Granular Liquids with Zero Surface Tension.

New experiments with spherical glass beads show that liquid behavior can arise simply from rapid collisions among a sufficiently dense stream of particles. The experiment was undertaken by Xiang Cheng, Heinrich Jaeger and Sidney Nagel and their colleagues at the University of Chicago, experts on discovering novel effects with granular materials (see http://www.aip.org/pnu/2005/split/725-3.html and http://www.aip.org/pnu/2005/split/759-2.html).

If one or two beads are dropped from above on a horizontal surface, they will bounce back in the direction from which they came. If, however, many beads are dropped all at once---constituting a dense granular stream hitting a target---then something else happens: the grains deflect out laterally in the form of a very thin, symmetrical sheet or cone as if they were a liquid.

Indeed, the experiments using granular matter quantitatively reproduce results obtained with streams of water. However, with beads, the “liquid” is one in the limit of vanishing surface tension. (To ensure there was no cohesiveness between the beads, which range in size between 50 microns and 2 millimeter, they were baked in a vacuum oven beforehand, evaporating any lurking moisture.) During the short interval the beads inside the stream collide with each other in front of the target, liquid-like conditions are established whose observable consequence are the thin sheets.

This novel, zero-surface-tension liquid state, the experimenters believe, might be of interest to physicists at the Relativistic Heavy Ion Collider (RHIC), where heavy nuclei colliding at high energies (see http://www.aip.org/pnu/2005/split/728-1.html) form a plasma of quarks and gluons that also resembles a liquid. Intriguingly, the collision pattern produced by the completely classical, macroscopic granular liquid can match that produced by the quark-gluon plasma. (Cheng et al., Physical Review Letters, 2 November 200,http://www.nscl.msu.edu/magnesium40)