Buckling of bilayer laminates: A novel approach to ... fileBuckling of bilayer laminates: A novel approach to synthetic papillae Research goals •Discover details of papillae expression

Buckling of bilayer laminates: A

novel approach to synthetic papillae

Sachin S. Velankar, Derek Breid, Sourav Chatterjee, Jiani Niu,

Victoria Lai, Rachmadian Wulandana

Department of Chemical Engineering

University of Pittsburgh

Collaborator: Roger T. Hanlon, Paloma Gonzalez, Justine Allen,

Trevor Wardill

Marine Biological Laboratory

Woods Hole, MA

Buckling of bilayer laminates: A novel approach to synthetic papillae

Research goals

• Discover details of

papillae expression and

elucidate biomechanics

• Practical implementation

of synthetic papillae

• FEM modeling of

complex materials

Current Impact

• Developed easy-to-use

method for simulating

elastic instabilities

• New platform for

reversibly morphing

surfaces

• Image analysis software

for analyzing active skin

mechanics

Planned Impact

• Elucidate biomechanics

of papillae expression

• Easy-to-use method for

simulating shape changes

of muscular hydrostats

Status quo

• Cephalopods can

camouflage themselves by

expressing papillae to change

the texture of their skin.

• The biomechanics of

papillae expression is not

known. Muscular hydrostat

and skin buckling are two

hypotheses.

• No demonstrations of

reversible surface texture at

the flip of a switch in synthetic

systems.

New insights

• Cuttlefish skin can strain few

10% in-plane. Indentation

modulus is ~ 1kPa.

• Reversible surface texture

can be realized even with

small strain compression by

driving elastic instabilities.

Main Achievements : Numerical

modeling of muscular hydrostats:

Easy-to-use; can predict shape

changes due to complex

arrangements of muscles

How it works: Specify muscle

arrangement as am orientation field.

Specify shrinkage along muscle

direction & volume-conserving

expansion orthogonally. Solve FEM

model

Limitations: Muscle force model is

inexact: only shape changes can be

predicted; no forces or speeds

Model of constant-volume muscle

deformation raising a papilla

Main Achievement: Composites of

shape-memory alloys (SMAs) and

elastomers as a platform for

reversibly morphing surfaces. Rapid,

low-voltage operation.

How it works: Compressive strain in

elastomer can be harnessed to drive

buckling instabilities.

Limitations: “Tall” features difficult

to realize. Actuation is fast;

deactuation is limited by cooling

speed

Surface features at the flip of a switch

Main Achievements: Software for video

analysis of active skin

How it works: Image correlation algo-

rithms optimized to handle large changes

in mean color of the skin.

Limitations: Possible errors due to 3D

nature of papillae Rectangular grid distorts as skin

deforms

6V DC, ~30 s

Cephalopod camouflage

• Octopus, cuttlefish

• Texture control

• Mechanisms

– Compression-induced

skin buckling

– Muscular hydrostats

Picture courtesy Roger Hanlon

Kier, 1989

3: Program goals

• Biomechanics of papillae expression (collaboration with Roger

Hanlon)

– Examine musculature of papillae

– Measure mechanical properties of skin

• Implementation of synthetic texturing surfaces

– Mimic papillae using synthetic materials

– Implement surfaces that will texture in predetermined ways

• Numerical simulation

– Develop and validate random imperfection method for elastic instabilities

– Develop simulations for muscular hydrostats (added)

4: Progress towards goals

• Biological experiments

– Indentation experiments with AFM and non-AFM

– Automated video analysis of skin motion

• Implementation of synthetic papillae

– Development of SMA/elastomer composites for morphing surface

applications

– Implementation of morphing surfaces by buckling

• Numerical simulations

– Detailed validation of new technique for elastic instabilities

– Developing FEM simulations of muscular hydrostats

• Other related research enabled by this grant







applications






AFM trials

• Insufficient indentation

• What are we indenting?

– Single layer epithelial layer is

few microns thick

– Does the “zero” displacement

already indent the epithelial

layer

• Skin motion is a huge problem

New instrument: non-AFM

• Same principle as AFM but with 500 micron indentation

– Cantilever deflection probed optically

– Milligram forces can be measured

• Smaller with vibration table

Indentation results

• Modulus is ~ 0.86 kPa +/- 0.2

– Soft !

