Development of a Nanometer Crossflow Differential Mobility
Analyzer (NCDMA)
Jerry R. Boyle
Advisor: Suresh Dhaniyala
Problem:
The importance of nanoparticles is constantly growing. Nanometer size particles,
especially from combustion sources, have recently become a health issue, as these
particles are small enough to enter and stay in human lungs and may be carcinogenic.
Also emerging fields such as nanotechnology require an accurate measurement of the
physical characteristics of nanoparticles. Here we propose to develop a technique to
accurately size nanoparticles in real-time with a compact, inexpensive instrument.
Current Limits:
Currently there is one commercial instrument that is widely used, the TSI
NDMA[1]. The device takes polydisperse aerosol (air with a distribution of particle sizes)
and classifies the particles based on their electrical mobility, which is related to the size
and charge of the particle[2]. This is achieved by introducing polydisperse aerosol into the
NDMA and charged particles traverse radially through clean sheath air due to an applied
electric field (Figure 1).
Figure 1. Schematic diagram of the NDMA[1].
The electric field strength can then be varied to change the size of monodisperse aerosol
(air with particles of one size) exiting the NDMA. The NDMA is accurate but also
expensive and the technique used results in a low concentration of particles in the
monodisperse aerosol due to a large sheath air to aerosol flow ratio and the difficulty of
charging nanometer particles (only a small fraction of the particles are able to be
charged).
The effects of this low concentration can be seen in experiments which involve
conditioning the particles and analyzing its effects, such as in the case of
thermodesorption[3]. Particles of a known size are heated to expel the outer volatile
surface then passed through a cooled tube to deposit the volatile material. However, as
the particle size decreases, more particles are lost due to increased thermophoretic
deposition. Therefore, there is a need for a new technique which can output a
monodisperse aerosol with a high concentration of particles with high size resolution.
One option to improve performance is to increase the aerosol flow rate, however, this will
often lower the sizing resolution[4]. This paper proposes to employ a modified electrical
classification technique to generate monodisperse aerosol while outputting large
concentrations of monodisperse particles.
Methodology:
Rather than a coaxial aerosol-sheath flow system, a new flow system is proposed
(Figure 2). This new technique will introduce sheath air into the aerosol flow through the
inner porous tube as the flow passes through an electric field. The resulting radial drag
force will oppose the electrical force and particles of one size will focus since the forces
are related to particle size. This will enable a higher aerosol to sheath air flow ratio, thus
a higher concentration of particles in the monodisperse aerosol. Initially a 2-D model was
constructed where the flow field is calculated using potential flow theory (Figure 3) and
particle trajectories are calculated assuming stokes drag and electric force (Figure 4). A
MatLAB script was written to calculate particle trajectories and range of focused
particles. Having produced desirable results (good particle separation at low sheath to
aerosol flow ratios) the model was tested using Fluent, a computational fluid dynamics
program.
Figure 2. Basic Schematic of new proposed flow system.
This required learning how to operate Fluent as well as Gambit, the drawing tool that
accompanies it. I have previous experience with a previous version of Fluent and
AutoCAD (a computer aided drawing tool), which helped me quickly learn and utilize
these new software programs. The results produced by Fluent agreed with the results of
MatLAB (see figure 5) thus a 3-D model was constructed in Fluent. The results were
different than those of the 2-D case because of the differences in the flow and electric
field between concentric tubes (3-D) and the flow and electric field between parallel
plates (2-D). However, the results still showed desirable particle separation, thus a
prototype is currently being designed that will be simple to build and set up with the
equipment that will be used to test it.
.016
Height From Porous Tube (m)
.014
.012
.01
.06
.08
.04
.02
0
-.125
-.075
-.025
0
.025
.075
.125
Distance From Begining of Porous Tube (m)
Figure 3. Streamlines of 2-D flow.
Combined uniform flow and corner flow.
.175
60 V Potential Difference Applied
0.01
100 nm
Height of Particle From Porous Tube (m)
0.009
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
40 nm
0
0
0.05
0.1
Distance of Particle From Begining of Porous Tube (m)
0.15
Figure 4. Particle Trajectories for 40 nm (solid line) and 100 nm (dashed line).
The smaller, more electrically mobile particles are focused and separated from the larger, less
electrically mobile particles which are pushed towards the outer surface due to drag forces.
Figure 5. Comparison of MatLAB and Fluent results. Results show similar trends and focusing
ranges.
Future Work:
The prototype will be tested and compared to the performance of the NDMA, as
well as to the theoretical models. The results will be analyzed and further modifications
will be made to optimize performance. The new device will then be tested to check for
optimization and agreeance with a modified theoretical model to accommodate for any
significant changes. A research paper will then be written to report and analyze the
results and findings of this project.
Timeline:
Summer 2004
- Finish designing prototype and submit design to machine shop
-Test prototype and compare performance to existing DMA as well as the theoretical
model
Fall Semester 2004
- Finish testing prototype
- Redesign prototype to optimize performance
- Modify theoretical models to account for changes in design
- Test new device and compare performance to first prototype, existing DMA, and
modified theoretical model
- Prepare a paper to be submitted for publication
- Finish final draft of thesis by end of fall semester or early spring semester 2005
References:
1.
D. -R. Chen, D. Y. H. Pui, D. Hummes, H. Fissan, F. R. Quant and G. J. Sem.
“Design and evaluation of a nanometer aerosol differential mobility analyzer
(Nano-DMA).” J. Aerosol Sci. Vol. 29 (1998): 497-509.
2.
Hinds W. C., Aerosol Technology: Properties, Behavior, and Measurement of
Airborne Particles., Wiley, New York, 1999.
3.
S. Philippin, A. Wiedensohler and F. Stratmann. “Measurements of non-volatile
fractions of pollution aerosols with an eight-tube volatility tandem differential
mobility analyzer (VTDMA-8).” J. Aerosol Sci. Vol. 35 (2004): 185-203.
4.
D. R. Collins, A. Nenes, R. C. Flagan and J. H. Seinfeld. “The Scanning Flow
DMA.” J. Aerosol Sci. Vol. 31 (2000): 1129-1144.