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Xio-Lun Wu rises from his chair and picks up a tall hourglass from his desk. He holds the glass by its top chamber, and the "sand" inside (actually sand-grainsized beads of glass) flows smoothly into the bottom. Oddly enough, though, when he grips the bottom chamber, the flow of sand stops dead.

Wu, an assistant professor of physics, may seem to be breaking the law of gravity, but instead he is playing with the properties of granular materials such as powders, sand, or tiny glass beads. The movement of this kind of material remains poorly understood by physicists, who until recent years believed that sand in an hourglass flowed continuously, just like water. Today, physicists study granular materials as a unique, separate state of matter, neither liquidnor solid.And it turns out that these materials move in unexpected ways.

Wu's experiment, for example, depends on the interaction between particles in the hourglass and the air they move through. When Wu holds the bottom chamber of the hourglass, the heat from his hand causes the air inside to expand. This expansion creates airpressure that pushes up on the particles, counteracting gravity and stopping the flow. "The question," Wu says, "is, 'Why can such a small pressure support such a large amount of mass?"'

The answer is that small particles like sand, pulled down by gravity, form a system of fragile "arches" across the narrow opening of the hourglass. Just as an architectural arch can hold up a great mass by distributing the weight over many stones that support one another, the arches inside the hourglass support the weight above by distributing it equally over many particles. "Really, what matters is the ratio between the particles and the neck size, " Wu says. "If the neck is large, it's difficult to form a stable arch." More precisely, arches will form inside an hourglass when the particles are between one-twelfth and one-half of the neck's width. These arches, however, quickly crumble. As a result, the flow of sand through the hourglass "ticks"--that is, it stops briefly and then starts again, over and over, at regular intervals as the arches form and then break apart.

In Wu's custom-designed hourglass, which has a very narrow neck, even a slight rise in air pressure from below--like the one when Wu warms the bottom chamber with his hand--is enough to stabilize the arches and stop the flow entirely. On the other hand, the increase in air pressure from above when Wu warms the top chamber prevents the formation of arches and forces the glass beads to spill through the hourglass continuously.

Scientists have long known that the effect of air pressure on granular material can have serious consequences. In Germany during the 1960s, air trapped in a silo caused thousands of tons of cement powder to "fluidize" and spill out onto an outdoor plant floor. Wu keeps a straight face, save for a reserved smile, as he reveals the ending to the story--after the spill, it rained, wetting down the cement powder that covered the entire factory floor.

By refining science's knowledge of the dynamics of granular motion, Wu's research may help prevent such disasters. In fact, a thorough understanding of granular flow could have many practical applications, ranging from more stable, spill-proof methods for storing and packing laundry detergent to a technique for keeping smaller bits of cereal from settling to the bottom of the box. After all, there are always rewards that come with learning how to go with the flow.

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