Everyone knows that water is the mortar that holds together the grains of sand in a sand castle.
Far fewer people probably know that when the water evaporates, the sand castle still holds together because salt residues create tiny bridges between the sand grains, locking them in place.
What nobody, not even sunbathing physicists, has understood completely is how sand and other kinds of granular materials change from something that behaves like a liquid when it's dry into something that acts like a solid when it's wet.
That transformation is a little less mysterious now than it once was thanks to research by a pair of Notre Dame physicists and their student assistants.
The idea came to Notre Dame Assistant Professor Peter Schiffer when he was assembling a swing set for his young son. The instructions called for pouring sand into a hollow base to weigh the set down and keep it from tipping over. Schiffer went to a home center and bought bags of sand, but when he opened them he found that the contents were damp. Even using a funnel it was a nightmare trying to force the clumped sand through the base's one-inch fill hole.
An experimental physicist, Schiffer had been talking for some time with fellow ND Assistant Professor Albert-Laszlo Barabasi, a theoretical physicist, about collaborating on a research project, possibly looking into the physics of that troublesome material mud. The stubborn swing set sand suggested a simpler research question: How does granular clumping begin?
Physicists have long known why individual wet sand grains clump together. It's a consequence of nature's tendency to conserve energy. Because it takes more energy for the water coating a grain of sand to have an interface with air than to join with the water coating a nearby grain, the two films merge, sticking the grains together. What the Notre Dame researchers wanted to understand was how much liquid it takes to make that phenomenon chain-react through a pile of sand or similar material.
When granular materials are poured onto a horizontal surface, they pile up in pyramid-like mounds. The chunkier the pieces are, the steeper the sides of the mound will be. But the pile's steepness, technically termed its angle of repose, also depends on the amount of moisture in the material. As any sand castle builder knows, a turned-over bucket of wet sand can make an sturdy parapet while a bucket of dry isn't good for much beyond a dune.
To pinpoint how much liquid it takes to make a difference in the properties of granular materials, Schiffer and Barabasi developed an experiment. A small bucket-like container was filled with tiny plastic beads of the kind used in sand-blasting-like operations. To the beads, they added a small amount of vacuum pump oil. Actual sand and water weren't used because sand varies too much in shape and composition, and water evaporates too fast.
A trapdoor in the center of the bottom of the container was then opened to allow the beads to drain out. When the beads ceased draining, the student assistant in charge of the experiment would calculate the angle of repose. The procedure was repeated thousands of times using varying amounts of oil.
The results, reported in paper that made the cover of the June 19,1997, issue of the science journal Nature, showed that even a tiny amount of liquid — in the case of the plastic beads only about a drop per quart of beads — is enough to change the properties of granular materials dramatically. This wetness threshold was much lower than previous studies had suggested.
The physicists' findings, detailed by Barabasi and graduate student Reka Albert, might eventually prove useful not only to industries where materials are stored in piles — like coal at power plants or grain in silos — but in the manufacturing of such products as pills, where it's important to understand how tiny grains of chemicals adhere.