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Water Exhibits Strange Behavior When Trapped Between Friend and Foe

water molecules

Though it covers more than three-quarters of the earth's surface, water¿particularly the intricacy of its molecular behavior¿continues to surprise scientists. According to a report published today in the journal Science, if stuck in a situation it doesn't like, water will thrash about in frustration.

A hydrophilic (or water-loving) surface exposed to water becomes wet. A hydrophobic surface, in contrast, forces the water to minimize contact by congregating in beads. But Steve Granick and colleagues at the University of Illinois, Urbana, wanted to know what the fluid would do if stuck between two opposing surfaces, one hydrophilic and the other hydrophobic. The conduct of the five to 20-molecule-thick film of water they studied in this manner turned out to be anything but simple. Instead of merely wetting the surfaces or moving away (so-called dewetting), the water undulated wildly, forming what the researchers describe as a "flickering, fluctuating complex."

According to the report, such fluctuations are "peculiar to water and are not observed with nonpolar fluids or with a polar fluid such as ethanol." To determine whether bubbles of dissolved gas present in the water might account for this strange behavior, the team repeated their experiment using degassed water. But they got the same results. In explanation, Granick notes that "while surface energetics encouraged the water to dewet the hydrophobic side of the interface, the hydrophilic side held the water in place, resulting in a fluctuating film of capillary waves."

The findings may help scientists understand similar tug-of-war situations that water experiences in nature. In certain proteins, for example, some parts of the molecule may be hydrophobic and others hydrophilic. "The non-mixing of the two," Granick remarks, "is a major mechanism steering protein folding and other self-assembly processes."

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