ARCGNVE
MASSACHUSE TTS NSTITUTE
OF TECHNOLOLGY
JUN 2 4 2015
LIBRARIES
Probing the Liquid-Vapor Interface in
Microstructured Heat Exchangers
by
Edgardo Farias
Submitted to the
Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
Bachelor of Science in Mechanical Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2015
Massachusetts Institute of Technology 2015. All rights reserved.
Author..
signature redacted ...................
Department of Mechanical Engineering
May 13, 2015
Certified by..................
Signature redacted
EvelVn N. Wang
Associate Professor of Mechanical Engineering
Thesis Supervisor
Signature redacted
A ccep ted by .........................................................
Anette Hosoi
Professor of Mechanical Engineering
Undergraduate Officer
Probing the Liquid-Vapor Interface in Microstructured Heat
Exchangers
by
Edgardo Farias
Submitted to the Department of Mechanical Engineering
on May 13, 2015, in partial fulfillment of the
requirements for the degree of
Bachelor of Science in Mechanical Engineering
Abstract
This thesis describes two aspects of a project designed to understand the liquid-vapor
interface in microstructured heat exchangers. The two aspects include: design and
fabrication of a custom vacuum chamber faceplate and the investigation of the liquid
meniscus shape on microstructured devices. The faceplate for the vacuum chamber
consisted of two metal components that serve to house and seal a viewport. Addition
of the viewport to the chamber was of interest so that experimentation within a pure
environment could be conducted.The second component of this project was to map the
meniscus profile of water on three different device geometries under various conditions
by laser interferometry. The first experiment was a transient study where a droplet of
water fully evaporated from the surface. The purpose was to determine how the profile
changes as evaporation progresses. As evaporation occurs a more curved meniscus is
established within the liquid which causes a greater capillary pressure. The second
experiment was a steady state study with the samples partially submerged in water.
This aimed to determine the profile that arises when evaporation is balanced by
fluid replenishment. The profile that arises after the first several microstructure unit
cells remains constant for the remainder of the microstructured region of the sample
and the meniscus has the highest curvature near the fluid front, indicating a higher
capillary pressure. The final experiment was varying heat applied to the surface. The
aim was to determine how the applied heat flux changes the steady state profile. With
higher temperature more fluid evaporates from the surface, resulting in an increase
of meniscus curvature with increased temperature.
Thesis Supervisor: Evelyn N. Wang
Title: Associate Professor of Mechanical Engineering
4
Contents
Contents
5
1
7
. . . . . . . . . . . . . . . . . . .
7
1.2
Project Overview . . . . . . . .
. . . . . . . . . . . . . . . . . . .
9
1.2.1
Vacuum Chamber Design
. . . . . . . . . . . . . . . . . . .
9
1.2.2
Meniscus Imaging.....
. . . . . . . . . . . . . . . . . . .
9
.
.
.
.
.
.
M otivation . . . . . . . . . . . .
11
2.1
Faceplate Design
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
2.2
Faceplate Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . .
13
2.3
Sealing Existing Chamber . . . . . . . . . . . . . . . . . . . . . . .
13
.
.
.
Vacuum Chamber Design
15
Meniscus Mapping
Device Preparation .
...
. . . . . . . . . . . . . . . . .
15
3.1.1
Device Fabrication
. . . . . . . . . . . . . . . . .
15
3.1.2
Cleaning . . ....
. . . . . . . . . . . . . . . . .
16
. . . . . . .
. . . . . . . . . . . . . . . . .
17
.
3.1
Calibration
3.3
Laser Interferometry
. .
. . . . . . . . . . . . . . . . .
19
3.4
Experimental Set Up . .
. . . . . . . . . . . . . . . . .
19
3.5
Transient Response . . .
. . . . . . . . . . . . . . . . .
21
3.6
Steady State Profile . . .
. . . . . . . . . . . . . . . . .
21
.
.
.
3.2
.
3
1.1
.
2
Introduction
5
3.7
Heating Response
22
4 Conclusions
27
5
Appendix A: Faceplate Designs
29
6
Appendix B: Fabrication G-Code
35
7
Appendix C: Dryout Profiles
37
8
Appendix D: Steady Profiles
41
9
Appendix E: Heating Profiles
45
Bibliography
51
6
Chapter 1
Introduction
1.1
Motivation
Two-phase heat transfer has gained importance in electronics thermal management
due to the bottleneck of high heat flux dissipation [1].
Various methods exist to
dissipate heat from electronic components, but do not meet the growing demand for
high heat flux dissipation. Methods of removing heat include natural convection,
forced flow, and phase change.
Devices that incorporate forced flow include air-
circulating fans and pump driven fluid loops. However, such systems are limited to
a heat flux dissipation of 100 W/cm 2 [2]. Higher dissipation can be obtained under
phase change heat transfer. Phase change, in particular the change from liquid to
vapor, takes advantage of the high latent heat of vaporization, resulting in dissipation
heat flux values as high as 1000 W/cm 2 [3].
In his thesis, Xiao summarizes the
various methods of employing phase change cooling: pool boiling, flow boiling, jet
impingement and spray cooling [4]. Pool boiling, although a passive process, is limited
by the critical heat flux of the working fluid above which a high operating temperature
is required
[5].
Flow boiling also suffers limitations due to high pressure drops that
occur as vapor bubbles expand [6]. Both jet impingement and spray cooling supply
fluid to a surface, which then removes heat by evaporation.
7
These two methods
have the disadvantages of being difficult to implement and requiring high operating
pressures, respectively [4].
An alternative method to cooling is the use of capillary assisted-pumping in microstructured devices [1]. These devices allow for high heat flux dissipation while
removing the need for an active circulation system. Instead, fluid capillary forces
drive fluid flow across the surface as evaporation continuously occurs on the surface.
