Propagating front in a vibrated granular layer

NEW: "Propagating front in an excited granular layer", W. Losert, D.G.W. Cooper, and J.P. Gollub, submitted to Phys. Rev. E, preprint available at cond-mat/98112089 (1998).

Abstract: A partial monolayer of ~ 20,000 uniform spherical steel beads, vibrated vertically on a flat plate, shows remarkable ordering transitions and cooperative behavior just below 1g maximum acceleration. We study the stability of a quiescent disordered or ``amorphous'' state formed when the acceleration is switched off in the excited ``gaseous'' state. The transition from the amorphous state back to the gaseous state upon increasing the plate's acceleration is generally subcritical: An external perturbation applied to one bead initiates a propagating front that produces a rapid transition. We measure the front velocity as a function of the applied acceleration. This phenomenon is explained by a model based on a single vibrated particle with multiple attractors that is perturbed by collisions. A simulation shows that a sufficiently high rate of interparticle collisions can prevent trapping in the attractor corresponding to the non-moving ground state.

 Bistability: a moving front transforms an amorphous solid to a granular gas after a small perturbation

	Front propagation during the amorphous-to-gaseous transition at 80Hz with a=0.93g. Images captured every 3.5s are shown at left; absolute differences between images taken 0.5s apart are shown on the right to highlight moving particles. (a,d) Amorphous initial state; (b,e),(c,f) unstable front propagation
	Hysteresis of the amorphous-to-gaseous phase transition. Minimum accelerations for a perturbed and a spontaneous amorphous-to-gaseous transition are shown (closed symbols). The freezing and evaporation points of the crystalline phase are also shown (open symbols).
	Growth of the gaseous area A for two peak accelerations of the plate at various bead coverages and vibration frequencies. Smaller frequencies lead to faster melting. Lower coverage leads to faster melting for large areas only. Larger accelerations always lead to faster growth.

  In addition, preliminary experiments on binary mixtures of different size particles are carried out to study size segregation and changes in energy sharing. The tendency to segregate is found to be strongly frequency-dependent.