The device consists of a wheel in the center of a shallow circular container, which is filled with small translucent cylinders as flexible as hard rubber. The tiny cylinders also were marked with a line on one end, which allowed scientists to establish that some rotated instead of simply sliding. Their optical properties then change whenever and wherever they build up stresses. Experimenters can see the resulting flashing stress chain patterns by illuminating the apparatus from underneath and observing it through cross-polarizing filters. Those filters screen out any light whose optical properties have not been stress-distorted.
Scientists can videotape the process and use a computer to convert the patterns of light into color.
As dramatic as the images are, the apparatus is far more than just an attention-getting display. Using a calibration device, investigators also can use it to painstakingly measure the forces on individual grains, as well as following how the grains move and orient themselves within their essentially two-dimensional world. Behringer acknowledged that most granular particles exist in three-dimensional space, and that most are made of hard substances rather than squeezable ones. But he said separate 3-D experiments with hard glass beads have produced some analogous results. Behringer has conducted stress fluctuation experiments both at Duke and at the Ecole Superieur de Physique et Chemie Industrielle of Paris, where he spent a 1997 sabbatical.