Grain, sand, coal, and other granular materials can "freeze" and "melt" just like other materials, according to physicists at Duke. This "phase change" isn't caused by temperature change, the researchers found, but by shaking or vibrating the material in a particular way.
Led by Robert Behringer, James B. Duke Professor of physics, the scientists studied the behavior of granular materials by shaking and vibrating plastic beads in a precise way inside a Plexiglas chamber. They published their findings in the journal Physical Review Letters. "What is rather striking about this is that vibration doesn't make a granular system move as expected toward a more fluid-like state, as it would in conventional matter," says Behringer. "Instead, it has the reverse effect: making it move toward a solid-like crystalline state." The transition to a more solid state occurs as particles are pushed so close together that they interlock and become immobile. "If there isn't a sufficiently large shear, solid-like or jammed granular material will not move," he says.
Their method may apply beyond the laboratory to help resolve some very common engineering problems, says Behringer. "Suppose that you have an embankment and you want to understand its stability. If you push it past the point where it will start to avalanche, what would those dynamics be? You might think that vibrations would destabilize the bank, when in fact they could stabilize it."
Their finding might also call into question a common industrial method for loosening materials jammed inside hoppers. "In some cases, devices are used that cause vibrations," says Behringer. "That might not only be an ineffective technique, it might actually compact the material even more."