The water marbles

Powder coated drops of water float on water and roll onglass.

On a roll: powder makes marbles from water

Physicists in France have been playing marbles with water. By giving water droplets a non-stick coating, Pascale Aussillous and David Quere of the College de France in Paris turned them into beads that behave as if they were made from very squishy rubber and roll over glass without wetting it. The coated droplets even float on the surface of water, like a pond insect.¹

These liquid marbles experience so little friction as they roll over glass that very small forces - electrical or magnetic forces for example - can move them. And they don't leave any fluid behind, as water normally does.

They might be useful in the emerging technology of microfluidics, in which tiny quantities of liquid are shunted around on a surface like a silicon chip for chemical or biological analysis. These 'labs on a chip' are being groomed for environmental monitoring, medicine and forensic science.

Aussillous and Quere's secret is a water-repelling powder. Mixed with water, the powder tries to escape, rising automatically to the surface of small droplets, like flour coating a lump of dough.

Powder-coated water droplets just a millimetre or so across form into nearly perfect spheres that sit on top of glass. A normal water droplet forms a lens shape because of water's attraction to glass.

This approach hasn't been tried before probably because people have focused instead on making non-stick surfaces, devising solid materials to which droplets would not adhere. Alternatively, electrical forces or jets of air have been used to levitate small droplets over a surface.

Because the liquid marbles are so soft, they change shape when they move at high speed. The researchers saw them morph from spheres to peanut shapes, disks and doughnut shapes at speeds of around a metre per second.