Mechanical and Electrical Characterization of Carbon Black-doped
Closed-cell Polydimethylsiloxane (PDMS) Foam
By
Jessica A. Herring
!~LIA Ju-~~
MASSACHUSETTS INTITIT(TE
OF TECHNOLOL-3V
Submitted to the
Department of Materials Science and Engineering
in Partial Fulfillment of the Requirements for the Degree of
JUN 08 2015
LIBRARIES
Bachelor of Science
at the
Massachusetts Institute of Technology
June
@ 2015
2015
Jessica A. Herring
All rights reserved
The author hereby grants to MIT permission to reproduce and to
distribute publicly paper and electronic copies of this thesis document in whole or in part
in any medium now known or hereafter created.
Signature redacted
......
.
..
Signature of A uthor ................................................... ....
Je
7. erring
Department of Materials Science and Engineering
May
Certified by .............................................
Siq nature redacted
1, 2015
Jeffrey Lang
Professor of Electrical Engineering
Thesis Supervisor
Signature redacted
Certif led by ..................................................
6
lge Yildiz
Professor of Nuclear Science and Engineerif and Materials Science and Engineering
I'
Thesis Reader
.
Accep
A-
Signature redacted
ted by ......................
bV. .
..
.............. ..........
' eoffrey Beach
Professor of Materials Science and Engineering
Chairman, Undergraduate Thesis Committee
Table of Contents
1. Introduction ............................................................................................................................7
2.
Background ............................................................................................................................ 10
3. Materials and M ethods ........................................................................................................ 13
a. Pure SF15 ....................................................................................................................15
b. Carbon Black-doped SF15 ....................................................................................... 16
c. M echanical and Electrical Property M easurem ent ............................................17
4. Results ..................................................................................................................................... 18
a. M echanical properties ............................................................................................. 18
b. Piezoresistive Properties .........................................................................................19
i. 4 w t% ratio ...................................................................................................20
ii. 4.5 wt% ratio ................................................................................................26
iii. 5 wt% ratio ....................................................................................................32
iv. 5.5 w t% ratio .................................................................................................38
5. D iscussion ............................................................................................................................. 44
a. Ram p ......................................................................................................................... 44
b. Sinusoidal Frequency Sw eep .................................................................................44
6. Current Explorations ........................................................................................................... 45
7. Conclusions and Further Recom m endations .................................................................48
8. A cknow ledgem ents ............................................................................................................. 50
9. Bibliography .......................................................................................................................... 51
io. A ppendix ............................................................................................................................... 53
2 1P a g e
Table of Figures
Figure 1. The lateral line of a Blind Mexican Cavefish...........................................................
Figure
2.
8
Array sensor made using CB-PDMS foam, solid PDMS backing, and solid Ag-CB-
PD M S as electrical contacts....................................................................................................
9
Figure 3. Representation of the percolation threshold......................................................
11
Figure 4. Images of 4 wt% CB-PDM S foam ...........................................................................
12
Figure 5. Wiring arrangement and compression testing....................................................
14
Figure 6. ADMET mechanical testing instrument used in experiments. ..........................
15
Figure 7. M old used to shape foam ........................................................................................
16
Figure 8. Compression test results for all tested samples.................................................
19
Figure 9. Compression to 5 mm, or 8%, and back down to equilibrium. ........................
20
Figure
10.
Voltage measured as a function of strain. .........................................................
21
31 Page
Figure ii. Hysteretic behavior is observed for each frequency. .........................................
22
Figure
12.
Normalized stress and voltage as a function of time.........................................
23
Figure
13.
Stress and Voltage, averaged over all tested samples, as a function of time...... 25
Figure
14.
Compression to 5 mm, or 8%, and back down to equilibrium. .......................
26
Figure
15.
Voltage measured as a function of strain.............................................................
27
Figure 16. Hysteretic behavior is observed for each frequency. .........................................
28
Figure
29
17.
Full test procedure results ....................................................................................
Figure 18. Sensitivity is very low for the 4.5 wt% sample. ..................................................
31
Figure
19.
Compression to 5 mm, or 8%, and back down to equilibrium ........................
32
Figure
20.
Voltage measured as a function of strain...........................................................
33
Figure
21.
Mechanical behavior is similar across all tested frequencies...........................
34
41 Page
Figure
22.
Entire test procedure result for the 5 wt% samples..........................................
35
Figure
23.
At 0.5 Hz, the 5 wt% samples can sense differenes in pressure........................
37
Figure
24.
Compression to 5 mm, or 8%, and back down to equilibrium.........................
38
Figure
25.
Hysteretic behavior is observed, with some spikes in the recorded data. ......... 39
Figure 26. Frequency independence is again seen in the 5.5 wt% sample.......................
40
Figure
41
27.
The entire procedure results for the 5.5 wt% samples. ....................................
Figure 28. Sensitivity to all frequencies can be seen in the 5.5 wt% samples..................
43
Figure
Underwater array sensor testing apparatus......................................................
46
Figure 30. Results of an underwater pressure variance test...............................................
47
29.
51 Page
Mechanical and Electrical
Characterization of Carbon Black-
doped Closed-cell
Polydimethylsiloxane (PDMS) Foam
Abstract
Carbon Black-doped Polydimethylsiloxane (CB-PDMS) can be used as a pressure
sensing material due to its piezoresistive properties. The sensitivity of such a sensor is in
part dependent on the stiffness of the material. A closed-cell CB-PDMS foam is being
explored as a possible flexible, lightweight, and waterproof underwater sensing material
for use in unmanned underwater vehicles and other hydrodynamic sensing purposes. The
percolation threshold for conduction through the CB-PDMS foam is theorized, and a
number of different concentrations based on the theorized threshold are explored in
order to determine the optimum weight percent of Carbon Black dopant to achieve a high
sensitivity, low stiffness sensing CB-PDMS foam. Sinusoidal mechanical pressure patterns
were applied and voltage response measured. An optimum dopant weight percent out of
the concentrations tested was found at 5.5 wt% CB-PDMS.
