Effect of particle hydrophobicity on the properties of liquid water marbles

We study both the static properties and dynamic behavior of liquid water marbles coated with silica nanoparticles of varied hydrophobicity. The static properties are characterized by the variation of marble height and diameter with increasing marble volume, such that the effective surface tension gamma(eff) of the marble can be obtained. The dynamic behavior of liquid marbles includes their impingement on a solid surface and their compression between two parallel glass plates. Marbles coated with particles of intermediate hydrophobicity exhibit maximum gamma(eff) values and enhanced mechanical robustness. Due to particle detachment from or particle rearrangement at the air-water interface caused by the impact, the dynamic surface tensions gamma(d) of liquid water marbles are different in magnitude to those of gamma(eff). In fact, gamma(d) plays an important role in determining the contact time and oscillation period during the impact and rebound processes. Our results show that both the static effective and dynamic surface tension depend on the hydrophobicity of the particles coating the marble surfaces.