Soft Materials Structure, Processing, Properties

MSE 190

Prof. Kendra Erk

Mechanics Spectrum

Hookean SOLIDS

Newtonian FLUIDS

Mechanics of complex fluids & soft materials? RHEOLOGY

Food

Personal care

Biofluids

Paint

Lubricants

Pastes

Rheology

• Study of the deformation and flow of soft materials, the “in-between” materials

• Word inspired by the Latin phrase panta rhei, “everything flows”

• Fusion of solid mechanics and fluid mechanics

• Best example of an “in-between” material is silly putty…

Developed by accident in 1940s during research into synthetic rubber substitutes for the USA during WWII. Mixture of boric acid and silicone oil. http://www.youtube.com/watch?v=GxdfoJoWNE4 (silly putty has nothing to do with smoke rings or boat captains as far as I know)

Silly Putty, “the real solid liquid”

Silly Putty’s Structure-Property Relationships

Deformed slowly

Chains slide past each other & reorganize liquid-like response

Deformed quickly

No time for reorganization solid-like response

Highly entangled!

Structure comprised of long, flexible polymer chains

Cool soft material property: Viscoelasticity

• Materials that exhibit both viscous and elastic behavior when undergoing deformation

• Silly Putty has viscoelastic properties, due to its structure of entangled polymer chains

• Other examples?

Mixture of corn starch and water “it’s eating you!”

http://www.youtube.com/watch?v=Lb9kt1z3jAA&feature=fvwrel

http://www.youtube.com/watch?v=3zoTKXXNQIU&feature=related

The physics behind the corn starch and water effect…

http://www.youtube.com/watch?v=JGfynrsdaV0

The physics behind the corn starch and water effect… particle jamming!

Nature 487, 2012

Viscoelastic properties result from the mixture’s structure

Particle jamming up close

Plastic beads moving through a funnel.

http://www.youtube.com/watch?v=lWSJwZhqoQw

Another cool soft material property: Yield stress fluids

• Materials that behaves as a rigid elastic solid at low stresses but flows as a viscous fluid at high stress

• Examples in daily life?

Q: What structural factors of a yield stress fluid could cause its elastic-to-viscous transition with increasing applied stress?

HINT: peanut butter is a suspension of peanut particles in oil

HINT: think about the structure-property relationships of cornstarch and water mixtures

A: particle jamming and subsequent un-jamming!

Soft materials are highly deformable at room/body temps (unlike most metals or ceramics)

And your life depends on it!

http://www.youtube.com/watch?v=9puAj9jshAk&NR=1&feature=fvwp

http://www.youtube.com/watch?v=ihg_Adxh5UQ

Blood cells deforming through a capillary…

But not with Sickle-cell disease…

Cells are soft… but not too soft… Another cool property: strain-stiffening

1

10

100

1000

0.01 0.1 1

Actin

Collagen

Fibrin

Vimentin

G (

Pa

)

The actin in cells becomes stiffer when deformed – preventing damage!

My idea: can we design a synthetic polymer gel that stiffens like actin?

G (

Pa)

~ St

iffn

ess

deformation

1

2

3

0.01 0.1 1 10

G' /

Go

Collagen

FibrinNeuro-filaments

Physically associating gel

Erk, et al., Biomacromolecules, 2010.

2

0 max

expGG

Stiffening controlled by bridge length & stretch

**Can design synthetic gels with tunable stiffening!

PMMA x-links PnBA bridges

Yes!

Another way lives depend on soft materials properties…

http://www.youtube.com/watch?v=L0hg6hvFfZ8

These shear-thickening fluids have similar features to the cornstarch & water mixtures we discussed earlier…

Soft material mechanics Food

Personal care

Biofluids

Paint

Lubricants

Pastes

Prof. Kendra Erk Materials Engineering

Soft materials are important in every-day life. Unusual mechanical properties due to the material’s structure: - Viscoelasticity - Yield stress fluids - Large deformation - Strain-stiffening - Shear-thickening