Thin streams of liquid commonly break up into characteristic droplet patterns due to the surface tension driven Plateau-Rayleigh instability.  Remarkably, very similar patterns are observed when initially uniform streams of dry granular material break up into clusters of grains, even though flows of macroscopic particles are considered to lack surface tension.   Using high-speed video imaging in the co-moving frame (i.e., by dropping an $80,000 high-speed video camera alongside the stream), measuring grain-grain interactions with Atomic Force Microscopy, and controlling the properties of the grain surfaces, we demonstrate that cluster formation is driven by minute, nanoNewton cohesive forces due to a combination of van der Waals interactions and capillary bridges between nanoscale surface asperities.  These cohesive forces correspond to an equivalent surface tension four to five orders of magnitude below that of ordinary liquids. As a result, gravitational stretching of the stream dominates and the stream ruptures into irregular (but compact) clusters rather than pinch off into smooth, spherical droplets.

Check out movies or read more about these experiments: Nature News&Views article (by Detlef Lohse and Devaraj van der Meer from the Physics of Fluids Group at the University of Twente), UofC Press Release, NSF News, MSNBC, Discover Magazine, Scientific American...

1. •John R. Royer, Daniel J. Evans, Loreto Oyarte, Qiti Guo, Eliot Kapit, Matthias E. Möbius, Scott R. Waitukaitis, and Heinrich M. Jaeger, "High-Speed Tracking of Rupture and Clustering in Freely Falling Granular Streams", Nature 459, 1110-1113 (2009). pdf of article / pdf of supplementary material / link to News&Views piece about this article