The regulation of surface nanobubble generation via solvent exchange on different substrates

Surface nanobubbles (SNBs) play an essential role in fluid transport, interfacial reactions, electrochemistry, material fabrication, surface cleaning, etc. Controllable preparation of SNBs is very crucial for their applications, such as controlling material preparation and increasing the efficiency of surface cleaning. Ethanol-water exchange, as a typical example of solvent exchange, is one of the most widely used methods to produce SNBs. By controlling the conditions, it is possible to manipulate the size and number density of nanobubbles. In this work, we systematically investigated the formation of SNBs on hydrophobic and hydrophilic surfaces by ethanol-water exchange at different rates using atomic force microscopy. Results indicate that the exchange rate has a significant effect on the size and number density of SNBs. On both hydrophobic surface of highly oriented pyrolytic graphite (HOPG) and hydrophilic surface of mica, formed nanobubbles exhibit smaller sizes and higher number density with increased exchange rates. But at the same solvent exchange rate, SNBs generated on HOPG surface were in greater number and width than on mica, while their heights change much less. To understand the effect of exchange rate on nanobubble production, we proposed a mechanism of non-uniform nucleation and diffusion-driven growth. A simulation of the gas concentration near SNBs was performed, and the results show good agreement between numerical calculations and experimental data. We believe that our results will contribute to the understanding of nanobubble generation and stabilization processes. It also provides a new perspective on the factors that affect the generation of SNBs, which allows the production and application of nanobubbles to be controlled in a reliable manner.