Physicists watch as sand forms droplets

Fire a squirt-gun skywards and the liquid stream will start to break up into smaller droplets due to surface tension. Intriguingly, this same behaviour is also observed in flows of sand even though granular matter is thought to be collection of grains that exert no forces on each other. Now — with the help of their $80,000 video camera — physicists in the US have developed an explanation for this puzzling similarity.John Royer and colleagues at the University of Chicago attribute this behaviour to the roughness of individual grains of sand. They propose that coarse surfaces lead to a combination of van der Waals interactions and capillary forces, causing grains to become attracted. Although 100, 000 times weaker than surface tension in liquid, these interactions closely resemble droplet formation in water jets, say the researchers.Recent studies have revealed instabilities in the flow of granular materials but the minuteness of the forces involved have rendered the clusters too short-lived to observe. Royer and his team avoided this problem by combining high-speed photography with sensitive measuring of forces. By “dropping” the camera alongside a stream of sand, they were able to capture high-quality images at 1000 frames per second and record the sand dynamics as it fell a metre in less than a second. “We now have a magnetic release, though at first I literally held it up by and then let go,” John Royer told physicsworld.com.