Influence of the concentration of dissolved air in unsaturated water on the interaction between hydrophobic surfaces

The effects of the dynamic increase of air concentration dissolved in unsaturated water on the interaction between hydrophobic surfaces were investigated by atomic force microscopy (AFM) using a colloidal probe. The experiments were performed at ambient temperature in unsaturated water. Initially, at an air saturation of 57.7%, the hydrophobic interaction was significantly weaker than that observed independently in air saturated water. Consequently, when the concentration of dissolved gas increased to an air saturation of 99.5%, the observed jump-in distances and adhesion were found to increase with time over the course of many hours. Finally, the hydrophobic interaction was stronger than that in air saturated water. The increased forces as a consequence of increasing gas concentration are attributed to the bridging between nanobubble covered surfaces, because under identical conditions initially no surface nanobubbles were detected by AFM on highly oriented pyrolytic graphite surfaces in unsaturated water, while after several hours nanobubbles could be discerned. It is hypothesized that air diffuses into the water in the AFM liquid cell and that nanobubbles are nucleated after the dynamic increase of the gas concentration. This work contributes to the understanding of nanobubble nucleation, interaction forces between nanobubble covered surfaces in aqueous media under variable dynamic conditions and thus to facilitate future applications exploiting nanobubbles.