Liquid marbles

Abstract

The transport of a small amount of liquid on a solid is not a simple process, owing to the nature of the contact between the two phases. Setting a liquid droplet in motion requires non-negligible forces (because the contact-angle hysteresis generates a force opposing the motion), and often results in the deposition of liquid behind the drop. Different methods of levitation-electrostatic, electromagnetic, acoustic, or even simpler aerodynamic 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 hydrophobic powder. The resulting 'liquid marbles' are found to behave like a soft solid, and show dramatically reduced adhesion to a solid surface. As a result, motion can be generated using gravitational, electrical and magnetic fields. Moreover, because the viscous friction associated with motion is very small, we can achieve quick displacements of the droplets without any leaks. All of these features are of potential benefit in microfluidic applications, and also permit the study of a drop in a non-wetting situation-an issue of renewed interest following the recent achievement of super-hydrophobic substrates.

Dynamic wetting and heat transfer at the initiation of aluminum solidification on copper substrates

“…Under these conditions the aluminum droplet exhibited a considerably different behavior. As shown in Figure 12, the droplet impinged on the substrate (1 ms), spread (7 ms), and then bounced away from the substrate (13, 25 and reported with the coated water droplets [49]. The authors of this study reported that the wetting of these droplets on glass was suppressed by coating them with a monolayer of hydrophobic powder.…”

Levitation of a drop over a moving surface

“…Configurations promoting non-contact have been explored, e.g. encapsulating the drop by a hydrophobic powder (Aussillous & Quere 2001) or oscillating the liquid surface to periodically renew the air film (Couder et al 2005).…”

Organization of microbeads in Leidenfrost drops

“…Finally, it has been shown that drops coated by hydrophobic microparticles, i.e. liquid marbles 15,16 , can also levitate 17 . Moreover, the critical temperature above which the Leidenfrost effect takes place does not exist for liquid marbles, i.e.…”