Mixing air into sand to get fluidlike flow.

One way to get sand or some other granular solid to flow like a liquid is by shaking it up. Industrial engineers have long used such a "fluidized bed" strategy to keep powders moving during the manufacture of pharmaceuticals and other products.

Now, physicist Robert P. Behringer and his coworkers at Duke University in Durham, N.C., have demonstrated quantitatively that air plays a central role in this process. Their experiments show that just a little bit of air trapped in a granular material allows the shaken grains of the solid to travel in the distinctive swirling patterns characteristic of convective fluid flow (SN: 6/26/93, p.405).

The researchers report their findings in the June 5 Physical Review Letters.

To study this convective motion, Behringer and his colleagues confined sand grains or glass beads to the narrow gap between two cylinders, one placed inside the other. When such an apparatus, partially filled with a granular material, is repeatedly shaken up and down, the enclosed material's upper surface develops a slant across the entire ring. In other words, particles heap up on one side of the ring that confines them.

This heaping results from the upward motion of particles within the material to the top of the pile. The particles then avalanche down the slope to reach the ring's opposite side before resuming their motion back up to the top.

The researchers found that for grains smaller than 1 millimeter in diameter, the amount of heaping decreases rapidly when air pressure is reduced below 1 percent of atmospheric pressure. Essentially no heaping occurs at zero pressure, suggesting that granular fluid flow requires the presence of air. For larger particles, air tends to leak out instead of getting trapped.