Water drops not only bounce off duck's feathers, lotus leaves, and other water-repellent surfaces, they do it in a surprising way, says a team of researchers in the 20 June issue of Nature. The work could pave the way to new materials that never get wet, such as stay-dry glass.
When a water drop hits a surface, part of it usually splashes away. The rest stays behind and creeps out laterally. But a droplet that lands on a water-hating, or hydrophobic, surface such as wax resists spreading. A roughed-up hydrophobic surface becomes super water-repellant. Because a droplet can't penetrate the miniature topography, it is almost spherical.
Now David Quéré of the Collège de France in Paris and two colleagues have seen an even more extreme case of hydrophobia. Upon hitting a special water-repellant surface, the droplet deforms in spectacular style and is entirely hurled off. Using high-speed photography of about 10,000 frames per second, Quéré 's team witnessed droplets deforming into shapes reminiscent of ice hockey pucks and baseball bats. The researchers' fast frames also allowed them to measure how long a bouncing drop touched the surface. To their surprise, they found that the droplet's approach speed didn't matter. However, weight did have an impact. To Quéré, that means a bouncing drop behaves more like a spring than a liquid blob.
The work "gives a beautiful analog" to a bouncing ball, says Lakshminarayanan Mahadevan of the University of Cambridge, U.K. "Here you have liquid balls which do the same thing." The trick is the peculiar nature of the surface, vital because it forces the droplet to retract on itself, he adds, noting that the finding "might have some relevance in situations where the time of contact is an important factor--for example, in cooling towers." Quéré believes there are real prospects for creating hydrophobic-coated roughed surfaces that could remain dry. He has already discussed car windshields with a leading French glass producer.