Spontaneous liquid marble formation on packed porous beds

The encapsulation of aqueous and organic solvents with particles used to form liquid marbles implies there are attractive interactions between the particles and those different liquids. This is often masked, however, by the impact of the droplet kinetic energy on marble formation. We investigated droplet wetting and evaporation when drops were gently placed (without rolling or shaking) on beds of silanised glass beads. Particle coating of the drop surface occurred within seconds of liquid contact with the particle bed. This ruled out evaporation from causing the particles to appear to rise up the surface of the drop as it was reduced in volume. Particles attach to the fresh liquid surface created during the droplet oscillations immediately after contact. The further ordered advance of the particles over the drop surface and the close-packed arrangement of the particles revealed the role of capillary forces in the coating process. By minimising the kinetic energy of the droplet contact with the particles, we found that maximum particle coating occurs at liquid surface tensions just above the critical wetting tension of the beads.