Physics dives into the basics of everyday life

Chicago Tribune

You might think scientists would have figured out by now the answers to such basic questions as why water droplets splash, why sand castles collapse and what makes shaving cream so creamy.

But no one has solved those puzzles, and that's why physicists at the University of Chicago are launching an $1.8-million study of how fluid flows and why things fall apart.

The technical term for their subject is "catastrophic deformation," which describes many processes we see every day.

Take those water droplets. Normally, a drop of liquid makes a splash when it strikes a flat surface. But the Chicago researchers discovered a few years ago that when air pressure is low, a drop of alcohol makes no splash. It just spreads out with nary a ripple, and no one knows exactly why.

A grant from the W.M. Keck Foundation will try to find out. The point is not just to understand everyday mysteries but to gain insight into practical matters ranging from how avalanches start to how car engines can use fuel most efficiently.

"Physics is really good at explaining problems that are linear, with clear borders, where all the forces are local," said University of Chicago physics professor Sidney R. Nagel, one of the grant's four main investigators.

"But so much of everyday life is governed by systems that are not linear, not bordered, and not in equilibrium," he said. "Right beneath our nose there can be a deep physics problem."

To tackle those problems, the researchers will be using imaging tools such as the Advanced Photon Source at Argonne National Laboratory and sophisticated microscopes that can capture three-dimensional images of changes inside living cells. About $1 million of the grant will go toward developing such instruments.

Many of the measurements require capturing very fast events that happen on a small scale. For example, understanding why liquid splashes depends in part on figuring out what happens in a split-second at a tiny point where a sphere of liquid meets a flat surface.

Somehow, the liquid must be interacting with the surrounding air pressure in a way that makes a splash of tiny droplets, said Wendy Zhang, an assistant professor in physics at the university.

"What is the air doing to create the splash?" Zhang said. "We have some theoretical ideas, but we don't know."

The smallest scale of living cells is of primary interest to university biophysicist Margaret Gardel. A cell's confines are full of complex molecules that must act in complex harmony for a creature to survive. For example, the cell's inner supports, or cytoskeleton, must be rigid yet flexible, in order to give a cell its shape but permit cell division.

"The cell has evolved molecular machines and materials that can dynamically change their shape, sometimes very catastrophically," Gardel said.

Some of the molecules in cells have physical properties that are similar to Silly Putty or shaving cream, Gardel said. Both of those materials are solid but also can be spread across a surface almost as a liquid.

By taking images of the cell's insides in action, Gardel hopes to shed light on the dynamic materials that keep our bodies working.

The researchers' work thus far has shown that even the most common events can spur deep physics questions. In 1997, Nagel and colleague Tom Witten published a paper on why coffee stains form perfect rings -- a surprisingly thorny question that arose one morning from Nagel's bleary-eyed contemplation of his kitchen counter.

It seems a nicely whimsical sort of problem, but Nagel eschews such language. He said conundrums that look easy at first glance often are the most profound.

"That's what my whole research life is about," he said.

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