61 Page
Introduction
Unmanned underwater vehicles (UUVs) are used to collect data of various kinds,
such as locating underwater mine locations and shipwrecks, or mapping ocean floor
topography (1). They rely on the use of sensors to navigate underwater autonomously. Most
UUVs use acoustic or optical sensors to navigate
(2),
but these methods of undersea
visualization are not energy efficient: they must send out a signal in order to read a
response from the environment (3) (4), and these signals have the potential to interfere
with the activity of undersea life, such as dolphins, that also use echolocation to navigate.
(5) For these reasons, there is desire to design a new type of sensor for use in UUVs.
A new type of MEMS (Microelectromechanical System) sensor is proposed that
does not rely on acoustic or optic signals. The proposed sensor mimics the lateral line, a
biological feature of a number of species of fish. The lateral line is composed of a series of
neuromasts located beneath the fish's skin. These neuromasts are connected to exterior
pressures via pores in the fish's skin. Pressure differentials under the fish's pores stimulate
the neuromasts, allowing the fish to sense local changes in pressure between neuromasts.
These changes in water pressure are caused by vortices in the water, which are created in
response to flow against an object. (6) These vortices can be analyzed to determine the
location of the object giving them off. This ability to measure pressure differentials allows
the fish to create a map of the area it occupies at any given time, allowing some blind
species of fish, such as the Blind Cave Fish in Figure 1. (7) (13)
71 Page
10C
TLL
HMC
API
AP2
Ap
Figure 1. The lateral line of a Blind Mexican Cavefish. Neuromasts, shown
as black dots, allowfor the measurement ofpressure differentials.
Originalimage editedfrom Bleckmann (7).
This proposed sensor utilizes the conductive nature of Carbon Black (CB) and the
sensitivity to small strain of a silicone elastomer, PDMS (Polydimethylsiloxane). The PDMS
is used as a pliable matrix to hold the conductive particles. When the composite material is
strained, it exhibits a piezoresistive response. This resulting change in electric response
can be used to sense changes in pressure between different points on the sensor. These
pressure differentials can be related back to hydrodynamic stimuli in the water caused by
objects, allowing the sensor to determine the location of these objects by interpreting the
pressure differentials along the sensor. (8)
81 Page
Current designs for this sensor are shown in Figure X. Blocks of silver-doped solid
CB-PDMS are used to reduce contact resistance to wires, providing ample contact area for
recording piezoresistive response from the CB-PDMS foam blocks molded between. These
blocks of CB-PDMS foam are held on a backing of PDMS.
I
11 1
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2
3
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Figure2. Array sensor made using CB-PDMSfoam, solid PDMS backing, and
solidAg-CB-PDMS as electricalcontacts. This sensorwas designed to mimic
the lateralline in fish. The red box highlights the CB-PDMSfoam, while the
blue highlights the solid Ag-CB-PDMS used as an electrical contact. Image
credit:Jeff Dusek, 2015.
91 Page
This design mimics its biological counterpart seen in the fish, allowing the flexible
silicone sensor to read a number of pressure differentials using 4-point measurements. Its
flexibility has benefits: it can withstand impact better than a rigid sensor, and it can be
easily configured to match the surface it is placed on, such as a boat or other hydrodynamic
vehicle. Its flexibility also gives it higher sensitivity to small pressures, providing a
significant piezoresistive response at lower strains.
In order to achieve higher sensitivity, a lower elastic modulus was desired for the
sensor. For this reason, CB-doped PDMS foam is being explored. Its feasibility depends on
its ability to detect small strains and provide significant piezoresistive response. The ability
to waterproof the sensor is also crucial; any coating must not diminish the sensitivity of the
sensor. (9) In order to remove the necessity for a waterproof coating completely, a closedcell foam is being explored.
Background
Silicone rubbers are polymeric elastomers, which exhibit non-linear elastic behavior
different than linear elastic materials. For this reason, linear elastic models do not capture
the behavior of elastomeric materials. The mechanical behavior of the sensor will therefore
be non-linear elastomeric, exhibiting recovery of shape even after being subjected high
stresses and large strains. The mechanical behavior of silicone, however, is dependent
upon its processing conditions before testing and rate of strain during testing.
CB-PDMS, both the foam and its solid counterpart, are piezoresistive, meaning that
under some change in strain, there will be a change in resistance in the material. This
10 | P a g e
piezoresistive nature is theorized to occur by the connecting network of CB particles in the
PDMS matrix. Once a certain threshold of CB dopant is reached, enough conductive
pathways will form to allow conduction through the material. Under strain, these
conductive pathways are disrupted, but reform after some time. It has been theorized that
the piezoresistive nature of this material can be attributed to the collapse of conductive
pathways upon the straining of the material. (1o) If this is the case, the resistance of the
material would increase upon straining, as the collapse of these pathways would hinder
conduction.
Below the
Percolation
Threshold
Above the
Percolation
Threshold
* -Fill Particle
D -Bulk Phase or Matrix
Figure3. Representation of the percolation threshold, the
concentration at which particles in a matrixform connected
networks. Image from TDA (n).