These microstructured devices allow for thin film evaporation at low operating temperatures. This method of using evaporation is advantageous over methods like jet
impingement and spray cooling as it occurs passively yet still allows for high heat flux
dissipation. This project aims to study fluid behavior as evaporation occurs from microstructured devices. The devices are fabricated with controlled surface geometry,
integrated heaters, and integrated temperature sensors. Varying device geometries
and varying heating conditions were used to study how the fluid's meniscus shape
changes under the various testing conditions. This area of study is of interest because the meniscus shape and its curvature drives fluid flow through the device as
evaporation occurs.
Previous work using similar microstructured devices has resulted in models for
liquid propagation across the surface of the device [7]. These models predict the
propagation rate as a function of device geometry: pillar height, diameter, and pitch.
Additionally, the models take into consideration the capillary pressure and viscous
losses. The models that were developed agree well with experimental results and
allow for design guidelines in selecting device geometry. Although there have been
studies on liquid propagation [8], it is important to study the effect of evaporative heat
transfer on the liquid-vapor interface and the liquid propagation in a heat transfer
system.
8
1.2
Project Overview
This project consisted of two main components: the fabrication of parts for a desktop
vacuum chamber and the characterization of microstructured devices.
1.2.1
Vacuum Chamber Design
Experiments conducted in a pure environment allow for more accurate results of the
fluid meniscus shape. In order for the ambient evaporating environment to be pure the
experiments need to be conducted in a vacuum. A small desktop vacuum chamber was
previously designed and fabricated to achieve this purpose. The chamber, however,
lacked a viewport. Without a viewport experiments within the chamber do not allow
for optical access to the meniscus.
For this portion of the project a replaceable
chamber faceplate that incorporates a 1-inch viewport was fabricated. Additional
modifications were made to the chamber to seal pre-existing leaks.
1.2.2
Meniscus Imaging
The second portion of the project was to use laser interference microscopy to study
the meniscus of a fluid as it evaporates from the surface of the devices. Three main
experiments were conducted to achieve this: a transient study in which a droplet
of fluid fully evaporated from the surface, a steady state study with the samples
partially submerged in water, and a steady state study with varying heat fluxes
induced on the surface. These experiments utilized water as the working fluid. Laser
interference microscopy enabled measurements of the meniscus profile along three
different directions of interest along the samples. Additionally, three different device
geometries were used throughout the study. Comparison of meniscus profiles from
the experiments provided quantitative insight into how device geometries and applied
heat fluxes affect the fluid's meniscus shape and curvature.
9
10
Chapter 2
Vacuum Chamber Design
The purpose of performing experiments in a vacuum chamber is so that a pure environment can be established.
Removing the air from the chamber allows for the
removal of all non-condensable gases that would otherwise interfere with the evaporation process. In a pure environment, evaporation will then only be a result of
temperature differences and not buoyancy differences. With experiments run in the
chamber the evaporating environment will be the the same as the working fluid, resulting in a better analysis of the meniscus profile. Adding a viewport in the existing
chamber then enables the use of laser interference microscopy to study experiments
performed within the chamber. Although the experiments conducted for this project
did not take place in this chamber, this chamber will be used for future experiments.
This portion of the project consisted of two components: fabricating a new faceplate
that contains a glass viewport and sealing all leaks so that the chamber holds vacuum.
2.1
Faceplate Design
Several constraints needed to be taken into consideration when designing the faceplate. The microscope objective lens to be used for interference microscopy imaging
has a working distance of approximately 5mm. Therefore, there could be a maximum
11
Figure 2-1: Section view of the faceplate design. Features for the indium wire o-ring
and bolted joints can be seen.
distance of 5mm from the device surface to the objective lens. Several materials need
to fit within this 5mm distance so that the viewport can be properly constrained.
These materials are: the glass viewport, the metal holding the viewport on either
side, and the acrylic plate which holds the device.
The design of the faceplate consists of two metal pieces which serve to sandwich
the glass viewport in place. Additionally, indium wire is placed between the glass and
metal so that the faceplate can be sealed in order to hold vacuum in the chamber.
Indium wire is very ductile and forms a seal when compressed between two materials.
Detailed drawings of the design are provided in Appendix A. The baseplate is bolted
to the existing chamber and has a center slot for the glass viewport. A secondary
metal piece is bolted onto the baseplate in order to seal the viewport in place. The
fabricated faceplate design houses a 1-inch diameter viewport. A center section view
of the design is shown in Figure 2-1. A secondary faceplate design was prepared
that incorporates a 2-inch viewport. This design, however, was not fabricated. The
increased viewport size in the second design allows for both a wider variety of lenses to
be used and for a larger viewing area of the sample during experimentation. Detailed
drawings of the second design are also provided in Appendix A.
12
Figure 2-2: Assembled and tested chamber with the fabricated faceplate attached to
the chamber.
2.2
Faceplate Fabrication
The faceplate was designed for fabrication via CNC milling. All the toolpaths used
for cutting were generated by writing custom G-Code; this portion of the project
required learning how to write and interpret G-Code. A sample piece of commented
code is available in Appendix B. Custom programs were written for each feature in
the two faceplate pieces. Two additional steps were taken after CNC machining: deburring all edges and tapping holes. The final assembled chamber is shown in Figure
2-2.
2.3
Sealing Existing Chamber
The existing chamber had leaks that needed to be sealed in order for vacuum to be
held adequately. If leaks were present ambient air will seep into the chamber, resulting
in a non-pure environment with non-condensable gasses, deviating the evaporation
process from ideal conditions. Two methods were used to determine leaks and their
locations. The first was to use a Pirani guage and a LabVIEW program to view, in
13
,Arvin
-
25001
2000CL
15001000-
500-
0)
5
1L
15
25
20
3
35
4
445
Time (Hours)
Figure 2-3: Increase in the chamber pressure after bring brought to vacuum and left
to sit for two days.
realtime, the pressure in the chamber. This was useful to determine if a leak existed
or not. A precise method of finding the location of the leak was accomplished with
an Adixen ASM 142 helium detector. All outlets of the chamber were first covered
by Apiezon wax, a sealant. Segment by segment the sealant was removed. Helium
was then sprayed onto the exposed outlets. If helium entered the chamber from these
outlets then the detector would sound an alarm signaling that a leak was found.
