Milner Research Group
Dynamics of glassy systems
Why is Tg in polymer thin films different from its bulk value?
Many experiments show that polymer films remain molten near a free surface at temperatures well below the bulk glass transition, an effect that persists over tens of nanometers. What makes near-surface segments more mobile, and what sets the range of the effect?
Glass transition temperature Tg versus film thickness for thin polystyrene films of varying molecular weight. (from Dalnoki-Veress et al., Molecular weight dependence of reductions in the glass transition temperature of thin, freely standing polymer films. Phys. Rev. E (2001) 63, 031801).
What makes dense, nearly-glassy liquids so sluggish?
How can we understand the origins of glassy behavior in dense liquids, in terms of the scarcity of free volume in which the particles may move locally? Can we identify on a purely geometrical basis which particles are "most likely to move"?
The Voronoi construction (black lines) defines the regions of space nearer to each given particle. The centers of hard-sphere particles (green) cannot enter the exclusion zones (red) of other particles. The free volume of particle 1 (white area, figure b) is the region over which its center can translate with the other particles fixed.
How do forces propagate in jammed granular solids?
Hard particles pressed tightly enough together "jam" in a random packing, and will bear stress. If the packing is "barely tight enough", many particles do not make "good contact", and the forces propagate via inhomogeneous "force chains". How does this inhomogeneity vary as the pressure is relaxed?
A two-dimensional pack of plastic beads under in-plane stress, viewed through crossed polarizers. Stress-induced birefringence causes beads under load to light up. (From B. Behringer, Duke Univ. www.phy.duke.edu/~bob/).