Manipulating Trapped Nanobubbles Moving and Coalescing with Surface Nanobubbles

Trapped nanobubbles are observed nucleating at nanopits on a pitted substrate, while surface nanobubbles are usually formed on the smooth solid surface in water. In this work, trapped nanobubbles and surface nanobubbles were captured by a tapping-mode atomic force microscope (AFM) on a nanopitted substrate based on the temperature difference method. A single trapped nanobubble was manipulated to change into a surface nanobubble, then to change into the trapped nanobubble again. At the same time, surface nanobubbles can be moved to merge into a trapped nanobubble. Our results show that the scan load and the size of the scan area were the main factors that significantly affect the mobility of surface/trapped nanobubbles. The coalescence and mutual transformation of the two kinds of nanobubbles indicate that trapped nanobubbles and surface nanobubbles have the same chemical nature, which also provides vital experimental proof of the existence of nanobubbles in the course of contact line depinning. Our results are of great significance for understanding nanobubble stability and providing guidelines in some industrial applications.