Going with the flow

Is sand a solid, a liquid, or a gas' It's a question that has plagued scientist for centuries. If a jogger runs on a beach, sand acts as a solid and supports their weight. Put it in an hourglass, and sand pours through the nozzle, much like a liquid. If a child in a sandbox trips and kicks up a cloud of sand, the separated grains collide and interact as though it's a gas. So what state of matter is sand? "In my opinion, the answer is all of the above," answers Ken Kamrin, an associate professor of mechanical engineering who received tenure earlier this month. "I think if we want to develop a better understanding of granular media, we have to think about it as broadly as possible and come up with models that treat it like a solid, liquid, or gas." Granular materials are ubiquitous. They make up the soil, sand, and gravel we live on, the grains and flour we use in cooking, and the powders we use for medicine.