Leaks were found at the top of the chamber at the bolted joints. The top surface
of the chamber was flattened and smoothed using the CNC mill to remove these leaks.
Secondly, polytetrafluoroethylene (PTFE) tape was wrapped around the threads of
the feedthrough fittings and then indium wire was applied under the bolt head in
order to seal the top of the joint. The helium detector was used again to verify
adequate sealing.
After the faceplate was fabricated the Pirani gauge was used to see if any leaks
were present. The chamber was brought to vacuum and left to sit for two days. Over
a 42.7 hour time period the pressure in the chamber increased from 7.24x10 0 Pa to
2.51x10 3 Pa, shown in Figure 2-3. This averages to a pressure increase of 58.6 Pa/hr,
an acceptable rate for the proposed experiments.
14
Chapter 3
Meniscus Mapping
3.1
3.1.1
Device Preparation
Device Fabrication
The devices used for these experiments were fabricated at MIT. The fabrication process involves two sets of various fabrication techniques to make features on either
side of a silicon wafer
[9].
The first sequence uses photolithography and reactive ion
etching to create pillars of equal height and spacing. A Scanning Electron Microscopy
image in Figure 3-1 shows these pillar features. After the pillars are made the wafer is
flipped upside down in order to deposit platinum and gold layers using photolithography and ebeam evaporation. This creates an integrated heater and 4 resistance
temperature detectors (RTDs). The integrated heater is used to heat the sample by
supplying power to the device via an external power supply. The RTDs have a resistance based on the temperature of the device and are used to determine temperature
during experimentation. These devices are characterized by 3 dimensions: pillar diameter, distance between adjacent pillars (pitch), and pillar height. Geometries for
samples used in these experiments are given in Table 3.1.
15
2,00'.8
TO mAo.- 10.0' SiuW A- I.L-
Figure 3-1: Scanning Electron Microscopy image of a device with similar features to
those used in this study [9].
Table 3.1: Device Geometries
Diameter (Am)
Pitch (pm)
Height (mm)
3.1.2
Sample 1
3
40
8
Sample 2
5
40
8
Sample 3
7
40
8
Cleaning
Before samples can be used they must be cleaned to remove any impurities or contaminations present on the surface. This was accomplished by both chemical and
plasma cleaning. The samples were first bathed in acetone for 10 minutes. They were
then sequentially cleaned with methanol, isopropyl alcohol, and deionized water. The
samples were dried using compressed air before being placed in a plasma cleaner (supplied with oxygen gas) for 15 minutes. The sample can be used for experimentation
after removal from the plasma cleaner. The sample retains wetting properties for 30
minutes to two hours post cleaning. After wetting characteristics have diminished,
the samples must be re-cleaned before further experimentation.
16
U
3
2
I
Figure 3-2: Circuit diagram for calibrating each device RTD [9].
3.2
Calibration
The devices are fabricated with an integrated heater and four RTDs. When electric
current is passed through the heater the temperature of the device increases via Joule
heating. This heat generation then spreads throughout the device. Use of the RTDs
allow for determining the temperature of the device as they exhibit temperature
dependent resistance. The resistances of the 4 RTDs must be correlated to the device
temperature so that temperature can be determined during experimentation. This is
done by placing the device in an oven where it is allowed to reach thermal equilibrium
at a known temperature. Typically this process takes about two hours for equilibrium
to be reached. A simple circuit is made using a known reference resistor such that
the resistance of the device can be measured, Figure 3-2.
The current passing through the circuit segment with the two resistors in series is
I=
U
Rref + R
(3.1)
The current passing through the smaller loop is equivalent to the current in the
larger loop so that
V = IR
V=( R
U
+)R
Rre + R
17
(3.2)
(3.3)
.
........
......
1480
-+-RTD1
RD
-- RTD4
1460
1420
-
~1380
~1360 ---
'
0-1400--
13401320
13%30
40
50
70
80
Temperature (C)
Figure 3-3: Calibration of sample 3 at four known temperatures to determine RTD
resistance as a function of temperature. A linear behavior is observed.
Resulting in a device resistance of
(3.4)
R = Rref
This is process is repeated several times at various temperatures so that a calibration curve can be made. An example calibration curve is shown in Figure 3-3. The
device resistance scales linearly with temperature resulting in an equation of the form
R(T) = Ro + OT
(3.5)
During experiments the temperature can then be determined by
R(T) - Ro(
T
=
(3.6)
A LabVIEW program and two power supplies were used during the calibration
process to supply U and V. The LabVIEW programs calculates R of each RTD for
use in determining the linear equation above. Matlab's Curve Fitting Tool was used
to determine the linear fits.
18
..
14-Reflected
waves 80'
Out of phase
Reflected
waves
in phase
Path length
difference =
Path length
difference=
of V2
of2
.............
Figure 3-4: Formation of interference fringe patterns in a fluid layer [10].
3.3
Laser Interferometry
The height of the fluid layer is determined using Laser Interference Microscopy. This
technique utilizes a monochromatic laser light that shines through the layer of liquid.
When the light hits the liquid surface, some of the light is reflected and some is
transmitted through the liquid. Because the height of the liquid varies, the distance
traveled by the light also varies.
Constructive and destructive interference occurs
with the transmitted and reflected light, resulting in alternating bright and dark
fringe patterns, as shown in Figure 3-4. The relative thickness of the fluid can then
be determined by counting the number of fringes. The height between consecutive
dark-dark or light-light fringes is
h=
(3.7)
2n
where n is the refractive index of the fluid (1.33 for water).
