Manipulating Trapped Nanobubbles Moving and Coalescing with Surface Nanobubbles

Abstract

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.

Toward a Comprehensive Understanding of the Anomalously Small Contact Angle of Surface Nanobubbles

“…Surface nanobubbles (SNBs) have become a prominent research topic in the field of surface science since their discovery by atomic force microscopy (AFM) in 2000. , Various methods have been employed to generate SNBs, including solvent exchange (SE), , temperature difference (TD), , and spontaneous formation during immersion. ,, Numerous factors influencing SNB formation have been extensively investigated, such as system temperature, , gas oversaturation of the liquid, , and physical/chemical properties of the substrate. , Moreover, the potential applications of SNBs in drag reduction of microfluidics, − froth-flotation, , and protein adsorption , have been explored. However, there is still a lack of consensus regarding the anomalous long lifetime of SNBs, − although gas oversaturation and contact line pinning were thought to be the main factors contributing to the stability of SNBs. Additionally, the factors influencing the abnormally small contact angle (CA) of SNBs require further investigation. ,− A water droplet on an HOPG surface may exhibit a liquid phase macro contact angle of 65°, while the gas phase CA of surface nanobubbles is expected to be around 115°.…”