Particles in flowing granular material can cluster like a stream forming droplets as it comes out of a faucet. The clustering is due to attractive forces that create a weak surface tension in grains of glass, scientists report in the June 25 Nature.
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Previously, scientists thought grains did not display surface tension, or if they did, the effects were too small to change the flow of the particles. “But this experiment says that if we look very carefully, we find surface tension is almost zero, but it is not exactly zero,” says Heinrich Jaeger of the University of Chicago, an author of the new study.
Clustering is a key problem for transporting powders and granular materials, including fertilizers and pharmaceutical drugs, Jaeger says. Understanding how ultra-low surface tension can cause clusters could help researchers better predict how grains and powders will flow through pipes or even parts of the body, and to know when and whether the material will clump.