Complementary correlation between surface microbubble and droplet shapes

Previous atomic force microscopy studies have suggested that surface micro- and nanobubbles exhibit a flat shape. In this study, we directly observed surface microbubbles formed in an NH3BH3 solution using an optical microscope. No flat microbubbles were observed. Instead, on an SiO2/Si substrate, we discovered a relationship where the sum of the contact angle of a microbubble and the contact angle of a droplet equaled ∼180°. This relationship allowed us to control the shape of surface microbubbles by manipulating the wettability of the surface and the surface tension of the liquid, similar to droplet control. We were able to produce almost perfectly spherical microbubbles. Conversely, on a Cu foil, this relationship did not hold, although we still observed the formation of nearly spherical microbubbles. In this scenario, the shape of microbubbles appeared to be influenced by contact line pinning.