The air that surrounds you and fills your lungs with each breath is accurately described by a detailed, microscopic theory, the kinetic theory of gases. That theory, dating back to the late 1800s, correctly predicts the macroscopic features of an ideal gas, such as its temperature and pressure, based on the motions of all its atoms or molecules. No such comprehensive theory exists for granular gases--collections of larger particles such as dust grains in space. Another baby step on the way to such a theory was taken recently by experimental physicists Florence Rouyer and Narayanan Menon of the University of Massachusetts at Amherst, who studied the motions of a "gas" of steel ball bearings and determined that a consistent distribution of ball velocities was maintained over a range of conditions.

The study of granular materials has burgeoned over the past two decades or so. The motion of soil in an earthquake or avalanche is granular, as are many industrial processes involving foodstuffs, pharmaceuticals and other chemicals. The rings of Saturn and the interstellar dust and particles that formed the planets are granular gases. Although they move in a mixture of gas and liquid, powdered catalyst particles used in the multibillion-dollar petrochemical industry also behave in some ways as a granular gas. Yet granular materials remain poorly understood compared with conventional solids, liquids and gases.

   
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