Surface and Capillary Forces Affecting Air Bubble−Particle Interactions in Aqueous Electrolyte

The interaction between hydrophilic silica particles and air bubbles in aqueous electrolyte has been studied by colloid probe atomic force microscopy. The interaction was found to be monotonically repulsive on approach. The silica surface was also hydrophobized by dehydroxylation and by treatment with octadecyltrichlorosilane (OTS). In these cases a repulsion was observed at long range with an attraction evident as the bubble−particle separation decreased. For a freshly prepared OTS−silica surface the intervening thin film rapidly collapsed, resulting in particle engulfment or establishment of a three-phase line. For an aged OTS−silica surface a stable film was formed which could be ruptured as the loading force was increased. In all cases adhesion resisted bubble−particle separation. This behavior was rationalized in terms of either attractive surface forces or capillary forces and contact angle hysteresis operating after formation of a three-phase line (TPL).