3.4
Experimental Set Up
The same general set up was used for all the experiments.
holder constrains the device in place.
A custom-built device
The device holder includes pogo pins that
interface with the heater and RTD pads. A VisiTech Hawk laser source and a Nikon
19
. ...............
Figure 3-5: Representative image of fringe patterns that result due to varying fluid
thickness. Marked are the three measurement profiles directions: diagonal (AD),
horizontal (AB), and vertical (BC).
Eclipse LV 100 microscope project a 405nm laser onto the sample. A Prior Optiscan
II moving stage controls the position of the sample. Finally, a Hamamatsu EM CCD
digital camera records the images of the liquid layer on sample surface.
For the transient experiment a MicroSyringe Pump Controller applied a 20nL
droplet of water onto the surface of the sample. For the steady state experiments the
bottom 5mm of the sample was submerged into a pool of water so that continuous
wicking was established.
The heating experiments were also performed with the
bottom of the sample submerged, and electrical power was supplied to the integrated
heater via external power supplies.
Videos were recorded for each experiment at a frame rate of 10 frames per second.
The videos show the liquid meniscus shape in the form of fringe patterns. Individual frames of the video were analyzed to give meniscus profiles in three directions:
diagonal (AD), horizontal (AB), and vertical (BC), shown in Figure 3-5. All image
processing was done using ImageJ, wherein distances between fringes were made using
the measurement tools within the program.
20
3.5
Transient Response
For this set of experiments a droplet of water was placed at the bottom of the sample
surface. The fluid wicked up the microstructured surface and was then allowed to
evaporate to the ambient environment. Videos were taken from the time that the
droplet was applied to the time of full evaporation. The purpose of this set of experiments was to determine how the meniscus shape changes over time as evaporation
progresses until the sample dries out. The analysis shown in Figures 3-6 used a reference time of t=O that corresponds to a steady fluid profile (capillary recovery of
fluid balances lost due to evaporation). These plots are representative of the behavior
that was seen across the three samples; a full set of plots is available in Appendix C.
As time progresses there is greater loss of fluid due to evaporation and thus greater
capillary pressure is established by the higher meniscus curvature seen in Figures 3-6.
The higher meniscus curvature results in a higher capillary pressure. This behavior is
observed until the sample commences drying out and non-uniform evaporation occurs
(typically seen by a receding fluid front or by evaporation from individual pillars).
3.6
Steady State Profile
The steady state experiments aimed to determine the meniscus profile change that occurs when continual wicking balances evaporation. For these experiments the bottom
5mm of the samples was submerged in water. This allowed for continual replenishment of liquid in the device as evaporation removed fluid from the surface. Figure
3-7 shows representative steady profiles in the first five cells following the fluid front.
The first cell is labeled as distance of Opm. A full set of plots is available in Appendix D. A steady meniscus profile is reached within several cells from the liquid
front. The meniscus has the highest curvature near the liquid front, indicating that a
higher capillary pressure is needed to balance evaporation. The meniscus profile that
is established after the initial cells remains constant for the remaining sample cells.
21
.
..
..
.......
1.2
s
tt = 4.9
4
.
S
- -l -11.6
0.6
20.7s
-t
~0.4
s
23.3
9
*~0.2
0
0
2
23.3
018
-0t-0SW
--
t:
1.5
W
5
4.9s 2'11
8
6
10
12
14
16
Is
20
Length along AD (pin)
5
8
07
0.6
4
2
s
-
10s
4.9 s
'
t
20 PMs
t- 116s
tt-20.7s
0.5
0.3.
4
0
0
0
2
4
6
8
10
12
14
16
18
0
20
Length along AB (pm)
1
2
3
4
5
6
7
8
9
Length along BC (sm)
Figure 3-6: Representative interference microscopy images and meniscus profiles from
the dryout study where a droplet of fluid was allowed to evaporate from the surface of
the sample. The images shown are from sample 3 at two different times. The profiles
are in the diagonal, horizontal, and vertical directions.
3.7
Heating Response
The heating experiments had a similar goal as the steady state experiments, but with
the addition of heating of the surface to determine how the applied heat flux changes
the steady state meniscus profile and to investigate the effects of temperature driven
evaporation.
The set up is the same as the previous set of experiments with the
bottom 5mm of the samples submerged in water. Additionally, electrical power was
supplied to the integrated heater via a power supply to promote temperature driven
evaporation. Figures 3-8 and 3-9 show the steady profiles that occur after thermal
equilibrium is established in the device. Only the profiles that occur in the first cell
and fifth cell are shown; a full set of plots is available in Appendix E. The meniscus
profiles in the first cell are highly curved, whereas the profile in the fifth cell is more
stable and remains constant. Unlike the other sets of experiments, only sample 3 was
used in this study. Temperatures of the device were determined via the RTDs using
22
.
.....
.
...
. ......
. ..
..............
Ei 1.4
-WU0 pill
-40 pin
1.2
-
80 pml
11-01
pml
p20m
AA
160pm
S0.6
S0.4
0.2
o1
0
10
5
15
20
25
Length along AD (jim)
0.9
0.8
00 pm
1.6
0.7
-40 pin
1.4
0(.6
-80
0.5
0.4
-120
1.2
pml
pil
* 40 pm
80 pin
120 pm
160 pm
'U>
jr
S0.0
160 gm
= 0.6
*
0.3
.50.2
0I
0
2
4
6
8
10
12
14
16
18
20
6
8
10
12
14
16
Length along BC (Am)
Length along AB (pm)
Figure 3-7: Representative interference microscopy images and meniscus profiles from
the steady state study where the bottom 5mm of the sample was submerged in water
to allow for continual liquid replenishment as evaporation occurred. The images are
from sample 3 at two different locations. The profiles are in the diagonal, horizontal,
and vertical directions. Measurements were taken in successive microstructure cells
(center-center distance of 40pm) from the liquid front. The first cell following the
liquid front is labeled as distance of Optm.
