Streaming sand grains help define essence of a liquid

Water forms droplets because attractive interactions between molecules produce surface tension. If macroscopic objects - say, grains of sand - replace the molecules, the relative strength of this attraction would dramatically drop. What vestiges of liquid behaviour remain in such ultra-low surface tension limit?

Physicists seeking to answer this question have, for the first time, measured the nanoscale forces that cause droplet formation in a falling stream of tiny glass beads. John Royer, a graduate student in physics at the University of Chicago, devised a special apparatus for an $80,000 high-speed camera to image the rapidly changing behaviour of the streaming sand, much as a skydiver might photograph a fellow jumper in free fall.

Royer and his colleagues also measured grain-to-grain interactions directly with an atomic force microscope. They report in the journal Nature that the surface tension in this granular liquid is as much as 100,000 times smaller than that found in ordinary liquids.

Better understanding of the laws of physics motivates the Chicago experiments. Their results have also drawn the attention of a leading industrial firm, Particulate Solid Research Inc.