Is surface tension reduced by nanobubbles (ultrafine bubbles) generated by cavitation?
Nanobubbles (ultrafine bubbles) are produced by hydrodynamic or acoustic cavitation. They work as cavitation nuclei. Is the experimentally reported considerable reduction of surface tension of liquid water by nanobubbles real? It is theoretically suggested that nanobubbles partly covered with hydrophobic materials are concentrated at a surface of liquid water. A hydrophobic cap is directed toward a gas phase above a liquid surface. Uncovered surface of a nanobubble is directed into liquid water underneath the liquid surface. It is suggested that a liquid film is more easily ruptured by the presence of nanobubbles at the liquid surface, which reduces the value of surface tension in du Noüy ring method. A role of ionic surfactants on accumulation of nanobubbles at a liquid surface is also discussed.