (273K)
75.3006
DRH (298 K)
75.3000
A DRH (298-273)
0.0006*
*This value is not significant, given that the DRH at 298K is only significant to 1 decimal place
and the relative humidity sensors are only accurate within 1.5%.
67
F. LabVIEW program
Overview of program
We built a LabVIEW program to improve data collection from sensors near the trap. It takes
hardware and manual inputs, performs calculations, builds graphs, and records outputs for
further analysis.. The interface is shown in Figure 42.
For using the UPSI RH sensor, first open the capacitance RH sensor text file to start
collecting RH data from the UPSI sensor. (1) To begin using LabVIEW, the user presses the
'record continuously' button then (2) clicks the 'run' arrow. This brings up a dialog to create a
new file. (3) The user types in a file name followed by a .txt extension; the data is saved in a tabdelimited format, with each time point on a new line. Now, the program begins collecting data.
Figure 41: Starting the LabVIEW program
At any point, the user can change the sampling interval by typing a new number (in
seconds) into the 'sampling interval' box then clicking outside the box. Note that sampling
intervals less than -1 second may not improve data quality due to time limitations on the RH
sensors.
68
RHOe (calc frorn T.*, UPM)
I9.558N2
I
Figure42: User interface of Main.vi. Sampling interval is set to 2 seconds. Thermocouples agree
within 0.020 C, and RH sensors agree within 2%. RH(ice) is physically meaningless in this case,
since the screenshot was taken at room temperature.
69
Virtual instruments
The following virtual instruments are used for this program:
* Cap..to_RH.vi
* LabVIEWTempLogger.vi
* Main.vi
* Read RH sensor.vi
* RH readout.vi
" RH_T_calcs.vi
* RHwatertoice.vi
Temperature
Temperature is logged using the LabVIEWTempLogger VI. This VI opens the data acquisition
(DAQ) devices NI TC-01 and NI-901 1 using the DAQ Assistant. It sets their temperature input
ranges from -80 "C to +40 "C and specifies the thermocouple type E. It also sets the physical
channel of the device under "Channel Settings -> Details"; this can be altered by double-clicking
the DAQ Assistant, as shown in the screenshot below.
TC-o1
Ttrap_metal
DAQ Assistnt
data
Figure 43: DAQ Assistant screenshot
Temperature is recorded into the data log file and displayed at the specified sampling interval on
a waveform graph on the Main VI.
70
Relative humidity
The relative humidity is measured or calculated differently depending on which RH sensor is
being used with the EDB due to their output types (voltage versus capacitance). For the
Honeywell sensor, see RHreadout.vi. For the UPSI sensor, see Cap-jo_RH.vi.
* RH readout.vi measures the voltage from the Honeywell RH sensor and a 5V reference
*
*
*
"
wire, calculates the relative humidity, and outputs the RH.
CapjoRH.vi takes the input capacitance and converts it to RH. Capacitance is logged to
a csv file with the Agilent DataLogger program at a 1 second interval. LabVIEW opens
this file and uses the most recent line to calculate RH. It requires an input parasitic
capacitance to account for stray capacitance from the wires, as described in Appendix E.
The measured and parasitic capacitance are sent into a formula node containing the
transfer function, and outputs RH.
RHwatertoice.vi inputs the measured RH and temperature, performs calculations
described in Appendix E, and outputs the relative humidity with respect to ice.
RH_T_Calcs.vi calculates the relative humidity at a user-specified temperature given the
relative humidity at the measured temperature. It is not currently in use but could be
helpful in a situation with a known volume of air and water molecules, known
temperature, but no RH sensor at that location.
The Requested RH and Adaptive RH Control VIs do not function currently but would be
a good place to improve the software. They are wired in to the measured RH and the notyet-functional N 2 Flow Rates VIs.
Manual inputs
To improve data analysis and keep all the information synchronized without requiring separate
lab notebook entries, the LabVIEW program allows for several manual inputs. The user can enter
the DC and AC voltages applied at each time, as well as an open 'notes' section. The notes
section is a useful place to type when the spring point occurs, any irregularities, and the N 2 flow
conditions.
71
Data logging
Data is logged into the text file specified at the beginning of the run. Each time point is appended
to the end of the file, meaning that at any point the file-in-progress can be copied and analyzed
separately, while still recording new data.
