Shear Stress Measurements via Elastomer Micropillar Arrays · 2014-10-17 · Sensing and Controlling Motion with Polymeric Materials Symposium Shear Stress Measurements via Elastomeric

Sensing and Controlling Motion with Polymeric Materials Symposium

Shear Stress Measurements via Elastomeric Micropillar Arrays

Dr. Christopher J. Wohl, Advanced Materials and Processing Branch, NASA LaRC Dr. Frank L. Palmieri, Advanced Materials and Processing Branch, NASA LaRC Dr. John W. Connell, Advanced Materials and Processing Branch, NASA LaRC

Dr. Yi Lin, National Institute of Aerospace (NIA) Allen Jackson, Fabrication Technology Development Branch, NASA LaRC

Alexxandra Cisotto, NASA Langley Aerospace Research Summer Scholars Program, NASA LaRC Dr. Mark Sheplak, Interdisciplinary Microsystems Group, University of Florida

246th American Chemical Society National Meeting September 8-12, 2013

246th American Chemical Society National Meeting


Airflow & Shear Stress

2

0

y

wy

U

u(y) = 0.99U¥• For a 2D flow, the shear stress can be

approximated by the linear change of mean velocity with distance from the wall

U= Free Stream Velocity = Kinematic Viscosity

U

y



Research Objective

• The objective of this work is to generate a robust shear stress sensor capable of making accurate shear stress measurements up to 10 Pa with Pa sensitivity without airflow disruption.

3

Length, L

Diameter, d



Research Objective

• The objective of this work is to generate a robust shear stress sensor capable of making accurate shear stress measurements up to 10 Pa with Pa sensitivity without airflow disruption.

4

A Smooth Wall A Rough Wall

Image from: Potter, Merle C. Fluid Mechanics Demystified. The McGraw Hill Company, New York, 2009, p. 144.



State-of-the-Art Shear Stress Sensing

Indirect Measurements

• Hot Wire Anemometer

• Pitot Tube

• Whispering Gallery Mode

Direct Measurements

• Oil Interference

• MEMS devices

Our approach … “Capped” Micropost Arrays

Current shear stress measurement techniques that minimally impact airflow are difficult to implement, of questionable reliability, and sensitive to environmental factors.

Development of these sensors would alleviate many of these issues enabling shear stress measurement on a variety of surfaces including acoustic liner applications with sensors that: (1) Are robust, (2) Reduce complexity for integration into a wind tunnel model, (3) Enable measurement in 360°.



Previous Micropillar Shear Stress Sensors

W. Schröder, RWTH Aachen University Other Research

6

S. Große and W. Schröder Int. J. Heat and Fluid Flow 2008, 29, 830. S. Große, et. al. Meas. Sci. Technol. 2006, 17, 2689.

E. Gnanamanickam and J. Sullivan J. Micromech. Microeng. 2012, 22, 125015. B. Li, et. al. Cell Motil. Cytoskeleton 2007, 64, 509.

Novel Manufacturing Methods

Investigation of Bio-adhesion



Laser Ablation for Template Generation

• Generation of master templates amenable to micropillar array fabrication: laser ablation patterning

7

2 layers of electrical tape used to absorb energy that, when applied to the epoxy, led to large diameter, shallow side-wall angle ablation.

Dicing tape applied to enable facile removal of the electrical tape layers.

Glass Substrate

Dicing Tape

2 layers of electrical tape

Laser ablation patterning affords excellent depth control but has limited diameter control with a minimum achievable diameter of approximately 30 m.

Epoxy



Contact Lithography for Template Generation

• Generation of master templates amenable to micropillar array fabrication: contact lithography

8

Substrate (Si wafer)

Photoresist

Photomask with chromium

UV Light





9

Substrate

50 m

Photolithography offers greater capability for increased aspect ratios relative to laser ablation patterning. However, the available photomask patterns are far too dense.