11 1 P a g e
This so-called percolation threshold of a matrix and filler particles - in this case, a
silicone matrix with CB filler - occurs when the filler particles are at a high enough
concentration to form interconnected networks of touching particles within the matrix. In
order to approximate the volume fraction of CB needed to reach the percolation threshold,
each cell wall was modelled simplistically as a 3 D slab. The volume fraction of filler falls
between a range of 0.3-0.4 using a slab as a model
(12),
and so the value 0.30 was chosen to
approximate the percolation threshold for one cell wall, and by extension, the foam. This
was done to verify the volume fraction range of CB to be used to dope the silicone foam.
This calculation can be seen in Appendix A.
Figure 4. Images of 4 wt% CB-PDMSfoam, taken at 20x magnificationand ioox magnification, respectively. These images
were used to calculate the cell geometry to predict the percolationthreshold using Imagej software.
12 | P a g e
The approximate percolation threshold mass CB per volume foam was calculated to
be o.64 g for a
2.5
cm long cube using the dimensions of a cell wall measured using an
SEM. The cube samples were approximately lo g in mass, giving a weight percent of 6.4%.
This model is an over-estimation - the cell walls are thinner near the middle and thicker
near the joining edges, and as such will not behave exactly like a slab would. In addition,
previous work with this material has shown that weight percent ratios near 3-5 wt% had
been optimal. For these reasons, ratios from 4 to 5.5 wt%, in 0.5 wt% increments, were
studied.
Materials and Methods
Waterproofing the open-cell PDMS foam proved to be a challenge. (8) A number of
materials and procedures were employed to ensure that water would not disrupt the
mechanical behavior of the foam, such as Saran wrap and a pure, solid PDMS coating, but
Saran wrap was not a structurally sound solution and solid PDMS became the dominant
mechanical material in the device, limiting the foam's ability to sense pressure differences.
In order to combat this problem, a closed-cell foam was employed so that water could not
make its way into the foam material. A commercially available closed-cell foam was
procured from Smooth-On.
Smooth-On product Soama Foama 15 was used to make the sensors because of its
relative chemical similarity to the PDMS foam, its closed-cell structure, and its quick
production time. The PDMS foam required
20
minutes to cure at 120 degrees and a water
13 1 P a g e
bath for
24
hours at 8o C in order to leech out sugar, which was used as a sacrificial
scaffold. Soama Foama15 (herein referred to as SF1 5 ) is a low-density closed-cell foam with
a cure time of one hour in ambient conditions, making production of SF1 5 sensors much
more efficient than production of the previous PDMS foam sensors.
Figure5. Wiring arrangement(left) and compression testing (right) using Keithley
Sourcemeter and custom ADMET instrument.
14 1 P a g e
Figure6. ADMET mechanical testing instrument used in experiments.
Pure SF15
SF15 's production procedure was optimized by Smooth-On. Two parts, Part A and
Part B, liquid components provided by Smooth-On, were used. Two parts A and one part B
by weight were drawn out using a 3 mL syringe and released into a mixing cup. Both parts
were then mixed well together by hand for approximately io seconds. The mixture was
quickly poured into a mold before the material had become solid and no longer pourable,
which occurred after approximately
expanding approximately
2-3
2
minutes. The foam was then allowed to cure,
times its original volume when foamed fully.
15 1 P a g e
Figure7. Mold used to shapefoam. Foam poured under wires,
then wires were placed on top of expandingfoam. A top identical
piece was placed on top and screwed down until tightened to
ensurefull molding aroundwires.
Carbon Black-doped SF15
Two parts A was measured out by weight and released into a mixing cup using a 3
mL syringe. Carbon Black (herein called CB) was measured out by weight under a fume
hood and mixed in with Part A until uniform. Mechanical mixing provided good
uniformity, but the heat from mixing at times caused early curing and solidification of the
incomplete mixture. Samples were mixed by hand to avoid mechanical heat and early
curing. One part B was added to the mixed CB-doped A. This was mixed by hand for
approximately io seconds, and quickly poured into the mold. The foam was allowed to cure
16 1 P a g e
for one hour, expanding to 3-4 times its original volume. Adding larger amounts CB would
cause the foam to expand less, although still approximately in the range specified above.
The exact expansion of each sample was not calculated.
In order to measure piezoresistive responses from the foam, wires were molded into
the samples as well. As soon as the foam was poured into the mold, wires were placed on
top. The top of the mold was placed on top of the wires and bottom half of the mold. Four
wires were placed in the samples, allowing for four-point measurements of voltage to be
recorded with reasonable accuracy, even with the presence of contact resistance.
Mechanical and Electrical Property Measurement
Mechanical properties of the foam were measured using a custom-built ADMET
mechanical testing apparatus and the ADMET MTESTQuattro and NI LabVIEW software.
The measurement profile was programmed using the MTESTQuattro software and
implemented on the ADMET. Voltage was measured using a NI-DAQ board and a custom
NI LabVIEW program. A number of different profiles were constructed and used, and are
explained in the Results section.
The electrical response of the foam to mechanical pressures was recorded during
mechanical tests. A current was sent through the foam so as to record a significant voltage
difference measurement upon adding pressure to the sample. A Keithley
2602
Sourcemeter
was used to apply a current to the sample and a NI-DAQ board to measure the voltage
response in LabVIEW. A current of o.1 mA was applied in all cases except the 5.5 wt%
17 1 P a g e
samples, which were measured with 0.5 mA due to the magnitude of the recorded voltage
being too low to read precisely.
Mechanical testing was performed on each sample, recording the voltage measured
in the sample as a function of time in addition to force and position.
Results
Mechanical Properties
The mechanical behavior of the samples at high strain shows the beginning of
densification around 30% strain in each sample, in the range of 20-30 kPa stress. While
there is no true linear-elastic regime, an elastic modulus can be calculated for the regime
ranging from approximately 0.02 to O.3 due to its linearity. One expects that the
composite materials will have a higher elastic modulus than the undoped silicone foam
due to the Rule of Mixtures; however, due to the small difference in weight percent
dopant between samples, this difference should be small. This is evidenced from the data
as well.