23
.....
..........
..
.......
E 1.4
1.2
-OW
-- 0.43W
-043
E
W
0.96 W
0.8
0.6
--
l.66 W
04
S0
10
5
0
20
15
25
Length along AD (pm)
318
0,9
0.8
%J
1.6
-O W
0
4
W
0
0
-0
W
-0.
11 W
.9
1.-4
y
0A~
0.6
0.3.
0.4
0.20.2
0
0
0
2
4
6
8
10
[2
14
16
18
20
0
2
4
6
8
10
12
14
16
Length along BC (sm)
Length along AB (pm)
Figure 3-8: Interference microscopy image of sample 3 with an applied power of
1.66W. Meniscus profiles are shown in the three directions for sample 3 under varying
input power. These measurements were made in the first cell after the heating front,
L =Opm.
a LabVIEW program. The temperatures determined by each RTD for all heating
conditions are provided in Figure 3-10. The RTDs are located on the back of the wafer
along the center at vertical distances from the bottom of the microstructured area:
7.7mm, 10.9mm, 14.1mm, and 17.3mm. The temperature is approximately uniform
across the sample due to uniform heating, uniform evaporation on the wetted surface,
and high thermal conductivity of silicon. Increasing the power supplied to the device
results in a steadily increasing meniscus curvature. With a higher temperature more
liquid is able to evaporate from the surface and is balanced by the capillary pressure
generated as the meniscus curvature increases. An interesting observation was noted
in that the position of the fluid front remained constant for the supplied electrical
power reported in these experiments. We do not expect this for higher heat fluxes:
the increased evaporation rate caused by a larger temperature rise will overtake fluid
uptake by capillary action, lowering the position of the fluid front.
24
. ....
........
1.4
2W
1.2
1
V
-011
W
-0.43 W
20.
0,96 W
~06
~0.4
1,66W7
;0.2
5
(
10
15
20
25
Length along AD (sr)
0.9
L8
0.8
0.7
0.6
-0
"-OW
-0.11 W
-- 0.43 W
0.96 W
1.66 W
1.6
W
1.4
--. 43 W
(1.96 W
1.66 W
1.2
0.6
03
*
0
0.1
0.2
0
2
4
6
8
10
12
14
16
18
20
0
2
6
8
10
12
14
16
Length along BC (pm)
Length along AB (sim)
Figure 3-9: Interference microscopy image of sample 3 with an applied power of
1.66W. The steady state meniscus profile in the three directions for sample 3 is shown
under varying input power. The meniscus profiles shown remain constant for the
remainder of the the microstructure cells following this location. These measurements
were taken in the fifth cell after the heating front, L = 120pm.
45
RTD I
-*-RTD 2
4-RTD 3
RTD4
40
35
C. 30
25
20
0
0.25
0.5
0.75
1
1.25
1.5
1.75
Electrical Power (W)
Figure 3-10: Measured temperature of each RTD for the supplied electrical powers
on the sample 3 integrated heater.
25
26
Chapter 4
Conclusions
This projected consisted of two components: design and fabrication of a custom
vacuum chamber faceplate and the investigation of the liquid meniscus shape on
a microstructured device. The faceplate for the vacuum chamber consisted of two
metal components that serve to house and seal a glass viewport. The addition of the
viewport into the chamber allows for use of imaging tools to record experiments that
are conducted within the chamber. The components of the faceplate were machined
using a CNC mill, and the final assembly was checked for leaks to verify vacuum was
being maintained. The second component of this project was to map the meniscus of
water on three different device geometries under various conditions. The three sets
of experiments were a transient study in which a droplet of water fully evaporated
from the surface, a steady state study with the samples partially submerged in water,
and a steady state study with varying heat flux applied to the surface. The liquid
meniscus shape was monitored using laser interferometry where fringe patterns arise
due to different liquid layer heights. The videos taken during the experiments were
analyzed using ImageJ to provide meniscus profiles in three directions of interest.
The purpose of the transient study was to determine how the meniscus shape
changes over time: from the development of a steady profile until dryout occurs. As
time progresses there is greater loss of fluid due to evaporation and a diminished liquid
27
source. As a result a greater capillary pressure is established within the fluid which
causes a higher curvature meniscus profile. The steady state experiments aimed to
determine the meniscus profile that arises when evaporation is balanced by continual
fluid replenishment. It was observed that the profile that arises after the first several
cells remains constant for the remainder of the cells on the sample. Additionally,
the meniscus has the highest curvature near the liquid front, indicating that a higher
capillary pressure exists to balance evaporation.
The heating experiments had a
similar goal as the steady state experiments but with the addition of heating of the
surface. The aim of this was to determine how the applied heat flux changes the
steady state meniscus profile and to investigate the effects of temperature driven
evaporation.
With a higher temperature more fluid is able to evaporate from the
surface and the viscous loss of flow through the structures is balanced by the capillary
pressure. This resulted in a gradual increase of meniscus curvature as temperature
increased. Further work needs to be done in this project for analytical models to be
developed. Future work for this project are to expand the device geometries used in
order to quantify the effects of varying pillar height, pitch length, and pillar diameter.
Additionally, replication of the experiments with more fluids would give insight into
how the fluid properties affect the meniscus shape.
28
Chapter 5
Appendix A: Faceplate Designs
Two designs for the chamber faceplate were made. A faceplate with a 1-inch viewport
was fabricated. In addition, a 2-inch design was prepared. The second design design
allows for a 2-inch viewport. The only modifications that occurred in this design
are the dimensions; the same features are used. Detailed drawings of the designs are
shown in Figures 5-1, 5-2, 5-3, and 5-4.
