Lec 3 Flashcards
(14 cards)
Glass-Transition Temperature (Tg)
Tg is a measure of the energy required to break the bonds and fluidize the material
Above Tg -> fluid
viscosity vs MW
log viscosity proportional to log MW
Lower viscosity and lower MW makes mvm easier
Elastic vs viscous materials G
Elastic solids store energy → characterized by G’ (storage modulus).
Viscous fluids dissipate energy → characterized by G’’ (loss modulus).
G’ > G’’ → solid-like
G’’ > G’ → fluid-like
Alpha actinin vs filamin
Alpha-actinin (ACTN)
Forms parallel actin networks
Filamin (FLN)
Forms orthogonal actin networks
Affine material mech response
Non-affine (not aligned) networks have non-linear responses to shear
very low (or high) cross linker to filament ratio leads to Anisotropic filament bundling
How does crosslink density influence mechanics?
Crosslinking increases elastic modulus (G’), but does not affect dissipation timescale
What does G’ > G’’ indicate?
Solid-like behavior
What determines mesh size in actin networks?
Inversely proportional to square root of actin concentration
How does stress stiffening arise in a network
Stress stiffening arises not from bond deformation, but from entropy loss in semiflexible polymers (lose slack)
What does peak in G″ (loss) mean
timescale where maximum energy dissipation occurs
What does increasing applied stress do to a network
Stiffens
What happens to the loss tangent (G″/G′) when myosin is added?
It decreases, indicating the network becomes more solid-like
What does a decreasing loss tangent (G″/G′) indicate about the material?
That it is becoming more solid-like, storing more energy than it dissipates
How is the stiffening effect modulated by actin concentration?
Higher actin concentration → more filament overlap → stronger stiffening by motors