Ever shake a box of dishwashing detergent or instant rice so the grains inside will flow more easily through the opening? You might be making things worse, says physicist Robert Behringer of Duke University. He’s found that rather than loosening them, vigorous shaking will make granular materials, like sand, dirt, and even instant rice, solidify. Behringer, along with Karen Daniels, now at North Carolina State University, first stirred plastic beads in a see-through chamber. As expected, they jumbled up, becoming more loosely organized.
Behringer thought that vibrating the beads would break them up even more so they would pour like liquid. With just a little jiggling, the beads did become more disordered, but to his surprise, shaking the beads with gusto compacted them into an immovable state. Altering the rates of stirring and shaking made the beads switch back and forth between a fluidlike state and a frozen one. A solid, frozen state is a stable configuration for the beads, Behringer says. “Once the beads find their way to it, they won’t fall out of it easily.” But it’s not so easy for beads to get properly organized because “they tend to get in each other’s way. Shaking provides the necessary energy for the particles to do the rearranging,” he says.
The studies have practical effects. In industries like agriculture, clumped grains in silos are a daily headache. “They go to drain the material out of the silo, and nothing comes because it is jammed,” Behringer says. “The standard fix is to bang on the silo,” he says, sometimes with a sophisticated machine, sometimes with just a sledgehammer. The new results show this might not improve things—and could have the opposite effect.