Sandcastle science

One of the great joys of childhood is making sandcastles on the beach; and oddly enough, part of the fun is also watching a wave wash them away.

It’s a little startling to find out, then, that something instinctively understood by children–that damp sand sticks together–was only recently explained scientifically in 1997. Dr. Peter Schaffer, an assistant professor of physics at Notre Dame University, was trying to assemble a swing set for his two-year-old. The swing’s plastic pieces had to be weighted down with sand, and Schaffer noted that dry sand sifted easily through the tiny opening provided, but damp sand clumped around it. He wondered why–and decided to find out.

Schaffer and colleague Dr. Albert Laszio Barabasi, with the help of a few grad students, mixed polystyrene spheres, each 0.8 millimetre in diameter, with a tiny amount corn oil and vacuum pump oil, put the mixture into a container with a plugged hole in the bottom, then pulled the plug. The spheres drained out and formed a cone; the researchers then measured the angle of the cone’s sloping sides.

They found that as they gradually added more oil, the cone formed steeper and steeper sides–until, suddenly, the spheres started clumping together. What bound the spheres together–and makes wet sand good for building sandcastles–the researchers figured out, was tiny bridges of liquid.

Fluids prefer to assume the shape that has the lowest potential energy. The larger the surface, the more energy there is, so fluids resist having their surface area increased–the molecules at the surface of the liquid cling tightly to each other, making the surface act like a membrane. The oil formed bridges between the spheres because that formation has the lowest potential energy. Of course, add too much liquid and surface tension disappears because there is no air between the particles to create a surface to the liquid–and your sand castle washes away.

Sand is a bit more complicated–it’s not made up of perfect spheres, but of millions of tiny sharp-edged particles. Sand under pressure–buried deep underground, for instance–will hold together due to friction between particles and interlocking between the particles’ faces, no matter whether it is dry or wet.

But a layer of wet sand deep underground sometimes exhibits a property that puzzles researchers. In the Marina District of San Francisco during the 1989 Loma Prieta earthquake, a layer of wet sand underground, usually solid because of the immense pressure of the layers of rock and soil above it, liquefied in response to the earthquake’s vibrations, to the point that many buildings sank until their third floors were at ground level.

Exactly what caused this is poorly understood. Scientists do know that during an earthquake, shockwaves compress the soil faster than water can escape. This raises the pressure of the water, causing it to bear more and more of the load and the sand less and less. The result is a reduction of pressure between individual sand grains; eventually the friction holding them together vanishes and the sand liquifies. But it’s very difficult for researchers to find out exactly how those grains of sand interact with each other as the pressure between them approaches zero, because the sand itself has weight, which creates stress.

The solution is to send the sand into space, where gravity can’t interfere with observations. Two shuttle flights have already carried an experiment that examines what happens to dry sand when compressed in a fashion similar to that of an earthquake. (It revealed, among other things, that a layer of dry sand at low pressure can support twice as much weight as previously thought.) In an upcoming mission, the experiment will be repeated with wet sand. A column of water-saturated sand in a latex sleeve will be squeezed over and over between two plates. Each squeeze-and-release cycle will take about 10 minutes. Cameras will document how the column of sand deforms. Once the column is back on Earth, scientists will use CT scans to study its internal structure, then inject epoxy into it so it can be sliced and studied in further detail under the microscope.

Understanding the physics of sand is important not only because of the danger of soil liquefaction during earthquakes but because humans use so many granular materials besides sand, including grain, bulk cereals, coal, ash and many fertilizers. Knowing how to convince any of these materials to either stay in place or flow smoothly could lead to safer and more efficient ways of handling them.

It might even lead to better sandcastles.

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