Physics Colloquium - Wednesday, August 26th, 2009, 4:00 P.M.

Paul Umbanhowar
Dept. of Mechanical Engineering
Northwestern University

Granular materials (e.g. sand, beads, poppy seeds) are collections of discrete mesoscopic solid particles that interact via dissipative contact forces. Typically the work to slide or lift a grain a fraction of its diameter exceeds its thermal energy (~kT) by many orders of magnitude. As a result, granular media change configuration only with sustained external forcing and form weak solids when unforced. Granular media also resist easy description in terms of continuum equations because flows typically involve solid and fluid-like regions. I will discuss experiments that illustrate the often surprising and varied response of granular media to a variety of external forcing including shaking and shearing. I will also address the closely related problem of motion within granular media with examples from impact and locomotion. Lastly, I will describe a new technique for generating non-trivial velocity fields using frictional driving and explore its potential for generating new insights into the physics of granular media.