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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
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