– But still comparable to other animal tissues

– May be much larger in orthogonal direction

• Not much difference between the papillae and non-

papillae region

– 0.86 on papilla vs 0.7 off papilla

Challenge for mechanical measurements

• “Fresh” skin is too active to be tested

~ 27 hours ~ 40 hours + warmed

Video analysis of active skin

• Sepia officinalis neural stimulation

– Paloma Gonzales and Justine

Allen @ MBL

• Quantify deformation

– How much area changes?

– How much stretching?

– Homogeneous or not?

• Autocorrelation analysis to track

displacements

– Changes in mean intensity cause

trouble

– Image processing algorithms must

be optimized

Particle tracking

• 30 - 40 % change in area

• Peak rate ~ 0.3 s-1

• Skin is under sufficient tension that it

does not buckle

• Currently applying this to more video

including live animal







applications






Overall idea

• Other buckling instabilities can be harnessed to realize “snapping”

behavior, directional buckling, wave motion…

smart material

smart material smart material

with controlled

delamination

without

delamination

Buckling for reversible texture

• This is viable

• Can we do this using “synthetic muscles”

– Rapidly at the flip-of-a-switch

– Reversibly

– Spatially-reconfigurable texture

• Develop composites of shape-memory alloys and elastomers as a

general platform

Surface texturing via buckling

• Reversible compression can drive buckling instabilities

– SMA-elastomer composites: General platform for reversible texture

– The surface film can be patterned for controlling shape of buckle

– Low voltage

– Spatial and directional control

• Implementation movie

Reversible surface texture at the flip of a switch

• Actuation at 4.5 V

– Have done this with 2xAA

• Strain ranges from 1.5 - 3.7%

– Max strain ~4.5% for bare SMA wire

• Even small strain can induce surface texture if we use it to drive

buckling

– Other buckling transitions can be driven too

– Delamination gives high amplitude too

Surface texture at the flip of a switch

• Fully reversible over several cycles

• Spatially reconfigurable

• Low voltage operation

• Modest temperature

• No moving parts

• Lengthscale can be tuned from few micron to cm

• Examine fundamentals…

Shear lag model with plane strain

• Shear transferred from the SMA to the elastomer

via shear stress at wire/elastomer interface

• Stress transferred to the elastomer to film

via shear stress at elastomer/film interface

• Film is linearly elastic

• Elastomer is linearly elastic

2 wewR

x

f

ef

dh

dx

f

f f

uE

x

f w

ef e

u uG

H

Shear lag model

– Stress builds up “from the ends”

– Film compression reduces for thick

films or compliant elastomer

2

2

12

f

f SMA

wf

f e

hEu x

REuhE G

x H

uf

Message #1: Stress transfer is critical

• Simply having a “strong” actuator is not sufficient: stress transfer

to the surface needs to be adequate

• Generally applicable to any type of embedded actuator trying to

induce surface strain

Message #2: Partial excitation

• Film stress “builds up from the ends”. Short films wont develop

sufficient stress

– Analogous to short fibers in composites

– Limits the minimum spatial resolution of wrinkles







applications






Progress on simulations

• Complete: Original proposal on FEM simulations of

buckling instabilities

• Now: developing models of muscular hydrostats

Muscular hydrostats

• Muscular structure that deforms at constant volume

Kier, 1989

contract tangential

muscle fibers

contract axial

muscle fibers

Allen, 2009

Musculature of a papilla

• Physical picture from MBL group

Movie : Basia Goszczynska Schematic Justine Allen

Modeling of muscular hydrostats

• Volume conservation: muscle shrinks along the muscle fiber direction

and expands orthogonally

• Capture complex arrangement of muscles

– Specify mean muscle fiber orientation

– Contract in that direction + expand perpendicular

• Muscles and non-muscle tissue behave elastically

Force models

• Strain and strain rate

dependence of force

Van Leeuwen and Kier, 1997

active passive

Input parameters

• Force model parameters

are not known

– Many are micro-scale

Van Leeuwen and Kier, 1997

uniaxial random in-plane

Minimal computation approach for shape changes

• Thermal expansion approach

– Usually isotropic

– Make anisotropic to conserve volume

specify geometry &

muscle fiber orientation actuate new shape

Validation

• Tentacle extension

• Papillae expression

Caveats

• Volume changes are small but not zero

– Few percent is typical

• Force model is not real

– Shape changes are good but no kinematic information







applications






Two other related projects

• Thin films supported by viscous liquids

• Swelling behavior of polymer films

• Both relevant to developing 3D surface features

Viscous-driven wrinkling

– Convenient for metrology in molten polymers

film

prestretched rubber membrane

viscous liquid

Validation

• 5 micron plastic film floating on viscous polymer

• Buckling is rate-dependent

– Not an equilibrium phenomenon

0.03 s-1 0.006 s-1

quantitative

• small film length: shear lag model

– stress builds up “from the ends”