Common errors
The most common error is reading hardware devices. If a device is opened in more than one VI,
if it is not shut down properly at the end of the previous session, or if it is unplugged or briefly
overcharged, the device will not function properly. The easiest way to troubleshoot this error is
to restart the computer. If a restart is not possible, find whatever process is using the 'task' for
the physical or virtual channel of that instrument, and close the channel or unreserve the task.
When moving the LabVIEW software from one computer to another, there may be
discrepancies between virtual channel names even if attaching the same instruments. If the
software fails to read an instrument, open the Measurement and Automation Explorer (NIMAX), click on Devices, and check to see the port name and correct setup.
72
My System
5e-Test IS Test Panels... j
SData Neighbrhood__________________
.19 Devices ard interfaces
NIUB-600"De1"Serial Number
NI USB-6008 "Dv2"
uDriver Name
02x NI U5B-6008 "Dev2"
Driver Version
-0NI USB-TC01 "Dev3"
NI USB-9211A "Dev4"
Network Devices
VISA Error
ff AA Scales
A ng lnp*
Create Task.,
OxIB932CF
NI-DAQmx
.OO
9.7.010
Software
1±1
IVI Drivers
[B4 Remote Systems
Channel Nun.
Rate (Hz)
10[0
Dev3/ai0
Mode
Samples To Read
Ionl Demand
10
Measurement Type
[Thermocouple
Max
Inu Umit
Mn In Un it
50 1
Unts
deg
Thermocouple Type
CC Sorce
JBuilt-In
Figure 44: National Instruments Measurement and Automation Explorer (NI-MAX)
Another consideration is that, since the file updates at the specified sampling interval, it is
best not to place the file into the group Dropbox until the experiment is complete. Otherwise, this
updates everyone's Dropbox accounts with undesired frequency.
Programming in LabVIEW
Each input or calculation is a separate virtual interface (VI) called from the main VI. The VIs are
all saved under the same 'project' and pass data to one another through the 'connection diagram'.
The user interface for a VI is on the main screen ('front panel'), while the background and
calculations can be found on the 'block diagram', accessible by clicking control+T. For more
information on LabVIEW programming, see their online tutorials or attend a LabVIEW course;
they offer a 3-day intensive course on the MIT campus where one can become a Certified
LabVIEW Associate Developer (CLAD).
73
Potential improvements
LabVIEW can both accept inputs and send outputs to instruments. Potential improvements (from
the 'Suggestions for future studies' section) include the following:
I.
Flow controllers: The lab purchased an Alicat BB9-USB Multi-Drop Box. This can
be attached to multiple Alicat flow controllers. By integrating flow devices into the
LabVIEW program, VIs can be created to automate the relative humidity entering the
EDB. Users could set a desired RH, and a feedback system could adjust the dry
versus wet N2 until the chamber reached that RH. Alternatively, users could program
a set experiment, e.g. start with 100 sccm dry N2 and increase the proportion of wet
N2 every X minutes up to 100% RH. Finally, users could make the software perform
a quick switch from wet to dry flow if the particle started to drop due to too high of
RH and deliquescence. The software could interface with the flow controllers faster
and more rigorously than users and could free up hands to perform other particle
observations.
II.
Automated particle trapping: Once a particle is trapped, it remains stable unless it
gains or loses mass or the temperature decreases significantly (increasing the
viscosity of the air). When it gains or loses mass, the particle moves in the vertical
direction and can be re-stabilized by changing the DC holding voltage. This could be
automated in LabVIEW. It would require a USB-connected CCD camera with
resolution on the micrometer scale. A stable particle would be in the center of a
virtual array open which the camera focused. As a particle gained or lost mass, the
change in position would be noted by the camera. This could feed back to a
LabVIEW hardware output signal, with the DC power supply connected to the
computer. As the particle fell, LabVIEW could increase the DC power until the
particle became centered on its array again, and vice-versa. LabVIEW could then
record the holding voltage and calculate changes in particle aerodynamic diameter
automatically.
74
G. Other issues and solutions
Power
If the power source voltage appears to be jumping or a particle seems particularly unstable,
check the power lines for continuity or short circuits.
* Make sure the trap or power connections have not gotten wet; if wet, flow dry N 2 through
the inner shroud.
* Check to be sure the exterior to interior connections have not come disconnected and that
the copper insulation is not shorting the wires.
* Use a multimeter to check for continuity from the trap wires to the exterior wires.
If a wire is broken or shorted, it should be replaced as described in Appendix B.
RH
If the relative humidity appears to be incorrect, try the following steps:
" Check that the sensors are making contact with each feedthrough wire.
