The dynamics of water droplets has fascinated physicists for well over a century. Yet several aspects of droplet motion have remained untested because moving drops tend to leave traces of liquid on surfaces. Now a team of French physicists has developed a way of moving a liquid drop across a surface without wetting it. Pascale Aussillous and Davide Quéré at the Collège de France in Paris believe that their non-stick drops might also benefit the growing field of 'microfluidics' (Nature 2001 411 924).
The Paris team has called the non-wetting drops ‘liquid marbles’ because they can bounce and roll around surfaces without leaking. They created the marbles by adding a highly water-repellent powder made of moss spores that had been coated with silane to the water droplets. The powder spontaneously migrated to the surface of the drop, thereby preventing the water from interacting with any surface. Indeed, the resulting liquid marbles even floated on a pool of water.
By studying the motion of the non-wetting droplets on an inclined plane, Aussillous and Quéré demonstrated that liquid marbles behave differently to conventional fluid drops. Small water droplets, for example, can stick to a window while more massive ones slide down the glass. Liquid marbles, however, roll down the plane with a velocity that is determined by a competition between gravity and friction, which slows down drops that have a large surface area in contact with the plane. The Paris team discovered that large non-wetting droplets move more slowly than smaller ones, as predicted two years ago by L Mahadevan, now at Cambridge University in the UK, and Yves Pomeau at the Ecole Normal Supérieure in Paris.
Aussillous and Quéré found even stranger results when they tilted the plane further and photographed the drops with a high-speed camera. The forces acting on the rolling spherical droplets deformed them into a doughnut shape, as first predicted by Lord Rayleigh nearly 90 years ago. And when the marbles rolled off the edge of the plane, they transformed into a peanut shape. The French team is currently testing the robustness of the liquid marbles and is studying their motion in electric and magnetic fields.
