Improved nanobubble immobility induced by surface structures on hydrophobic surfaces.

In fluid flow on hydrophobic surfaces, boundary slip occurs at the solid-liquid interface and nanobubbles on the surfaces are believed to be the reason for it. Boundary slip is of practical importance in micro/nanofluidics to reduce the drag force in fluid flow. However, nanobubbles tend to move under external disturbance. Therefore, the decreased degree of nanobubble movement (nanobubble immobility) is of interest. In this study, nanobubble immobility is studied on both continuously and partially coated polystyrene films. Experimental results show improved immobility on both surfaces. The nanoindents generated by nanobubbles after immersion in a liquid for a period of time on both films and island-like structures on the partially coated film are thought to be the reasons for improved immobility. A model is developed to reveal the role of nanoindents and island structures in the improvement of nanobubble immobility based on contact angle hysteresis and surface tension. Analysis shows that both structures increase the initial force needed to move nanobubbles. Hence, nanobubble immobility is improved on both surfaces as compared with smooth hydrophobic surfaces.