Very Small Bubble Formation at the Solid−Water Interface

Abstract

The formation of very small gas bubbles (so-called “nanobubbles”) at structured solid−water interfaces has been studied using the tapping mode atomic force microscopy (TMAFM) imaging technique. Silicon oxide wafer surfaces were prepared with different degrees of nanometer scale surface roughness and hydrophobicity. Small bubbles do not form on smooth, hydrophilic, or dehydroxylated silicon oxide wafer surfaces immersed in aqueous solutions under known levels of gas supersaturation. Randomly distributed small bubbles were observed over the whole surface of observation on methylated surfaces of controlled roughness. Bubbles formed on rough, methylated surfaces were larger and less-densely distributed than those on a smooth surface of similar hydrophobicity. The process of bubble coalescence was observed as a function of time. The macroscopic contact angle, measured with respect to the aqueous or gas phase, is very different from the microscopic contact angle detected by TMAFM and appears to be due to the influence of line tension at the pinned three-phase contact line. The latter has a value of −3 × 10 -10 N and acts to stabilize the small bubbles, flattening them and thereby reducing the Laplace pressure.