Effect of heterogeneity size on three-phase contact line formation kinetics

Abstract

Flotation relies on the differences in surface wettability. Although solids with the same hydrophobicity are generally regarded as having the same capture probability, the heterogeneities found on particle surfaces could complicate bubble–solid interactions. For surfaces with the same hydrophobicity and different sizes of inclusions, the heterogeneity size is expected to affect bubble–solid interactions. In this work, Microelectromechanical system techniques were used to fabricate chemically heterogeneous samples with the same hydrophobicity on which discrete hydrophilic dots with different diameters were distributed. The collision and adhesion processes between free-rising bubbles and these chemically heterogeneous surfaces were studied using a high-speed video camera. For all surfaces, the time intervals between successive collisions and the bubble trajectories were similar before bubble rupture. In contrast, the time elapsed between maximum air bubble deformation and rupture varied, with the rupture time appearing random. Nevertheless, the terminal diameter and average expansion velocity after bubble rupture were negatively correlated with the hydrophilic dot diameter. Thus, for heterogeneous surface with discrete hydrophilic dots, the bouncing behaviour of air bubbles before rupture is independent of heterogeneity size, whereas the three-phase contact line expansion behaviour after rupture is related to heterogeneity size. These insights can provide guiding principles for improving the flotation.