18 1 P a g e
.
Compression of CB-doped SF15
Stress (Pa)
40000
30000[
20000[
0.1
0.2
0.3
0.4
Strain
-
Pure
-
4wt%
-
4.5wt%
-
5wt%
-
5.5wt%
-
'
10000[
Figure8. Compression test resultsfor all tested samples. Non-linearelastic behavior is
observed, in accordancewith expectations of a non-linearelastic materialsuch as PDMS.
Most samples exhibit similar magnitude of behavior, and any discrepancy may be due to
unfilled molding or airpockets in sample.
Piezoresistive Properties
The doped foam samples were tested mechanically while recording voltage
response from the material. A test procedure was developed to record changes in voltage
from a strain pattern in order to determine whether the sensor could respond quickly
enough to the strain pattern to detect the waves. Two strain patterns were tested: a
varying frequency sinusoidal test (amplitude: 8% strain), and a constant strain rate (5
mm/min,
20%
strain) ramp. Voltage was measured as a function of time, as was force and
position of the platen.
19 1 P a g e
.....
. ...........
4 wt% ratio
Ramp
This simple ramp was performed at a strain rate of 5 mm/min. The sample was
compressed to
20%
strain and then returned to o% strain. Hysteretic behavior is
observed: the amount of force required to strain the sample is larger when compressing
than when decompressing. An elastic modulus, though not a linear model, can be
calculated for this range of strain in order to predict mechanical behavior.
Stress-Strain (Low Strain)
4 wt% ratio CB-SF15
20000
y = 89469x - 989.16
15000
10000
U)
5000
0
0.05
-0.05
-5000
0.1
0.15
0.25
0.2
Strain (Fraction)
Figure 9. Compression to 5 mm, or 8%, and back down to equilibrium. Elastic modulus
given by slope offitted line.
20 | P a g e
"Auxiliary (Volts)"
0
-0.05
0.1
0.05
0.15
0.2
0.25
-8
-10
-12
a)
ii'
0
-14
II
-16
18
-20
Strain (Fraction)
Figureio. Voltage measured as a function of strain. Hysteretic behavior observed, although
large amounts of noise are present.
Hysteretic behavior is observed in voltage response as well. Even with the large
amount of noise present, the voltage observed in compression follows a similar pattern to
that observed during tension, and each regime behaves similarly. Higher compression
results in a higher recorded voltage.
21 1 P a g e
SinusoidalFrequency Sweep
Hysteretic behavior is clearly observed in the stress-strain plot (Figure n) below.
This test included two regimes, started from a pre-loaded condition: lo cycles of 0.5 Hz
frequency sinusoid, io cycles of i Hz frequency sinusoid, and the same for frequencies of 2
Hz and 4 Hz. The frequency shows little influence on the hysteretic behavior; the
material behaves similarly mechanically for these frequencies.
Stress-Strain (Sinusoidal Sweep)
4 wt% ratio CB-SF15
16000
14000
12000
10000
8000
Ln
6000
4000
2000
0
-0.02
0
-2000
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
Strain (Fraction)
Figuren. Hysteretic behavior is observedfor eachfrequency.
22 1 P a g e
At 0.5 Hz frequency, the material senses with some clarity the changes in stress.
However, at 1 Hz, the ability to sense changes in strain becomes much more difficult,
although possibly due to noise levels. At the highest tested frequencies,
2
Hz and 4 Hz, 4
wt% CB-PDMS cannot detect accurately the changes in applied pressure.
Average Stress and Voltage vs Time
4 wt% CB-SF15
4
(kPa/2)
M-Stress
/8
)
Voltage
Li.~
Ib It
Il~k iiiii
i
I
'LLIJ
Time
Figure12. Normalized stress and voltage as a function of time. Responses are difficult to see
at such a large scale.
23 1 P a g e
Average Stress and Voltage vs Time
4 wt% CB-SF15
0.5 Hz
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Average Stress and Voltage vs Time
4 wt% CB-SF15
1 Hz
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-Stress
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44
4
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24 1 P a g e
.........
. .....
Average Stress and Voltage vs Time
4 wt% CB-SF15
2 Hz
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Average Stress and Voltage vs Time
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Figure13. Stress and Voltage, averaged over all tested samples, as a function of time. The 4
wt% ratio CB-PDMS is able to sense 0.5 Hz waves, but fails to sense accurately the other
frequencies.
25 1 P a g e
~
I
~
I,
II.I
....... .......-
-
E
4.5 wt% ratio
Ramp
Stress-Strain (Low Strain)
4.5 wt% ratio CB-SF15
18000
16000
14000
y
78657x - 1028.4
12000
C'
10000
8000
6000
4000
2000.-
-0.05
-2000
0.05
0
0.1
0.15
0.2
0.25
Strain (Fraction)
Figure 14. Compression to 5 mm, or 8%, and back down to equilibrium. Elastic modulus
given by slope offitted line.
The same experiments were performed on a 4.5 wt% sample of CB-PDMS. Again,
mechanical hysteretic behavior is observed and an elastic modulus calculated. A
hysteretic trend is also observed in the voltage response.
26 1 P a g e
--=--
---
-
--
I - --
-
--.-
--
-
-
----------- - =L
"Auxiliary (Volts)"
-8
0.05
-0.05
0.1
0.15
0.2
0.25
-9
-10
-11
C0
-12
0
-13
-14
-15
-16
Strain (Fraction)
Figure15. Voltage measured as a function of strain. Hystereticbehavior observed, although
large amounts of noise are present.
