"The response of a granular medium such as sand to shear forces is an important fundamental property of these materials, which are different from ordinary solids and fluids," explains Gollub, a physicist who is well known for his research in nonlinear dynamics (including chaos), condensed matter physics and fluid dynamics. "Granular materials are also believed to be important in earthquake dynamics, in that slipping events occur along faults in the earth's crust, which are often separated by a layer of sand."

In the experiments at Haverford, Satoru Nasuno, a visitor to the campus from the Kyushu Institute of Technology, Arshad Kudrolli, a former research associate at Haverford, and Gollub studied friction in granular materials in a novel way that revealed, for the first time, the frictional forces within individual slipping events that occur when the internal stress in the material reaches a critical level. These events resemble, among other things, mini-earthquakes lasting only 40 milliseconds. The researchers also detected microscopic precursors of the slipping events. Though their materials and conditions were quite different from those prevailing at real earthquake faults, the results show behavior that is in many ways analogous, Gollub says.

These results, which were published in the August 4 issue of Physical Review Letters, were the subject of invited lectures at several international meetings this past summer, including a Gordon Research Conference in New Hampshire and a Symposium on Localization Phenomena in Geophysics at Columbia University. In September they were also presented to the Department of Earth and Planetary Sciences at the Massachusetts Institute of Technology, and will be discussed at the Institute for Theoretical Physics at Santa Barbara in October.

The work is being continued this year by physics major Anthony Bak '99 in Jerry Gollub's laboratory.