The stability and morphology of nanobubbles on homogeneous surfaces with different wettability

Abstract

Surface nanobubbles were observed to survive with apparent long-term stability and small contact angles in contrast to larger microbubbles. Although the contact line pinning and hydrophobic attraction model can well explain the stability of pinned surface nanobubbles, their experimental morphology features on different surfaces are yet to be presented. With a linearized attraction potential related to surface wettability, in this work, we developed a model that distinguished the stability of nanobubbles and the instability of larger microbubbles reaching diffusive and mechanical equilibrium on homogeneous surfaces. The dynamic evolutions, stability, and morphology of nanobubbles on surfaces with different wettability were investigated. Its results demonstrated that bubbles with different initial sizes and gas oversaturation would present four dynamic states: shrinkage to dissolution, shrinkage or growth to the stable equilibrium, and expansion to burst. The bubbles at stable equilibrium have typical footprint radius ranges, limited heights, and small contact angles, as measured in experiments. Furthermore, with the increase in hydrophobicity, the stable contact radius of nanobubbles becomes larger while the required degree of gas saturation is alleviated. At the same contact radius, the height and contact angle of the bubble on hydrophilic surface are larger than that on hydrophobic surface, both increasing with the growing contact radius. These theoretical predictions are consistent with experimental observations and would be meaningful to understand the stability of surface nanobubbles.