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 this corresponds to a surface tension that is 100, 000 times weaker than 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.

Glassy free fall

In the experiment, glass spheres were fed through a funnel and began to accelerate under gravity. Almost immediately, the stream began to elongate and, after a metre of free fall, thin bridges of just a few grains wide begin to appear in the stream. By two and a half metres, these bridges had started to rupture as the clusters continue to separate.

To determine the strength of this cohesion, the researchers recorded the forces between individual grains by bringing them into contact and pulling them apart with an atomic force microscope. The grains were typically 150 µm in diameter and forces were of the order µN, or 100, 000 times smaller than an equivalent surface tension in water droplets.

These findings constitute the latest in a series of fluid experiments by Royer and his colleagues using their high-speed camera. Some other recent examples include the splashing of liquid droplets impacting a solid surface and the pinch-off of a bubble underwater.

The University of Chicago physicists intend to now develop this research by studying the flow of different materials including highly charged grains. They hope that an improved understanding of clustering dynamics and how it varies between materials could eventually lead to a sensitive tool to study cohesive interactions in grains and powders. “These findings could be very relevant to numerous industries that handle powders and grains, including the pharmaceutical and chemical processing industries,” said Royer.

These findings are published in the latest edition of Nature (Nature 459 1110).