Slippery When Dry

Flip flop drops. Silhouette photos of water drops on a new material show that the material becomes water repellant when wet.

Flying in the face of intuition, scientists have created a coating material that is water-loving when dry but repels water once wet. The new technology may prove useful in medical diagnostic or other devices where tiny streams of fluid could be turned on or off by switching the surface behavior of a material.

If you've watched water bead up on a freshly waxed car, you've seen how hydrophobic surfaces act. When water drops onto a hydrophilic surface like a windshield, on the other hand, it spreads out and runs in rivulets. Some researchers have made polymer coatings that become more wettable in the presence of water and more water-repellant in the presence of organic solvents. But no one expected the opposite might be possible.

A new polymer may do the trick. Polymer scientists Kenneth Wynne and Umit Makal of Virginia Commonwealth University in Richmond were working to create antimicrobial coatings by incorporating a well-known antimicrobial molecule called hydantoin into fluorine-containing polymer chains. When Makal tested the behavior of water on one of these coatings, a standard test of new materials, he saw that water drops spread, wetting the surface when first touched to the coating. But when Makal withdrew the drop and set it down in the same spot again, the drop remained much more beadlike. The surface had become water-repellent where the original water drop had been.

"It's unprecedented, this kind of behavior," Wynne says. "Poor Mr. Makal had to do the experiment several times because I thought the instrument was broken.” Wynne and Makal propose 26 April in Langmuir that when water touches the material, the polymer side chain rearranges, exposing the hydrophobic fluorine-containing groups to the surface and causing it to repel water. The effect is completely reversible, Wynne says, by drying the surface.

"It's the kind of work that you love to see in science," says surface and materials chemist Greg Ferguson of Lehigh University in Bethlehem, Pennsylvania. Ferguson can imagine several applications, including water purification, for this sort of material. But, he adds, "the field is young. So, sorting out the basic understanding of what's driving this is a very fascinating work in itself."

Related sites
Langmuir paper abstract
Wynne's website
Ferguson's website

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