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U. CHICAGO (US)—How do liquids behave when there’s a dramatic drop in surface tension—the physical property of liquid that allows droplets to form? For the first time, physicists have measured the nanoscale forces that cause droplet formation in a falling stream of tiny glass beads.

Freely falling granular streams behave similarly to water flowing from a faucet. These granular streams behave like dense, cold fluids with ultra-low surface tension (cohesion between individual molecules).

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 behavior remain in such ultra-low surface tension limit? Scientists at the University of Chicago think the answer could lead to a better understanding of the laws of physics.

John Royer, a graduate student in physics at Chicago, devised a special apparatus for an $80,000 high-speed camera to image the rapidly changing behavior 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.

The study was funded by the National Science Foundation.

University of Chicago news: www-news.uchicago.edu