29
5.50
2.20
A
0.50
1.20
CN
0.25
0.40
0
Cf)
0
00.)251
y
0
(N
0
D7
\C'48
cli
0.025
00
0
A
SECTION A-A
#8 Tap Holes ;Equal spacing of 45 degrees
#8 Clearance holes
DIMENSIONS ARE IN INCHES
TOLERANCES:
FRACTIONALt
PROPRIETARY AND CONFIDENTIAL
THE INFORMATION CONTAINED IN THIS
DRAWING IS THE SOLE PROPERTY OF
<INSERT COMPANY NAME HERE>. ANY
REPRODUCTION IN PART DE AD A WHOLE
WITHOUT THE WRITTEN PERMISSION OF
<INSERT COMPANY NAME HERE> U
PROHIBITED.
NAME
DATE
DRAWN
ANGULAR: MACNV BENDt
TWO PLACE DECIMAL i
THREEPLACE DECIMAL t
CHCE
ENG APPR.
MATER tAL
Q-A-
MFG APPR.
COMMAENTS:
NEXT ASSY
USED ON
APPLICATION
FINISH
SIZE
, G. NO.
iA! acelateOnejnch
DO NOT SCALE DRAWING
WBGI - SHED
- -FI
REV.
RE
Figure 5-1: Dimensioned drawing of the 1-inch faceplate base. This component is
bolted onto the existing chamber.
30
A
0.75
4.00
__
0.348 7
01.00
0.402
00.80
o
0.691
A
SECTION A-A
8 Clearance Holes; equal spacing of 45 degrees
PROPRIETARY AND CONFIDENTIAL
THE INFORMATION CONTAINED IN THIS
DRAWING IS THE SOLE PROPERTY OF
<INSERT CMANY NAME HERE>. ANY
REPRODUCTION N PART OR AS A WHOLE
ITHOUT THE WRITTEN PERMISSION OF
<INSERT COMPANY NAME HERE> IS
PROHIBTED
MTERLAL
FINISH
--
-
APPLICATION
HCE
ERG APPR.
MFG APPR.
G.A
COMTENTS
S
USED ON
NEXTASSY
DATE
NAME
DRAWN
-
DIMENS IONS ARE IN INCHES
TOLERANCES:
FRACTIONAL!
ANGULAR: MACH t
BEND
TWO PLACE DECIMAL
THREE PLACE DECIMAL
I:A
DO NOT SCALE DRAWING
D
kkPae lat Onench
eIc
REV.
---
Figure 5-2: Dimensioned drawing of the 1-inch faceplate cover. The component is
bolted onto the faceplate base.
31
5.50
4.298
3.298
A
2.20
1.20
177-7
V/
0.50
0.25
LO)
0.025
0
0.40157
0.03937
I0
- LO
0.025
0
U-O
SECTION A-A
SCALE 1: 1
#8 Tap Holes; equal spacing of 45 degrees
8 Clearance Holes
DIMENSIONS ARE IN INCHES
TOLERANCES:
- P
ERACTEONALC
ANGULAR: MACE
BEND
TWO PLACE DECIMAL
PROPRIETARY AND CONFIDENIAL
THE INFORMATION CONTAINED IN THIS
DRAWING IS THE SOLE PROPERTY OP
<INSERT COMPANY NAME HERE>. ANY
REPRODUCTION IN PART OR AS A WHOLE
WITHOUT THEW
WRITTEN PERMISSION OF
<INSERT COMPANY NAME HERE> IS
PROHIBITED.
NAM
DAT
DRAWN
CHECKED
ENG APPR.
-fDAPPI.
Q.A.
MATERIAL
NEXT ASSY
USED ON
APPUCATION
FINISH
DO NOT
SIZE
SCALE DRAWING
A
Wa
O
CeP
WEIGT:
wSCALTIT
lteIwoInch
E
)w7ETOFI
Figure 5-3: Dimensioned drawing of the 2-inch faceplate base. This component is
bolted onto the existing chamber.
32
4.00
02.00
0.75
01.80 1
A
Q.1 77
D.401 57
-
1
---1Z>
0.69095
L
0
0
I
A
SECTION A-A
SCALE 1: 1
8 Clearance Holes; equal spacing of 45 degrees
DIMENSIONS ARE IN INCHES
TOLERANCES:
FRACTIONAL
ANGULAR:MACHt
TWOPLACEDECIMAL t
THREE PLACE DECIMAL :t
BEND+
PROPMETARY AND CONFIDENTIAL
THE INFORMATION CONTAINED IN THIS
DRAWING IS THE SOLE PROPERTY OF
<INSERT COMPANY NAME HERE>. ANY
REPRODUCTION IN PART OR AS A WHOLE
WITHOUT THE WRITTEN PERMISSION OF
HINSERT
COMPANY NAME HERE> IS
PROIRTED.
USED ON
APPLICATION
DATE
CHECKED
ENG APPR.
MFG APPR.
O.A
MATERIAL
NEXT ASSY
NAME
DRAWN
AD
kD
SCALE 1:1
;WEIGHT.
s
DO NOT SCALE DRAWING
A BaEEPTateFwonch
REV.
SHEDT I OF I
Figure 5-4: Dimensioned drawing of the 2-inch faceplate cover. The component is
bolted onto the faceplate base.
33
34
Chapter 6
Appendix B: Fabrication G-Code
Custom G-Code was written for each step of the manufacturing process. The code
was used to fabricate each component on a CNC mill. The sample code provided here
was used to mill the central hole for one of the components.
