Loose sand can pack so strongly and densely that it can support a house, yet it can also flow easily, as in an hourglass. Physicists in the Netherlands have now accentuated the liquid behavior of granular materials by reducing the sizes of the particles and the forces between them. The result is a bizarre substance that ejects towering jets of grains when objects drop into it.
Granular materials play important roles in many natural and industrial processes, from the movements of sand dunes and avalanches to the manufacture of pharmaceuticals and concrete (SN: 7/26/03, p. 56: Available to subscribers at Mastering the Mixer). The new findings are helping scientists discriminate when conditions favor liquid or solid behavior, says Detlef Lohse of the University of Twente in Enschede. Researchers have long struggled to determine when to use the equations of liquid behavior for granules.
In their experiments, Lohse and his colleagues observed what happens when a gumball-size steel sphere drops into a washtub-deep bed of fine, loosely packed sand. To make the sand more fluid, the team used particles less than a twentieth the diameter of average beach-sand grains. They also took the innovative approach of bubbling air through the sand bed before the tests until the material became extraordinarily “fluffy,” Lohse says. “That’s the new trick” to minimize attractive forces between the sand grains, he notes.
When the scientists dropped the metal ball into the bed of sand from a half-meter or more above its surface, the ball hit the sand with a splash and then plunged out of sight. About a tenth of a second later, a narrow geyser of grains shot up, gushing higher than the level at which the ball was released.
Such a jet results from the ball boring a hole into the sand bed. When the bed collapses into the borehole, pressure builds up on the inflowing sand at middepth and forces it back out through the breach in the surface, explains Michel Versluis of the Twente team. Similar jets are well-known in bona fide liquids and were first reported in beds of glass beads in 2001.
The Twente researchers also found the first evidence that a second jet simultaneously zooms downward into the tunnel bored by the ball. They describe their study in an unpublished report at the Internet site called arXiv (http://www.arXiv.org/abs/cond-mat/0406368).
The team speculates that the new findings may offer insights into what happens when asteroids or comets strike planets.
Sigurdur T. Thoroddsen of the National University of Singapore, the codiscoverer of the glass-bead jets, calls the Twente experiments “beautiful.” Learning about such jets is “important for basic understanding of the behavior of rapidly flowing granular materials,” he says.