27 1 P a g e
=
Sinusoidal Frequency Sweep
Stress-Strain (Sinusoidal Sweep)
4.5 wt% ratio CB-SF15
14000
12000
10000
8000
)
6000
Ln
4000
2000
0
0.02
-0.02
-2000
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
Strain (Fraction)
Figure16. Hysteretic behavior is observedfor eachfrequency.
For the 4.5 wt% material, sensing is difficult at even the lowest frequency. As
frequency was increased, the material's sensing ability dropped.
28 1 P a g e
- ---.........
----------
Average Stress and Voltage vs Time
4.5 wt% CB-SF15
10
8
~IIiuuI
M~
-Stress
CL0
ii
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(kPa/2)
-Voltage
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Time
Figure17. Full test procedure results. Sensitivity is difficult to see at this range.
29 1 P a g e
Average Stress and Voltage vs Time
4.5 wt% CB-SF15
0.5 Hz
10
4
8
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I
I
I
I
I
lI
6
-Stress
4A
C
I
: 2
(kPa/2)
Voltage
-4
Time
30 | P a g e
-
-
Average Stress and Voltage vs Time
4.5 wt% CB-SF15
2 Hz
1I I i
10
I
I
S
I
I
I
6
4
I
I
SI
I
I
I
I
I
I
I
I II I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I I
I
I
I
I
I
I
I
I
I
I I
I I I I
11 1
1111111|
I I I I I I
I I I
I I I
I
I I
I
I
I
I I
||
I I
I
I
I
I I
1
I I
I I
I
I
I
j
I
I
-Stress
CA
2ii
2
I
IIIIIIIIII
~
I
(kPa/2)
Voltage
-
I
I)
I
-2
Time
Average Stress and Voltage vs Time
4.5 wt% CB-SF15
4 Hz
II
10
I
I
I
I I I I
I
I
SI
I
I
II
I
I
6
(N
I
I
4
I
I
I
I
I
II
I
I
III
I I I I I
I
I
I
I
I I I
I
I
I
I
I
I
II|
|
I
I
I
I
I I
I I
I
I
I
I
I
I
I I
I
I
I
I
I
I
I I
I
I
I
I
I
I
I
I
I
I
-~
I
~a2
I
I
I
I
I
-Stress
(fl(~
OJO
U~)
4
2
I
-
I
I
I
I
I
I
I
I
I
I
I
I
I
I
15I tL
I
1 111
(kPa/2)
Voltage
I
11
61.
5
-4
Time
Figure 8. Sensitivity is very low for the 4.5 wt% sample, which barely senses even 0.5 Hz
waves.
31 | P a g e
_-___- -___ - ------
- ---------____ -___ ---
5 wt% ratio
Ramp
Stress-Strain (Low Strain)
5 wt% ratio CB-SF15
18000
16000
y=82629x-175
14000
12000
10000
8000
6000
4000
2000
-0.05
-2000
0.05
0.1
0.15
0.2
0.25
-4000
Strain (Fraction)
Figure19. Compression to 5 mm, or 8%, and back down to equilibrium. Elastic modulus
given by slope offitted line.
Similarly to previous experiments, hysteretic behavior was observed in both the
mechanical and electrical measurements. An elastic modulus was calculated.
32 | P a g e
-
- -
-MA
"Auxiliary (Volts)"
0
-0.05
0.05
0.1
0.15
0.2
0.25
-0.5
-1
-1.5
0
>
-2
-2.5
-3
-3.5
Strain (Fraction)
Figure20. Voltage measured as a function of strain. Hysteretic behavior observed, although
large amounts of noise are present.
33 1 P a g e
.
....
......
..........
Sinusoidal Frequency Sweep
Stress-Strain (Sinusoidal Sweep)
5 wt% ratio CB-SF15
18000
16000
14000
12000
10000
U)
8000
6000
4000
2000
0
-0.02
-2000
0
.
2
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
Strain (Fraction)
Figure 21. Mechanical behavioris similar acrossall testedfrequencies.
The 5 wt% material was capable of sensing 0.5 Hz stimuli, but higher frequency
measurements were difficult.
34 | P a g e
-I- , , ,
__ -
__
-
. ................
-
Average Stress and Voltage vs Time
5 wt% CB-SF15
10
IIIII1iIII
-Stress
)
-Voltage
4
'/)
3
2
11111
1
I
0
10
~
~<.L~.Id
20
'I
~ I2T3 0 ''!
U
ti
'lIY
6070
80
Time
Figure22. There is some sensitivity to 0.5 Hz waves in the 5 wt% samples, but sensitivity is
lost with higherfrequency. This is the entire test procedureresultfor the 5 wt% samples.
35 1 P a g e
(kPa/2)
Am
Average Stress and Voltage vs Time
5 wt% CB-SF15
0.5 Hz
1011
I
9
9
I f
I I
I
II
S III
II I
I
I I I I
II I I
III
i i
I
1 1
I
II
I
I I
II
I
I
I
III
I I I
I I I
I
I I
I
I
I
1111 I
I I I I I
I I I I I
I
I
I
8
6
1
I
I
I
ObM-Stress
(kPa/2)
-Voltage
3
23
43
38!
3
28
23
-1
Time
Average Stress and Voltage vs Time
5 wt% CB-SF15
1Hz
I I I I I I I I I
III
1
11
8
1
1
1
11
I
i
II
I
Lu
9
87
r'4
I I I I
li
i
I
I
I
I
1
1
I
l
/
I
l
I
I
I
1
I
5
4
Ln
-Stress
I
1
3
42
I
1
Il
1
i
11
1
1
(kPa/2)
-Voltage
1
0
43
4
S
4
.5
4
554.
