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?
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.
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/).