" Dry the sensors with N2.
* Test that the voltage coming from the power source is 5 volts, and that it is grounded.
* Open "RHReadout.vi" and check the (Honeywell) RH sensor versus the test wire.
*
"
Use the multimeter to test for continuity.
For the UPSI sensor, make sure the capacitor is not in contact with metal where an
electric field is present.
If a wire is broken or shorted, it should be replaced as described in Appendix B.
Unable to trap particle
If unable to trap a particle, first check the power wire connections. Then, try using a new set of
particles. Investigate the ambient laboratory conditions; under conditions of high relative
humidity (e.g. summer, with lab humidities sometimes over 80%), it is possible the particles
have already deliquesced before being inserted into the trap and are thus too large. Try drying the
inside of the trap with N2 and using fresh particles.
If the power works, the particles are new, and the ambient laboratory conditions are
acceptable, the problem may be the interior of the SVEL. Dismantle the trap, sonicate the parts
in acetone, and redo the Aerodag G coating.
Unable to see particle
If unable to see particles, try the following:
" Check the camera focus
"
Re-align the laser
"
"
Clean the windows
Be sure there is no condensation or freezing on the interior windows
75
H. Aqueous solution droplet generation
Liquid droplets can also be suspended in the EDB. We had limited success using a droplet
generator inherited from Bauer (2013). The principle of operation is as follows: A nozzle of
changeable diameter and a squeeze-bottle of water are stabilized using hydrostatic pressure.
Electrical tweezers pulse above the nozzle and form a droplet, which exits the nozzle through an
induction ring, becomes charged, and enters the top of the EDB trap (A. Bauer, personal comm,
October 2013). Many heights and distances can be changed to achieve hydrostatic balance; these
are shown in
Figure 45.
Droplet generator
Bottle
B
3
Filter
E
C
A
EDB
D
Lab Jack
Figure 45: Droplet generatorsetup. Changeable heights given by variables.N=replaceable
nozzle. Successful conditions:A =8.125 ", B=8 " C=16 ", D=8.75 ", E=50mL.
76
Methods of use are as follows:
1. Clean the glass nozzle with acetone; sonicate, attach nozzle to tubing, attach PM filter to
tubing, and squeeze more acetone through tubes using syringe.
2. Set up so nozzle is centered over EDB trap.
3. Balance heights D and E such that droplet almost forms at end of nozzle.
4. Start power and make droplets; view on microscope slide.
5. Attach inductor ring and ensure droplets still form. Inductance around 125V was used by
Feng (2000) to produce adequate charged droplets.
6. Dip nozzle into salt solution. Ensure droplets still form.
Note: If the trap becomes wet from cleaning or droplets, it will short the power.
Salt droplet size calculations
These calculations are used to determine the minimum concentration of salt needed for a particle
that can be suspended in the EDB; once dried out, the particle cannot be less than ~10 pm in
aerodynamic diameter.
1.
rfinal should be 5-25 pm for the EDB
rinitial is limited by the droplet nozzle size options, between 15-35 pm, and the EDB
trapping size.
4
Vinitial = -7r nitial
43
Vfinal = 3 irrinal
Vfinal
Vinitial
3
_
rfinal
rinitial
2. Largest nozzle droplet and smallest suspendable EDB residual:
- (
= 0.003
(35um) 3
Vinitial
The minimum salt concentration is thus 0.3% volume salt per total volume.
3.
Calculate mass from volume, using NaCl (density p = 2.16 g/cm 3) and water (density p
1 g/cm 3
m
p = V-m=
Vp
0.65 g NaCl ( 1 g H 2 0.
100 g H 2 0 1 mL H 2 0)
1cm 3 )
)1gH 2 0
0003(2.16 g NaCl)(
3
cm
0.65 g NaCl
100 mL H2 0
0.65 g NaCl in 100 mL H20 should produce trappable droplets using the 70pm nozzle.
77
=
These calculations are automated in an Excel spreadsheet. Other sizes of interest are shown
in Table 5: Droplet calculations.
Table 5: Dropletcalculations
Solute density
Final diameter
Initial diameter
Concentration
Mass solute
(g/cm )
(Ym)
(pUm)
needed (g/g)
needed (g/100
ml H20)
2.165
2.165
2.165
2.165
1.065
1.065
10
20
20
12
12
20
40
40
80
60
40
60
3.375
27
3.374
1.728
2.876
3.944
0.034
0.27
0.034
.018
0.029
0.039
3
78