G90 ;
absolute position control
G94 ;
units per minute mode
G57;
set coordinate axis
spindle speed
M03 S3000 ;
coolant on
M08 ;
#1
=
0.2 ;
z-height variable
x-distance variable
0.1875;
#2
"while" loop to position z-distance
WH[#1 LE .4] DO 1
G90 G01 XO YO F1 ;
postion at center
G90 GOl Z-#1 F0.1 ;
cut into material in z-direction
= #1 + 0.2 ;
increase z-distance for next loop
#1
"while" loop to cut circles
WH[#3 LT 0.75] DO 2;
position to cut in circle
G90 GOl X#2 F0.1 ;
cut semicircle
G02 X-#2 YO R#2 F1;
cut semicircle in the opposite direction
G02 X#2 Y0 R#2 F1 ;
35
increase radius for next cut
#2 = #2 + 0.1875
DO 2;
end of inner loop
;
end of outer loop
DO
end program
M02
36
Chapter 7
Appendix C: Dryout Profiles
Below are meniscus profiles in the diagonal, horizontal, and vertical directions for the
three samples. These profiles are from the dryout study wherein a droplet of fluid was
allowed to evaporate from the surface of the sample. A baseline of t=O was chosen
where a steady profile was observed prior to meniscus change due to evaporation.
37
---------------
Figure 7-1: Raw images of sample 1 during various times (t = Os, 9.9s, 21.5s, 23.9s)
in the evaporation process. These images were analyzed in ImageJ in order to plot
meniscus profiles.
20 pim
t=0 s0p
20 pm
t=13.5s
t=
2
5.9 s
pm=1.s
Figure 7-2: Raw images of sample 2 during various times (t = Os, 5.9s, 13.5s, 16.8s)
in the evaporation process. These images were analyzed in ImageJ in order to plot
meniscus profiles.
38
Figure 7-3: Raw images of sample 3 during various times (t = Os, 4.9s, 11.6s, 20. 7s)
in the evaporation process. These images were analyzed in ImageJ in order to plot
meniscus profiles.
1.2
1.2
-M-I-OS
t
.
2.9
9. S
t
=51.9
s
0.4
0.4
0.2
I; 0
0.2
0
2
4
6
8
10
18
16
14
12
4
2
0
20
6
8
10
12
14
16
18
20
Length along AD (pm)
Length along AD (stm)
.
1.2
a.8
4.9
11.6
0.6
S
20.7s
23.3 s
0.4
0.2
0
2
4
6
10
9
12
14
16
18
20
Length along AD (pm)
Figure 7-4: Temporal liquid profile in the diagonal direction for the three samples.
39
. ..............
S0.8
30.7
+r =Os
E 0.
1.6
E
Wt = 0 S
S0.6
0.5
+t
0.4
0.3
*-t
21.5 s
0
-t
23.9 s
03
=099 s
-t
=
16.8s
-
'.2
.1
0
0
4
2
6
11)
8
12
16
14
0
20
18
2
4
6
Length along AB (pm)
-
10
8
12
14
16
18
20
Length along AB (sim)
08
S07
-t= 4.9s
t
0.5
*t 11.6 s
--
t = 20.7 s
t= 23.3 s
02
2
I
4
6
6
12
t0
14
16
Is
20
Length along AB (pm)
Figure 7-5: Temporal liquid profile in the horizontal direction for the three samples.
1.5
1.5
-5.9s
9.9
E
0.9
-
21.5s
309
1
l3.s
-t=23.9 S
0.6
M 03
x
S0
(o
0
2
4
6
8
10
12
3
2
14
4
5
6
7
8
Length along BC (im)
Length along BC (pm)
1.5
E
+Wt=0s
t=4.9s
1 II,
t 11.6 s
1.2
-t
09
207 s
23.3s
t
06
0.3
0
0
1
2
3
4
5
6
0
9
Length along BC (jIm)
Figure 7-6: Temporal liquid profile in the vertical direction for the three samples.
40
9
Chapter 8
Appendix D: Steady Profiles
Below are meniscus profiles in the diagonal, horizontal, and vertical directions. These
profiles are from the steady state study wherein the bottom 5mm of the sample
was submerged in water to allow for continual liquid replenishment as evaporation
occurred.
Measurements were taken in successive cells (center-center distance of
40Am) from the liquid front. The first cell following the liquid front is labeled as
distance of Opm. The cell that contains the fluid front is only partially filled with
liquid, so was neglected in this analysis.
41
Figure 8-1: Raw images of successive cells in sample 1. These images were analyzed
in ImageJ in order to plot meniscus profiles.
20 pm
2L=0
L=0 pm
pm L=20
20 pm
pm
L
40 pm
L=120 PM
Figure 8-2: Raw images of successive cells in sample 2. These images were analyzed
in ImageJ in order to plot meniscus profiles.
42
.....
...
.............
..............
..
...
I
Figure 8-3: Raw images of successive cells in sample 3. These images were analyzed
in ImageJ in order to plot meniscus profiles.
i
1.4
.0
mil
[4
0 pm
40 pm
1.2
-40 ptm
l80
-- Jim
I
-120
pm
-AA
*
0.6
2 4
A
0,4
0.4
0.2
,Air
0.2
0 W-
.
0
20 pm
60 pm
0.8
160 pin
0 .6
5
10
20
15
10
0
25
15
20
25
Length along AD (pm)
Length along AD (pm)
1.4
.2
(Pill
40 pn
-80 pil
W
o
-
120 pm
160
mtn
0.6
0.4
-A-ni
0.2
0O
0
5
10
15
20
25
Length along AD (pm)
Figure 8-4: Steady state liquid profile in the diagonal direction for the three samples.
43
....
....
. ......
M
0.9
0.9
0.9
Ar-
pin
pm
-40
M
pm
+
0.4
-.
-. 8
Om
+40
E 0.6
- 0.5
A pm
211 pm
0.5
-
08
0.7
A
-80
pm
pm
120 pm
04
'
E
160
pm
03
~0
0
2
4
6
8
10
14
12
18
16
--
0
20
'
0
S01
2
4
8
6
10
12
14
18
16
20
Length along AB (pm)
Length along AB (pm)
'.0.9
0.$
E
M
+9
C 0.7
40pm
0.6
pm
-80
-0.5
-120
AV
pm
10pm
0.4
01.3
S0.2
S0
0
2
4
6
8
10
12
14
16
18
20
Length along AB (pm)
Figure 8-5: Steady State liquid profile in the horizontal direction for the three samples.