5
5
55
'
5 .5
-1
Time
36 1 P a g e
-;;;=.-
__
__
-
_ - .
=__4
.........
.........
..
.......
__ _ - -_
Average Stress and Voltage vs Time
5 wt% CB-SF15
2Hz
ii
10
9
ii
I
8
I
1
I
I
1
1
I
I
I
1
1
I
8
6
1
11
_
-',e
OD
CA
4-J
1 i1 I 1
I 511.
I
1 1
63.
M -1
54411
2
53.5
54
11
1
4.5
55
Ii
55.5
I
I
II
1
II
1
I
-Stress
Voltage
1 1
1131
15
17.51
.
5711
56.5
'56
57
(kPa/2)
8
57.5
58
58.
58.5
-1
Time
Average Stress and Voltage vs Time
5 wt% CB-SF15
4Hz
I
10
9
I
9
8
I
I
I
I I
I I I
I I
1
I I I I
I
7
I I
I I
I I
I I
I
I 1 I I
I I I I
I I
I I
I
I
I
I
I
I
I
I
I
I I
6
I
I
I
I
1
-
Stress (kPa/2)
-Voltage
W
59
.
956
16.
-1
Time
Figure 23. At 0.5 Hz, the 5 wt% samples can sense differences in pressure;however, with
higherfrequency, this sensitivity wanes.
37 1 P a g e
5.5 wt% ratio
Ramp
Stress-Strain (Low Strain)
5.5 wt% ratio CB-SF15
18000
y 88916x - 1392
16000
14000
12000
10000
8000
6000
4000
.0-
.05
0.1
0.15
0.2
0.25
-2000
-4000
Figure24. Compression to 5 mm, or 8%, and back down to equilibrium. Elastic modulus
given by slope offitted line.
38 1 P a g e
- -
-
_tIft_
M
"Auxiliary (Volts)"
0.5
0
0.05
1
0.2
0.25
-0.5
0)
-1
0
-1.5
-2
-2.5
Strain (Fraction)
Figure25. Hysteretic behavior is observed, with some spikes in the recorded data. These
spikes occurfor an unknown reason.
39 1 P a g e
SinusoidalFrequency Sweep
Stress-Strain (Sinusoidal Sweep)
5.5 wt% ratio CB-SF15
16000
14000
12000
10000
8000
6000
4000
2000
-0.02
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
-2000
Strain (Fraction)
Figure26. Frequency independence is again seen in the 5.5 wt% sample.
The 5.5 wt% material showed excellent measurable response to all frequencies
tested. There seems to be unexpected behavior with a large amount of noise collected
near the end of the procedure. This behavior is not seen in other samples, and may be
either due to a mechanical defect in the sample composition (wiring failure, material
failure, etc.) or instrumentation.
40 | P a g e
Average Stress and Voltage vs Time
5.5 wt% CB-SF15
18
16
14
12
10
E
CU8
-Stress
-Voltage
(kPa/2)
(mV)
6
4
0
0
10
20
30
40
50
60
70
80
90
Time
Figure27. The entireprocedure resultsfor the 5.5 wt% samples. Strange behavior occurs at
the end of the test. However, sensitivity to allfrequenciescan be seen.
41 1 P a g e
Average Stress and Voltage vs Time
5.5 wt% CB-SF15
I I I I I
18
18
HI I II0.5
16
I
II III I
I
I
~8
I
I
I
I
I
I
I
I
-S re sia
2
E
M
IIStress
(kPa/2)
8
-
6
-I-Voltage
(mv)
0
23
33
28
38
43
-2
Time
Average Stress and Voltage vs Time
5.5 wt% CB-SF15
1Hz
I I I I I I I I I iI I I I I I I I I
I I I
I I
I I
I I
iI
I
I
I I
I
I I I
16
18
14
-Stress
(kPa/2)
-Voltage
(mV)
>Time
43.5
44.5
45.5
46.5
47.5
48.5
49.5
50.5
51.5
52.5
53.5
-2
42
P a ge
..........
Average Stress and Voltage vs Time
5.5 wt% CB-SF15
2Hz
I II I I I I I I I I I I I I I I
18
14
12
-Stress
U,
(kPa/2)
-'U
-Voltage
6
53.5
54
54.5
55
55.5
56
56.5
57.5
57
58
(mV)
58.5
-2
Time
Average Stress and Voltage vs Time
5.5 wt% CB-SF15
4 Hz
,
-Stress
(kPa/2)
_
-Voltage
5103
39
51
53
59
9
(mV)
9
2
Time
Figure28. Sensitivity to allfrequencies can be seen in the 5.5 wt% samples.
43 | P a g e
Discussion
Ramp
The ramp test was performed at a strain rate of 5 mm/min. The sample was
compressed to
20%
strain and then returned to o% strain at the same rate. Hysteretic
behavior is observed, and the amount of force required to strain the sample is only
slightly larger when compressing than when decompressing. This is typical behavior of
elastomeric materials like PDMS, and is expected behavior in the CB-PDMS foam.
Hysteretic behavior is observed in voltage response as well. Even with the large
amount of noise present, the voltage observed in compression is very similar to that
observed during tension, and each regime behaves similarly. Higher compression results
in a higher recorded voltage.
Sinusoidal Frequency Sweep
Hysteretic behavior is clearly observed in the stress-strain plots. This test included
four regimes, started from a pre-loaded condition of 20% strain: io cycles of 0.5 Hz
frequency sinusoid of amplitude
2
mm, followed by the same number of cycles at the
same magnitude for frequencies i Hz,
2
Hz, and 4 Hz. The frequency shows little
influence on the hysteretic behavior; the material behaves similarly mechanically for all
frequencies.