Photoresist




Designed Photomask for Contact Lithography Master Template Fabrication

Close-up of Pattern Schematic

10

Pitch of stress-relief

grid: 2 mm

Width of grid

lines: 0.1 mm



Micropillar Array Fabrication: Soft Lithography

• Fabrication of micropillar arrays using soft lithographic techniques

11

Uncured Silicone Master Template Cured Silicone




• Fabrication of micropillar arrays using soft lithographic techniques: using laser ablation generated templates

12

% power kHz in/s Post Height

(μm)

90 60 2.5 115

90 60 2.0 123

92 60 3.0 62

92 60 2.5 45 - 85

92 60 2.0 132

95 60 3.0 85

95 60 2.5 123




• Fabrication of micropillar arrays using soft lithographic techniques: using contact lithography templates

13

Results from old Photomask 45-50 m

50°

90°

Results from New Photomask Aspect Ratio: 4:1



Force Calibration of Micropillars

• Force-displacement pillar calibration experiments using atomic force microscopy

14

1

1

2

2

3

3

~35 μm

~230 μm

a1 ~ 110μm a2 ~ 145μm

a3 ~ 175μm

Bending distance (m)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

No

rma

lize

d F

orc

e (

nN

)

0

20

40

60

80

100

120

140

Point 1

Point 2

Point 3



“Capping” Micropillar Arrays

• Fabrication of “capped” micropillar arrays: schematic of approach

15

Length, L

Diameter, d



“Capping” Micropillar Arrays

Sylgard 184 Spin Curve SEM Image of “Capped” Pillars

16

“Cap” film thickness can be easily varied based on spin-coater settings. With greater pillar spacing, the caps readily formed and were free standing, i.e., no pillar coupling.

0

2

4

6

8

10

12

14

16

18

20

22

24

26

0 1000 2000 3000 4000 5000 6000

Thic

knes

s (μ

m)

Spin Speed (rpm)



Future Work

• Micropillar array design refinement

– Identification of greatest efficacy methods for micropillar capping and signal enhancing dopant

– Characterize micropillar deflection

– Develop micropillar arrays for various wind speeds

• Characterization for signal implementation

– Determine measurement range, sensitivity, noise floor, etc.

• Use of PIV equipment and visualization techniques to see pillar deflection

17



Acknowledgements

• Research Assistance:

– John Hopkins-Ablation Laser Operations

– Vincent Cruz-Contact Lithography Assistance

• Discussion

– Xiaoning Jiang, North Carolina State University

• Funding:

– NASA Aeronautics Research Directorate NARI Seedling Program

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Supplementary Slides

19



Experimental Approach

• Pillar Array Design and Fabrication – Generate master templates using available lithographic

processes Identify requisite pillar parameters for shear stress measurements in subsonic flows

• Laser ablation of epoxy substrates

• Contact lithography of SU-8 coated Si wafers

– Fabricate micropillar arrays using commercially available elastomeric materials

• Pillar Calibration and Signal Transduction – Calibrate pillar deflection using atomic force microscopy

– Determine signal transduction approach

20

Concept

Pillar Parameter Value

Length, L L < 100 m

Aspect Ratio, L/d (diameter)

L/d ≥ 3

Pillar Spacing, s s > 2L

Deflection Limit, w w ≤ 0.1L





– This procedure required process refinement for several steps:

• Plasma exposure of Si wafers

• Dehydration baking

• SU-8 adhesion promotion layer

• SU-8 application

• Soft bake

• Photomask positioning and exposure

• Pattern development

21



• Generation of a macroscopic example of the micropost array sensor

Accomplishments

22

Pillar deflection was readily observed on the macroscopic sample.




• Evaluation of different silicones

23

Shore 00 0 10 20 30 40 50 60 70 80 90 100

Shore A 0 10 20 30 40 50 60 70

Ecoflex -

0010

Shore

Hardness:

00-10

Solaris

Shore

Hardness:

A-15

Ecoflex 00-

50

Shore

Hardness:

00-50

Silastic

T-2

Shore

Hardness:

A-42

Mold Star

30

Shore

Hardness:

A-30

Sylgard 184

Shore

Hardness:

A-50

Extra Soft Soft Medium Soft Medium Hard

Silicone Hardness (Shore A)

Tensile Strength (MPa)

Ecoflex-0010 00-10 0.83

Ecoflex-0050 00-50 2.17

Solaris 15 1.24

Mold Star 30 30 2.90

Silastic T2 42 5.52

Sylgard 184 50 7.07



Signal Transduction

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• Integration of Micropillar Array Sensors with Existing (Micro) Particle Image Velocimetry Instrumentation



Fluorescent Dye Doping of “Caps”

• Identification of most efficacious method for signal transduction: optical, piezoelectric, etc.

Visible Light Illumination UV Light Illumination

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Shear Stress Measurements via Elastomer Micropillar Arrays · 2014-10-17 · Sensing and Controlling Motion with Polymeric Materials Symposium Shear Stress Measurements via Elastomeric

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