Atomic Force Microscope Images of Nanobubbles on a Hydrophobic Surface and Corresponding Force−Separation Data

Abstract

Domains, apparently nanobubbles, have been observed on a hydrophobic glass surface in water using an atomic force microscope operated in tapping mode. Phase images show the domains to be softer than the underlying substrate. Complementary force curves between a silica colloid probe and the glass surface display features characteristic of the hydrophobic interaction, including a jump-in distance that is comparable to the height of the imaged domains. Images and force curves have been acquired over a range of pH conditions to probe the nature of the overall interaction and gain some insight into the conformation of nanobubbles on the sample surface. The bubbles appear to regrow following their removal by the application of high loads through either contact mode imaging or tapping mode imaging at high drive amplitudes. They are not present in a solvaphilic fluid (ethanol) but regrow following the subsequent reintroduction of H2O. The correlation between image and force data, supported by existing results in the literature, provides strong evidence to favor nanobubbles as the origin of the hydrophobic force.