Water will stick even to a duck's back---if you mix in trace amounts of tiny coiled molecules called polymers, researchers report in the 13 June issue of Nature. The new technique should help pesticides adhere to the leaves of plants and reduce pesticide waste and pollution by almost half.
A clean plant is a healthy plant, so the leaves of many species have evolved a water-repellent, or hydrophobic, waxy surface that helps rain water wash off dirt. But from a farmer's point of view, plant hydrophobia is counterproductive because it makes drops of liquid pesticide splash off the leaves. When a drop of liquid lands on a hydrophobic surface, the fluid's momentum spreads it into a flat pancake. Then, the drop recoils from the water-repellent molecules on the leaf's surface and snaps back, launching part of the drop into the air. As a result, up to half of the liquid pesticide sprayed on any given crop simply falls to the ground, where it goes to waste and damages the environment.
Now, materials scientist Vance Bergeron and co-workers at Rhodia Recherches in Saint Fons, France, have shown that adding only 0.1 grams of the springlike polymer polyethyleneoxide to a liter of water can prevent droplets from splashing. Bergeron's team believes that the coiling molecules resist the spreading of the drop, slowing its expansion across the leaf surface. Because the water drop spreads slower, it recoils less violently and very little of the water shoots back into the air. Bergeron notes that his method could also reduce wasted ink in inkjet printers and spray paint.
This versatility has experts impressed. "It is not often that you get such a range of applications from such a subtle effect," says Jacob Klein, a materials scientist at the Weizmann Institute of Science in Rehovot, Israel. Klein notes, however, that the polymer might not hold the droplet together, but instead chain the liquid to the surface. "They have given a fair enough explanation [of the effect], but no direct proof," Klein says. "They should clear that up."