Drops of knowledge

Liquids: Research into one of life's most mundane phenomena, the drip, yields aesthetic and industrial products.

Medicine & Science

December 01, 2003|By Michael Stroh | Michael Stroh,SUN STAFF

For most of us, leaky pipes are a major pain. For Sydney Nagel, they're a chance to plumb a long-standing physical mystery: How do drops drip?

To answer this question, the 55-year-old University of Chicago physicist spends hours hovering over the lab equivalent of a trickling kitchen tap, photographing and studying his short-lived subjects. Last month, persistence paid off.

Nagel and an international team of drip detectives reported in the journal Science that they had observed a new type of drop that might help engineers create ultra-thin wires and a new generation of electronic devices.

"You look at something day after day and realize there might be something interesting and fundamental left to be discovered," Nagel says.

Nagel has built a career uncovering complexity hidden in the ordinary. Over the years, for example, he has published papers on sand piles, coffee rings, wadded paper and other seemingly mundane subjects.

Chasing a phenomenon that most of us encounter in bathroom sinks and the ends of icicles might sound odd. But Nagel insists that understanding how streams of liquid break apart offers insight into mysteries that range from star formation to the growth of bacteria colonies.

Drops - not to mention their runny relatives, splashes and bubbles - are also of great interest to industry. Pharmaceutical companies such as Roche, for example, fund drop research in a hunt for new ways to deliver drugs. Hewlett-Packard, meanwhile, studies drops to find advanced new applications for its ink-jet printer technology.

"You would never think going home and turning on the faucet that something so familiar could contain so many stories," says Wendy Zhang, a theoretical physicist at the University of Chicago and co-author of the Science paper.

The scientific investigation of drops began more than 300 years ago, when French physicist Edme Mariotte started poking holes into the bottom of containers to study trickling water. In 1686 he published the first treatise on drop behavior, Traite du mouvement des eaux et des autres corps fluides.

Mariotte concluded that gravity gave rise to drops. But by the 19th century, scientists were gathering evidence that the phenomenon was far more complex than Mariotte imagined. Their new insight came from a new technology: photography.

The life span of a liquid drop, scientists long realized, was too fleeting for the eye. So John William Strutt, a British scientist better known as Lord Rayleigh, devised a way to bring a moving drop to a dead stop.

His secret was the Leyden jar, a primitive device that stored an electric charge and produced a brief but brilliant spark. Spending tedious hours in pitch darkness with his homemade flash and plate camera, Strutt captured the first images of a jet of water cleaving into drops.

His research in the 1890s laid the foundation for the ink-jet printer, while his technique popularized a powerful new scientific tool: stop-motion photography.

Others drop devotees followed.

In 1908, British physicist Arthur Worthington published A Study of Splashes. His Leyden jar flash photos of dripping milk, mercury and other liquids are considered the first comprehensive stop-motion study of the phenomenon.

By the 1930s, Harold "Doc" Edgerton at the Massachusetts Institute of Technology was building sophisticated, high-speed cameras that could snap 3,000 photographs a second.

His studies of water faucets, combined with the work of theoretical physicists, helped cement the growing scientific understanding of drops.

It turned out that many physical forces influence the behavior of a drop of liquid. Water trickling from a tap, for example, is initially glued together by surface tension, a cohesive force between molecules.

As more fluid pours in, the drop starts to resemble a tadpole, with a thinning neck and bulbous bottom. As the mass of the drop grows, gravity overpowers molecular adhesion.

The upshot: the thinning neck eventually pinches and the drop drips.

It might seem that only so much can be divined about the nature of a drop. But Jens Eggers, a researcher at the University of Bristol in England, says the discipline has actually expanded in recent years.

"The subject has been far from exhausted after more than 300 years of scientific research," he said in Reviews of Modern Physics.

One reason is that powerful new computers and cameras can capture more detail about drops than ever before.

At Purdue University, Osman Basaran studies drops with a Cordin 220 ultra-high-speed digital camera capable of shooting 100 million images per second. He also uses Purdue's IBM-SP2 supercomputer to simulate drop behavior.

"To see one drop dripping from a kitchen tap, you might solve half a million equations," says Basaran, a professor of fluid mechanics and chemical engineering.

He can afford these pricey tools in part because of increased corporate interest in drop dynamics. His studies, for example, are funded by companies such as 3M, Kodak and Roche.

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