2
2
2
1.4
-0 pm
-40 pm
0 pm
1.2
-120
U40pm
1.6
E1.2
160 pm
1.6
3
-
1.4
80 pm
120 pm
E
pin
0.8
0.6
0.8
O.6
0.4
0.4
-
160 pm
0
0
0
2
4
6
8
10
12
16
14
0
2
6
4
0
10
12
14
16
Length along BC (pm)
Length along BC (pm)
2
pml
pin
40 pm
-WO
-40
S1.4
E
1.2I4
A
-120
pm
160 pm
0.6
0.2
o1
0
2
4
8
6
Length along
10
12
14
16
BC (pm)
Figure 8-6: Steady State liquid profile in the vertical direction for the three samples.
44
Chapter 9
Appendix E: Heating Profiles
Fluid profiles are provided for sample 3. The device was heated via joule heating at
varying wattage supplied by an external power supply. For each supplied power the
fluid profile in three directions was measured. These measurements were taken at the
first five cells following the fluid front.
45
Figure 9-1: Raw images of successive cells for no applied power. These images were
analyzed in ImageJ in order to plot meniscus profiles.
i 0.9I
E1.4
:L
1.2
4(1AM
I
40pm
j
20 pm
pin
0.8
160
0.8
-a(0 m
0.7
-40
0.6
-80
0.5
pm
-a
120 pm
160 pm
0.4
0.6
f0.4
pm I
0.3
S0.2
.! 0
0
5
10
20
I5
0
25
4
2
6
8
10
12
Length along AB (pm)
Length along AD (psm)
2
A-0 pm
-40 pm
-80 pm
-120 pm
160 pm
6
4
E
.2
10.2
0
0
2
4
6
8
10
12
14
16
Length along BC (pm)
Figure 9-2: Steady state liquid profile for no applied power.
46
14
16
18
20
17!1,
Figure 9-3: Raw images of successive cells for 0.11W power input. These images were
analyzed in ImageJ in order to plot meniscus profiles.
'i 0.9
i 1.4
=L
1.2
-
1
0 Pin
+40 pin
0.0,
0-7
80 AI
0.
-20
0 .6
40
--
0.5
ymi,
120
0.4
--M
41-
In
.
0.4
ipm
40 pm
0.2
0.2
S0.
1
S0
0
5
10
15
0
25
20
6
4
2
8
10
12
Length along AB (pm)
Length along AD (pm)
2
0
1.6
1.4
-40 pm
1.2
-8
pm
120 pm
f0 o
i0.6
0
2
4
6
1
N
Length along
BC
12
14
16
(pm)
Figure 9-4: Steady state liquid profile for 0.11W power input.
47
14
16
18
20
Figure 9-5: Raw images of successive cells for 0.43W power input. These images were
analyzed in ImageJ in order to plot meniscus profiles.
1.4
Jim
0.9
40 pm
0.8
W0
80
0-7
-40
0.6
-80pm
0-5
04t
-120
+00
1.2
-
-120
m
E
pm
60 pm
0.6
pm
pm
_41
pm
160 pm
0.3
0.4
0.21
5
10
15
0
25
20
2
4
6
8
10
12
Length along AB (sm)
Length along AD (sim)
2
5.8
16
1.
-8040 pm
pm
1.2
I
AM
-
120pm
160 pm
-
0
.3-
0.6
S0.4
.20.2
0
2
4
8
6
10
12
14
16
Length along BC (sim)
Figure 9-6: Steady state liquid profile for 0.43W power input.
48
14
16
19
20
L
.I
.L .I
....
.....
.11 ....
.... -..
I
Figure 9-7: Raw images of successive cells for 0.96W power input. These images were
analyzed in ImageJ in order to plot meniscus profiles.
I
-i
=. 0.9
S1.4
S1.2
*-0
pm
in
-
0.8
~*40 pm
Z
80 Pml
.3<
60
0.6
0.6
40 pil
pil
pi
-120
04
-A
pm
-0
0.5
-
120pm
0.0i
U-0 pm
0.
160
pm
0.3
S0.4
0.2
<Air
1.2
~0
0
5
10
15
20
2
0
25
6
8
10
12
Length along AB (pm)
Length along AD (pm)
2
6
1.6
L4
r
- -it
pin
-40
pin
80 pill
.2
120 p
m
160
m
p
-a-
-A
0.8
0.6 *'
0.4
0.2
0
2
4
8
6
10
12
14
l(
Length along BC (pm)
Figure 9-8: Steady state liquid profile for 0.96W power input.
49
14
16
18
20
.......
.....
Figure 9-9: Raw images of successive cells for 1.66W power input. These images were
analyzed in ImageJ in order to plot meniscus profiles.
1.4
E,0.9
W0
1.2
I
0(.6
pm
140 Pin
160 pm
7
0.8
+0
0.7
--
0.6
-4 p42
o-5
0.4
-n
PI
4) pm
B (ps
160 p~m
(0.3
0.(4
0O.2
;0.2
o(.1
0
10
15
20
0
25
4
2
6
8
10
12
14
Length along AB (pm)
Length along AD (pm)
-2
10 pm
1.8
1.6
-40
1.4
jim
__80 PM
2
120
pmn
160
-m
S0.8
6
0O.
0.4
0.2
0
0
2
4
8
6
10
12
14
16
Length along BC (pm)
Figure 9-10: Steady state liquid profile for 1.66W power input.
50
16
18
20
Bibliography
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Heat Transfer. MIT Ph.D. Thesis. January 2013.
[2] Mudawar, I., Assessment of High-Heat-Flux Thermal Management Schemes,
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51
[9] Adera, Solomon, Personal communications. April 2015.
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52