At o.5 Hz frequency, the samples sense with some clarity the changes in stress and
strain. However, only the 5.5 wt% was able to sense all frequencies. Further
44 1 P a g e
recommendations for this material are to explore higher weight ratios of dopant in order
to verify the optimum weight ratio, and to explore the limits of the magnitudes of
pressure that the samples are able to detect. In addition, exploring the spectrum of
frequencies open to the optimized material would provide information on the kinds of
applications that this material may be useful in.
Current Explorations
Sensor arrays are being designed according to the specifications outlined in the
Materials and Methods section, then made in order to test performance underwater. Two
thicknesses of the CB-doped layer are being tested: 1/8" (thin) and 3/16" (thick). These
sensors are placed on a linear stage in a water tank, where the depth to which the
material is submerged can be controlled.
45 1 P a g e
/
1
Figure29. Underwaterarray sensor testing apparatus. The linear stage can be manipulated
vertically, allowing the user to adjust the water pressurethat the sensor is exposed to.
This sensor, placed inside of a 3 D printed container leaving the foam sensing
material exposed to the water without a waterproofing layer, is shown to respond to small
pressures (ioo's of Pascals) with noticeable changes in voltage response, allowing small
pressures to be sensed with confidence. Pressures near 100 Pa have been detected. This is
in contrast to previous open-cell PDMS sensors, which required a waterproofing layer to
be used underwater.
The sensor seems to saturate, however, at approximately 300-400 Pa (seen at the
20
second mark on the plots in Figure 30). Whether this is due to the mechanical
behavior or the piezoresistive properties of the doped material is still unknown.
46 1 P a g e
..........
1.11.1
----
Optimization of this sensor array is still in progress, but shows promise as a potential
lightweight, flexible sensing option for low pressure stimuli underwater.
Hydrostatic Pressure Varialion
350
300
250
a_
200
150
100
5
15
10
20
25
Tirne [s]
CBPDMS
35
30
45
40
50
Foam 5% Thin
0. 5
- -Thin I
-- Thin 2
-1
-
----
-
0
10
30
20
CBPDMS
40
Thin 3
Thin 4
50
6
Foarn 5% Thick
0.4
0-2
0
-
-. U
Thick I
Thick 2
- -Thick 3
-Thick
4
-
-0.2
0
10
20
30
TiMe [a]
40
50
60
Figure30. Results of an underwaterpressurevariance test. Pressurewas
ramped and voltage out was recorded. Dataprovided by JeffDusek, 2015-
47 1 P a g e
-
-
- -!M
Conclusions and Further Recommendations
This closed-cell CB-doped silicone foam has potential to be used as an underwater
sensor due to its ability to sense changes in pressure with a change in resistance, and
therefore a change in measured voltage output. It is flexible and elastomeric, allowing it
to deform over large strains without plastic deformation. This is a useful property for a
sensor that will be used over the long-term to sense fluctuations in water pressure.
Out of the concentrations studied, a 5.5 wt% ratio of CB-PDMS shows the best
sensing capabilities for frequencies between 0.5 and 4 Hz. Because this was the highest
concentration studied, tests involving a higher ratio of CB-PDMS should be done to verify
the optimum ratio.
Difficulties in measurement in the cubic sensors might be attributed to wellrecorded problems with contact resistance; this may explain the differing measurements
between the sensor array and the cubic sensors. The silver-doped CB-PDMS blocks used
in the array provide better contact than a wire, especially when in contact with a foam,
which is composed of air-filled cells that reduce contact area.
Current work on the array sensors will provide more accurate and precise sensor
results, and will show the viability of using the sensor underwater over the long-term.
Further recommendations from this point include: verifying the percolation threshold in
order to find the optimum weight percent ratio of CB to SF1 5 PDMS foam by exploring
higher weight percent ratios, and testing the electric response limits of the sensor as a
function of sinusoidal frequency to discover the sensor's wave-sensing capabilities, such
as how strong or weak - how high or low a pressure difference - the material can sense.
48 | P a g e
Determining the lifetime of such a material is also necessary to ensure that it will
survive in an underwater environment for the required span of time. Its durability in
saltwater or under mechanical fatiguing will determine its feasibility for use underwater
without a protective coating.
This material shows promise as a potential sensor material for underwater
hydrodynamic purposes. With optimization and further exploration, it could prove a
viable replacement for current acoustic and optical underwater sensing instruments.
49 1 P a g e
Acknowledgements
I would like to express gratitude to Professor Jeffery Lang for his wonderful
guidance and assistance throughout this project. Many thanks are due to Jeff Dusek and
Matthew D'Asaro as well for the amazing amount of support received from them both.
My appreciation goes to my thesis reader, Professor Bilge Yildiz, for her lovely support
and assistance. I am grateful for being able to join you all in this research, knowing that
there is promise to create something with such useful application and impact. Thank you
for being great mentors and supporters throughout the short time we have known each
other.
I would also like to thank my friends in the DMSE for supporting me in this
project: Joanna Chen, JJ Hernandez - you are such wonderful friends and classmates, and
I am honored to have been able to enjoy this time together with you. My gratitude
extends to the entire 2015 Course 3 class; it was a tremendous joy to study with you all.
To my friends outside of my department as well - to Simmons Hall and its
residents; to the Baptist Student Fellowship; to Professor John Belcher; to Stephanie
Chen, my longtime roommate; to Nadiah Jenkins, my longtime neighbor - I could not
have come this far this without your love and support.
I would like to thank my family for their unending devotion and love overflowing.
You are all some of my best friends in the world, and I love you all so dearly. I could not
have accomplished an MIT degree without you all, but more importantly, I would not be
who I am today without your love, support, guidance, and friendship.
