Physicists Seek Grains of Truth by Digging Into Sand
It’s odd how the everyday things are often the most mysterious. How mind springs from inanimate matter, for instance. Or the nature of space, or time.
Which came first, the chicken or the egg? What is matter, and why does it have mass? How exactly does gravity work? Scientists still don’t know what drives the erratic magnetic compass of the Earth, or where the moon came from, or why the sun boils up in cycles of 11 years. They don’t understand how proteins knot themselves into shape, or why snowflakes crystallize into ever-changing patterns.
So while some scientists explore the exotic frontiers of the universe, hunting for strange, wild specimens such as quasars, pulsars and quarks, others subscribe to what Dorothy found out when she returned from Oz: Sometimes the richest treasures are buried in one’s own backyard.
These days, physicists at the forefront of science can be found putting enormous energy into understanding something that finds its way into the backyards of many Southern Californians: sand.
Sand may seem like child’s play, but for physicists, it’s frustratingly hard to get a handle on. The problem is, it’s weird stuff. It’s made of solid grains, but it flows like a liquid. Dry, it floats around on the wind like dust, but wet, it can be sculpted into castles.
Actually, sand forms its own special category--not solid, not liquid, not gas, but “granular material.” Studying the behavior of sand tells scientists a great deal about the behavior of any pile of stuff--whether it’s piles of magazines, molecules, soil, salt or aspirin.
These granular materials exhibit bizarre behavior with no obvious explanation. For example, if you shake up a jar filled with granular material of different sizes, the biggest grains always rise to the top. That’s why the Brazil nuts always wind up on top of the tin of mixed nuts, and the peanuts on the bottom.
Somehow, granular materials manage to sort themselves by size.
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This odd property causes a major problem for many industries, including makers of pharmaceuticals, who often have to mix powders together, and need smooth, evenly mixed concoctions.
More interesting to physicists, there’s no good explanation for how inanimate matter can organize itself so readily. Over time, things normally mix, not unmix. If you leave a perfume bottle in the corner of the room, sooner or later all the perfume molecules will waft out, wandering willy-nilly about the house. If you leave the refrigerator door open, pretty soon the cold air will mix with the warm air of the room, and both room and refrigerator will be the same temperature.
But granular materials line up in tidy stacks. They appear to disobey the laws of nature. The question is, how?
Physicists also don’t understand exactly how a sandpile stands up. You would think that a mound of sand would exert the most pressure at the point where the pile is highest: in the middle. Surprisingly, the maximum pressure point is actually a ring that forms around the apex.
This might explain why children can build tunnels through sand castles without causing them to crash; somehow, invisible arches inside the castle must be supporting the load. Susan Coppersmith of the University of Chicago proposed in the journal Science that sand somehow distributes its weight unevenly, forming these “stringy” lines of force.
More recently, an article in the journal Nature explored the question of why wet sand behaves so differently from dry sand. Apparently, the water acts as an adhesive that makes the grains stick together, but the effect is surprisingly large.
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“Small quantities of wetting liquid can thus dramatically change the properties of granular media,” concluded physicist Peter Schiffer of the University of Notre Dame, and colleagues.
Physicist Per Bak of Brookhaven National Lab, meanwhile, has developed a new branch of science built on sand. Called “self-organizing criticality,” it explains how avalanches in sandpiles form, and how the pile retains, over time, the same general shape. Bak thinks the theory will also help to explain everything from earthquakes to stock market crashes.
Perhaps the poet was right: You can find a whole world in a grain of sand.
