News in Science

Bouncing drops make for smarter rainwear

Repulsive surface Scientists say they have found a way to make raindrops bounce faster off surfaces, opening the way to new water-resistant materials from clothing to aircraft wings.

Reporting in the journal Nature, a team at Boston University and the Massachusetts Institute of Technology (MIT) say they have slashed the amount of time a water drop stays in contact with a surface.

High-speed photography shows that a droplet first spreads out like a tiny pancake on the surface, pulls back due to its surface tension and then bounces off.

When it comes to wetness, what counts is the duration of this contact, a calculation based on oscillations in the drop.

To minimise that time, materials scientists have been focussing their efforts on repellent, or hydrophobic, chemicals that make surfaces less adhesive for water.

But the new study takes a physical approach.

Tiny ridges added to the surface break up the chubby symmetry of a droplet as it hits.

The droplet splits apart, forming smaller, highly irregular shapes that recoil faster compared with a simple fat blob.

"The upshot is that the surface stays drier longer if this contact time is reduced, which has the potential to be useful for a variety of applications," says James Bird, an assistant professor of mechanical engineering at Boston.

The material used, in lab conditions, was a silicon wafer coated with a hydrophobic chemical called fluorosilane, etched with microscopic ridges.

Contact times with it were 40 per cent shorter than for a non-treated surface, and the scientists hope to boost this to as much as 80 per cent.

According to the authors, the texture could be replicated in other materials too - a milling tool could do the same on steel or aluminium, and a drop-splitting surface could be made for fabrics.

One early area of use could be in airplane wings. Exposure to water droplets at high altitudes boosts the risk of dangerous ice build-up.

Butterfly wings and the leaves of some plants, such as nasturtiums, have minuscule veins on their surface that have the same drop-busting effect, the researchers found.

Tests showed that drops bounced off butterfly wings and nasturtiums faster than off lotus leaves, the fuzzy leaves considered the "gold standard" of non-wetting surfaces in nature.