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Computer model shows tiny drops still drip.

Raindrops falling on a waxed car hood bead up and roll away. But should those drops hit a cotton rag used to buff that metal, they flatten and stock, dampening the fabric.

Now two chemists have shown that what people observe in their everyday encounters with water also happens on a molecular scale. Using sophisticated computer software that they developed, Joseph Hautman and Michael L. Klein of the University of Pennsylvania in Philadelphia studied the behavior of water on charged and neutral surfaces. Even when present in small numbers, water molecules on a neutral surface form microscopic droplets that look and act like their much larger counterparts, they report in the Sept. 23 PHYSICAL REVIEW LETTERS.

"That's the surprise, the scaling, that it maps so nicely," says Klein. "You need relatively few molecules to produce macroscopic behavior." Raindrops, with more than a quintillion molecules, dwarf the researchers' minuscule computer droplets, which contain fewer than 100 molecules.

For their surfaces, Hautman and Klein simulated long hydrocarbon molecules called alkylthiols. In real life, when scientists dip a thin gold-covered plate into a solution of these molecules, the alkylthiols line up on the plate and "self-assemble" into a single layer that coats the plate. This "monolayer" serves as a surrogate for a simple membrane, says Klein.

For their study, they had the computer model make two kinds of monolayers. They created one monolayer where the molecules align with a charged end sticking out and also a slightly different one with neutral tips facing outward. The scientists then introduced 90 super-cooled water molecules on top of each layer. For the simulation, they heated the wetted monolayer. Then, using graphics software developed by graduate student John Shelley, they watched the reorganization of these computer-generated water molecules on a computer screen.

On the neutral surface, "the [water] droplet arises spontaneously," says Klein. Also, the droplet does more than look like visible drops (SN: 3/16/91, p.165). Scientists often assess the hydrophilic (water-loving) nature of a material by measuring the angle between the surface and the droplet. The more water beads up, the larger this contact angle, and the less wettable material. Hautman and Klein found that this angle in their simulations also correlated to wettability. Thus, one can use this computer model to predict a new material's hydrophilic properties, says Klein.

This study represented one of the first steps toward developing computer models of membranes and, ultimately of cells, he adds.
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Title Annotation:behavior of water on the molecular scale
Author:Pennisi, Elizabeth
Publication:Science News
Date:Oct 5, 1991
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