Contact Angles of Surface Nanobubbles: How Far Can MD Simulations Explain?

Despite a quarter-century of scientific endeavor, the properties of nanobubbles on solid surfaces remain elusive. In particular, the mechanisms behind the significantly larger contact angles (i.e., the flat shape) than those predicted by Young's equation are still under debate because the interfacial tensions cannot be precisely measured in experiments. In this study, for the first time, we calculated the solid-liquid, solid-gas, and liquid-gas interfacial tensions of surface nanobubbles using molecular dynamics (MD) simulations. It was found that Young's equation still holds for nanobubbles with diameters of less than 10 nm. We also found that the gas molecules adsorbed onto the solid surface just under the bubbles significantly decrease the solid-gas interfacial tensions, thus increasing the contact angles. However, this is still insufficient to rationalize the experimentally observed values (>150°). Our results clarify that factors not usually considered in MD analysis, such as surface charge, contact line pinning, and whether systems are open or closed, are crucial in explaining the unusual contact angles of surface nanobubbles and related characteristics.