Liquid marbles

The transport of a small amount of liquid on a solid is not a simple proces s, owing to the nature of the contact between the two phases. Setting a liq uid droplet in motion requires non-negligible forces (because the contact-a ngle hysteresis generates a force opposing the motion(1)), and often result s in the deposition of liquid behind the drop. Different methods of levitat ion-electrostatic, electromagnetic, acoustic(2), or even simpler aerodynami c(2,3) techniques-have been proposed to avoid this wetting problem, but all have proved to be rather cumbersome. Here we propose a simple alternative, which consists of encapsulating an aqueous liquid droplet with a hydrophob ic powder. The resulting 'liquid marbles' are found to behave like a soft s olid, and show dramatically reduced adhesion to a solid surface. As a resul t, motion can be generated using gravitational, electrical and magnetic fie lds. Moreover, because the viscous friction associated with motion is very small(4), we can achieve quick displacements of the droplets without any le aks. All of these features are of potential benefit in microfluidic applica tions, and also permit the study of a drop in a non-wetting situation-an is sue of renewed interest following the recent achievement of super-hydrophob ic substrates(5).