In the formation of droplets in a stream of falling sand, scientists have witnessed a dynamic that points beyond the boundaries of traditional physics, and may represent one aspect of a fifth state of matter.

“Here we have a material right underneath our noses, that everybody grows up playing with in a sandbox, yet it’s full of surprises for scientists,” physicist said Heinrich Jaeger of the University of Chicago.

The droplets formed because of instabilities in the subtle atomic forces that attract sand grains to each other. Something similar happens to water falling from a faucet, but the forces acting on those molecules are 100,000 times stronger.

Measurements of this phenomena, published Wednesday in Nature, overturn the previous explanation for sand droplets — that grains stick to each other after colliding — and quantify what’s called an “ultralow-surface-tension regime.” It’s entirely new territory for researchers, and just one of many dynamics governing the behavior of granular materials, which for reasons unknown to science act sometimes as solids, or liquids, or gases — or something in-between.

“You walk on the beach, and the sand supports your weight. Pick up a handful, and it runs through your fingers, like a liquid. But you can’t walk on water,” said Jaeger. “In the top of an hourglass, sand is this strange solid. It’s at the verge of being a solid; it flows through the middle as something like a liquid, and then it’s a solid again,” he said.

Since the early 1990s, Jaeger has treated granularity as both a form of matter unto itself and a model for investigating the dynamics of types of matter, as though molecules could be seen by a naked eye. Jaeger also sees in granularity a potentially universal dynamic, reflected in everything from highway traffic to crowd patterns to ecosystem function.

“You have many interacting particles. Energy is put in, sometimes they get stuck, and sometimes it flows,” said Jaeger. “If it flows, what properties does it have? With many interacting players, that behavior is typically very complex and crosses between solid- and liquid-like behavior.”

On a less-speculative level, research into granularity could be a boon for manufacturers. Most finished products and foods pass at some point through a granular stage — pellets of plastic, gravel in concrete, corn in a silo, powders in a pill, on and on. A report published by the Rand Corporation in 1986 found that granular industrial processes generally function at about 60% of capacity.

“Semingly modest changes in conditions, such as temperature, humidity, and surface conditions routinely cause earth bound devices to fail,” concluded the authors of a 2005 NASA technical report on the importance of understanding granularity to exploring Mars and the Moon.

The authors are scathing in their critique of industry, which in the absence of granular theory relies on “millennia-long trial-and-error practices that lead to today’s massive over-design, high failure rate, and extensive incremental scaling up of industrial processes because of the inadequate predictive tools for design.”

“Physicists have a rich toolbox for dealing with solids, liquids and gases. But we don’t have a manual for when the old categories don’t apply,” said Jaeger.

See Also:

Citation: “High-Speed Tracking of Rupture and Clustering in Freely Falling Granular Streams.” By John R. Royer, Daniel J. Evans, Loreto O. Gálvez, Quiti Guo, Eliot Kapit, Matthias E. Möbius, Scott R. Waitukaitis and Heinrich M. Jaeger. Nature, Vol. 459 No. 7250, June 25, 2009.

Video 1: John Royer. Because the formation of sand droplets happens so rapidly, and the sand must fall several feet, he arrived at the ingenious solution of filming it with a high-speed video camera that fell at the same speed as the sand. Video 2: DropDropG/YouTube

Brandon Keim’s Twitter stream and reportorial outtakes; Wired Science on Twitter.