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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 30: Poster: Micro and Nanofluidics

CPP 30.10: Poster

Mittwoch, 24. März 2010, 17:30–19:00, Poster C

On the shape of surface nanobubbles — Bram Borkent¹, •Sissi de Beer², Frieder Mugele², and Detlef Lohse¹ — ¹University of Twente, PoF, Enschede, the Netherlands — ²University of Twente, PCF, Enschede, the Netherlands

In recent years the puzzling appearance of surface nanobubbles on hydrophobic surfaces has attracted a lot of attention. On the one hand, stable Atomic Force Microscope (AFM) imaging of the bubbles should not be possible. Due to the large Laplace-pressure inside these small bubbles, they should dissolve immediately and therefore not exist at all. On the other hand, previous AFM experiments of surface nanobubbles suggested an anomalously large contact angle of the bubbles (i.e. much larger then the macroscopic contact angle) and a possible size dependence. We present precise measurements of the contact angle for nanobubbles of various sizes on smooth highly orientated pyrolytic graphite (HOPG) with cantilevers of different shape, spring constant and surface properties. We find that for all cantilevers the contact angle is constant, within the experimental error, down to of a size 20 nm, and its value is equal to 119o. This result, which is the lowest contact angle for surface nanobubbles found so far, is very reproducible and moreover 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 (probably due to precipitated matter from the cantilever onto the substrate). In this case much larger contact angles show up, similar to values reported in literature.