Prof. Bill Van Megen "Collective modes in hard sphere systems; cage effect"

It’s axiomatic that the slowing of diffusion, increasing sluggishness of flow and delay of the structural relaxation a fluid when its density is increased are due to the increasing congestion among the particles comprising the fluid. It is also generally accepted that these manifestations of dynamic slowing are due to the “cage effect”; the microscopic picture in which particles are trapped, at least momentarily, by their respective neighbours. An attempt to examine the role of caging more closely considers the spread of the displacement distributions of Brownian particles. These distributions are necessarily biased by the presence of neighbouring particles. Accommodation of this bias by those neighbours conserves the displacement distribution locally and presents a collective mechanism for exploring configuration space that turns out to be more efficient than the intrinsic Brownian motion. Blocking of the bias effects caging of some particles, however locally in space and time. This incurs, through the impost of global conservation of probability (the average number of caged particles), a delayed, non-local collective process. Both collective mechanisms incur delay or stretching of time correlation functions, in particular the particle number and flux densities. This presentation identifies and distinguishes these mechanisms in existing data from dynamic light scattering experiments and computer simulations on systems of particles with hard sphere interactions.

invited by Prof. Eckhard Bartsch