I thank God for the opportunity to have been able to study here at MIT for four
years, for the opportunity to graduate an engineer and a scientist, to be able to make such
wonderful and brilliant friends, and to be able to grow mentally and spiritually.
Thank you all for your support throughout this project and throughout the last
four years at the Institute. I could not have done it without you all - I give my many,
many thanks, and my warmest wishes for your futures.
This thesis was partially supported by the Center for Sensorimotor Neural
Engineering through an ERC Grant from the U.S. National Science Foundation.
50 1 P a g e
Bibliography
1. Crimmins, D.; Manley,
J. NOAA
Ocean Explorer. What Are AUVs, and Why Do We Use
Them?
http://oceanexplorer.noaa.gov/explorations/o8auvfest/background/auvs/auvs.html.
(accessed April
23, 2015).
Yaul, F. M.; Bulovic, V.; Lang, J. H. A Flexible Underwater Pressure Sensor Array Using a
Conductive Elastomer Strain Gauge. Journal of Microelectromechanical Systems 2012, 21,
2.
897-907.
3. Montgomery, J. C.; Coombs, S.; Baker, C. F. The Mechanosensory Lateral Line System of
the Hypogean form of Astyanax Fasciatus. Environmental Biology of Fishes 2001, 62, 8796.
4. Naeem, W.; Sutton, R.; Ahmad, S. M.; Burns, R. S. A Review of Guidance Laws
Applicable to Unmanned Underwater Vehicles. Journal of Navigation 2003, 56, 15-29.
5. Schrope, M. Whale deaths caused by US Navy's sonar. Nature 2002, 415, io6.
6. Dusek, J.; Kottapalli, A.; Woo, M.; Asadnia, M. et al.; Development and testing of bioinspired microelectromechanical pressure sensor arrays for increased situational
awareness for marine vehicles. Smart Mater. Struct. 22 (2013) 014002 (13pp)
doi:io.1o88/o 9 64 -1 7 26/22/1/o1 4 002. (accessed April
27, 2015)
7. Bleckmann, H. Role of the Lateral Line in Fish Behavior. In The Behavior of Teleost
Fishes; Pitcher, T., Ed.; Springer US, 1986; p 177.
8. Kottapalli AG, Asadnia M, Miao J, Triantafyllou M. Touch at a distance sensing: lateralline inspired MEMS flow sensors. Bioinspir Biomim. 2014 Dec;9(4):1-14. PubMed PMID:
25381677.
9. Woo, M. Development of a Porous Piezoresistive Material and its Applications to
Underwater Pressure Sensors and Tactile Sensors. Masters Thesis, Massachusetts Institute
of Technology, Cambridge, MA, 2013io. Y. Ishigure, S. lijima, H. Ito, T. Ota, H. Unuma, M. Takahashi, Y. Hikichi, H. Suzuki.
Electrical and elastic properties of conductor-polymer composites. The Journal of
Materials Science,
doi:10.1o2 3 /A:100
Volume 34, Issue 12, pp 2979-2985.
4 66 4 22 5 01 5 . (accessed March 10, 2015)
19990615,
11. TDA Research. Specialty Composites. http://www.tda.com/eMatls/composites.htm.
(accessed April
20, 2015)
51 1 P a g e
12
Sotta, P.; Long, D. The crossover from
2D
to 3 D percolation: theory and numerical
simulations. Eur Phys J E Soft Matter. 2003 Aug;11(4):375-87. PMID:
1501103.
Yoshizawa, M., Jeffery, W. R., van Netten, S. M., McHenry, M. J. The sensitivity of
lateral line receptors and their role in the behavior of Mexican blind cavefish (Astyanax
mexicanus). The Journal of Experimental Biology, 217(6), 886-895. 2014.
13.
doi:10.12 4 2/jeb.o94 5 9 9 . (accessed April
20, 2015)
52 1 P a g e
Appendix
Mathematica Code for Calculation of Approx. Percolation Threshold
The percolation threshold was calculated using a rough model of the cell wall as a
slab. Measurements for the dimensions of the hypothetical slab were taken from cell wall
average thicknesses at various points along the wall, and measurements were taken from
various walls.
The amount of mass of Carbon Black needed for a specific volume of foam was
calculated using the relative density of the foam and the approximate percolation
threshold for a slab (0-3 volume fraction). Using the size of the Carbon Black particles and
its density, the weight percent required to reach 0.3 volume fraction Carbon Black was
calculated. This number was calculated to be approximately o.64 g, or 6.4% for a
10
g,
2.5
cm long cube of CB-SF1 5 foam.
The calculation in Wolfram Mathematica software is given:
53 1 P a g e
Ir
p46::= fOamreLdenity ((2
g/
(0.025 O.025 * 0. 005 m3 ))/(3. 93 g / (. 015 3 M 3)))
cutp4el= 0.549618
vv:= 0.30 (0.165 /2 mm
ut>lE9= 1.33214x1O~
1. 33 x
ut
3.42851x 10
T
7=
r:=
outlo3=
gpp
7
=
Pi (2.1 x 10~0 m) 3)
3
Pi (2.1 x 10 8 m) 3) 2 x 10' g/m
.7 5 8 4 8 x 1 OA~
g
ir[7':= perathreshdensity
ltp1-=
In[721:=
CDt 72=-
mm * 0. 464 / 2 mm) 10~9 m 3 / mm3
m
M
fn77:=
1012
* 0. 464 /2
=
gpp
*
3 .42 x 1010
.
10-m3
0 .0355 x 10~" m
74743.6 g
ptdfoam
= percthreshdensi ty * foamreldensi ty
41080.5 g
3
Ir 731:= ptdfoaM * (0. 025 m)
outp73= 0.641882 g
54 | P a g e