Jamming in Liquids and Granular Materials

Many systems can develop a yield stress while in an amorphous state. For example, a supercooled liquid, when cooled sufficiently, forms a glass - an amorphous solid with a yield stress. Another common example is a granular material which will remain solid and not move even under the influence of moderate stresses. This accounts for why piles of grain or sand can exist with a non-zero slope even though gravity is acting to flatten out the upper surface. The solidity in that case is due to the system having become jammed. Similar jamming often inhibits flow out of a hopper or in conduits transporting material across a factory floor. Jamming is a ubiquitous phenomenon occurring in many different systems such as colloidal suspensions, foams and, of course, traffic. We tend to think of the jamming transition as being stress-induced. A “fluid” at constant density (or under a confining pressure) flows if the stress is above the yield stress but becomes stuck in an amorphous configuration if the stress is too low. The idea of temperature, per se, does not seem to be crucial to the transition. This makes it seems quite different from the formation of a glass out of a supercooled liquid by lowering the temperature. However, there are similarities between these two types of transitions, aside from the obvious fact that they both have to do with the complete arrest of dynamics and flow. An exploration of these similarities was the subject of a program at the Institute for Theoretical Physics in Santa Barbara held in the Autumn of 1997. A synopsis of this program was published that details some of the interesting ideas now current in that field.[1]