• “infinite” film length: empirical

– non-linear after initial instability

viscous liquid thickness H

film thickness h

film length L

Swelling of polymer films

• Crosslinked polymer film swollen with solvent

• Buckle delamination

– Swelling causes compressive stress

– Compressive stress causes delamination

– Tsukruk : nanoscale

• Fold formation : buckle has a concentrated

curvature

– Tall folds persist after drying

Variety of folding patterns • Large parameter space

– film modulus

– film thickness

– drop volume

– nature of solvent…

• Drop volume has an interesting effect

– Perimeter delamination and folding

Map of pattern development

• Thin films and/or large drop volumes induce delamination and folding

• Evaporation behavior is also complex: self-adhering folds vs. blisters

corrals

no

delamination

5: Scientific or technological transitions

• Initiative with BioE folks at U. Pittsburgh about creating muscular

hydrostats

• Talking with COMSOL for implementing random modulus into their

software

6: Interactions with other groups and organizations

• William Kier, UNC

• Paul Ashby, LBNL

• Matt Shawkey

• Jeff Urbach & Dan Blair

7: Papers published in 2012 • Zhang, J.-T.; Wang, L.; Lamont, D. N.; Velankar, S. S.; Asher, S. A. "Fabrication of Large-

Area Two-Dimensional Colloidal Crystals", Angew. Chemie-Int. Ed. 2012, 51, 6117.

• Wolf, M. T.; Daly, K. A.; Brennan-Pierce, E. P.; Johnson, S. A.; Carruthers, C. A.; D'Amore,

A.; Nagarkar, S. P.; Velankar, S. S.; Badylak, S. F. "A hydrogel derived from decellularized

dermal extracellular matrix", Biomaterials 2012, 33, 7028.

• Velankar, S. S.; Lai, V.; Vaia, R. A. "Swelling-induced delamination causes folding of

surface-tethered polymer gels", ACS App. Mat. Int. 2012, 4, 24.

• Nagarkar, S. P.; Velankar, S. S. "Morphology and rheology of ternary fluid/fluid/solid

systems", Soft Matter , 2012, 8, 8464.

• Medberry, C. J.; Crapo, P. M.; Siu, B. F.; Carruthers, C. A.; Wolf, M. T.; Nagarkar, S. P.;

Agrawal, V.; Jones, K. E.; Kelly, J.; Johnson, S. A.; Velankar, S. S.; Watkins, S. C.; Badylak,

S. F. "Hydrogels Derived from Central Nervous System Extracellular Matrix", Biomaterials,

2012, in press.

• Zhang, J.-T.; Wang, L.; Chao, X.; Velankar, S.S.; and Asher, S. A., “Vertical spreading of

two-dimensional crystalline colloidal arrays”, J. Mater. Chem. C, 2013, in press.

Invited presentations

• “Interfacial jamming phenomena in ternary liquid/liquid/solid systems”,

Georgetown University, April 2012.

8: Nature of the research

• Biological research is transformational

– Papillae are a key part of cephalopod camouflage. The mechanisms

underlying expression of papillae are altogether unknown. At the

conclusion of this research, we aim to discover the mechanism by which

papillae are expressed.

• Numerical research is transformational

– The numerical technique of random imperfections enables non-experts to

simulate complex buckling and post-buckling behavior using off-the-shelf

software. The method has broad applicability for simulation of elastic

instabilities. The ability to simulate elastic instabilities coupled to other

physical phenomena (thermal, electrical, viscous, transport) is a

tremendous advantage.

• Synthetic papillae research is evolutionary

– Research on surface-buckling is well-developed. Our research aims for

practical application of this research to realize synthetic papillae using SMA-

elastomer composites. This permits a wide range of surface textures to be

realized with rapid actuation, reversibility, and spatial addressability.

Acknowledgements

• Funding: AFOSR, Hugh DeLong

• Derek Breid, Sourav Chatterjee, Jiani Niu, Victoria Lai, Andrew

Flowers, Rachmadian Wulandana

• Lots of exchanges with Justine Allen, Paloma Gonzalez, Roger

Hanlon, William Kier

Documents

Buckling of bilayer laminates: A novel approach to ... fileBuckling of bilayer laminates: A novel approach to synthetic papillae Research goals •Discover details of papillae expression