On the Shape of Surface Nanobubbles

Abstract

Previous AFM experiments on surface nanobubbles have suggested an anomalously large contact angle θ of the bubbles (typically ∼160° measured through the water) and a possible size dependence θ(R). Here we determine θ(R) for nanobubbles on smooth, highly oriented pyrolytic graphite (HOPG) with a variety of different cantilevers. It is found that θ(R) is constant within experimental error, down to bubbles as small as R = 20 nm, and is equal to 119 ± 4°. This result, which is the lowest contact angle for surface nanobubbles found so far, is very reproducible and independent of the cantilever type used, provided that the cantilever is clean and the HOPG surface is smooth. In contrast, we find that, for a particular set of cantilevers, the surface can become relatively rough because of precipitated matter from the cantilever onto the substrate, in which case larger nanoscopic contact angles (∼150°) show up. In addition, we address the issue of the set-point dependence. Once the set-point ratio is below roughly 95%, the obtained nanobubble shape changes and depends on both nanobubble size and cantilever properties (spring constant, material, and shape).