Previous AFM experiments on surface nanobubbles have suggested an anomalously large contact angle theta of the bubbles (typically approximately 160 degrees measured through the water) and a possible size dependence theta(R). Here we determine theta(R) for nanobubbles on smooth, highly oriented pyrolytic graphite (HOPG) with a variety of different cantilevers. It is found that theta(R) is constant within experimental error, down to bubbles as small as R = 20 nm, and is equal to 119 +/- 4 degrees . 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 ( approximately 150 degrees ) 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).

In some cases water droplets can completely wet microstructured superhydrophobic surfaces. The dynamics of this rapid process is analyzed by ultrahigh-speed imaging. Depending on the scales of the microstructure, the wetting fronts propagate smoothly and circularly or-more interestingly -in a stepwise manner, leading to a growing square-shaped wetted area: entering a new row perpendicular to the direction of front propagation takes milliseconds, whereas once this has happened, the row itself fills in microseconds (''zipping''). Numerical simulations confirm this view and are in quantitative agreement with the experiments.

Shock wave induced cavitation experiments and atomic force microscopy measurements of flat polyamide and hydrophobized silicon surfaces immersed in water are performed. It is shown that surface nanobubbles, present on these surfaces, do not act as nucleation sites for cavitation bubbles, in contrast to the expectation. This implies that surface nanobubbles are not just stable under ambient conditions but also under enormous reduction of the liquid pressure down to −6MPa. We denote this feature as superstability.

The acoustic nucleation threshold for bubbles trapped in cavities has theoretically been predicted within the crevice theory by Atchley and Prosperetti ͓"The crevice model of bubble nucleation," J. Acoust. Soc. Am. 86, 1065 ͑1989͔͒. Here, we determine this threshold experimentally, by applying a single pressure pulse to bubbles trapped in cylindrical nanoscopic pits ͑"artificial crevices"͒ with radii down to 50 nm. By decreasing the minimum pressure stepwise, we observe the threshold for which the bubbles start to nucleate. The experimental results are quantitatively in good agreement with the theoretical predictions of Atchley and Prosperetti. In addition, we provide the mechanism which explains the deactivation of cavitation nuclei: gas diffusion together with an aspherical bubble collapse. Finally, we present superhydrophobic nuclei which cannot be deactivated, unless with a high-speed liquid jet directed into the pit.

We experimentally study the dynamics of water in the Cassie-Baxter state to Wenzel state transition on surfaces decorated with assemblies of micrometer-size square pillars arranged on a square lattice. The transition on the micro-patterned superhydrophobic polymer surfaces is followed with a high-speed camera. Detailed analysis of the movement of the liquid during this transition reveals the wetting front velocity dependence on the geometry and material properties. We show that a decrease in gap size as well as an increase in pillar height and intrinsic material hydrophobicity result in a lower front velocity. Scaling arguments based on balancing surface forces and viscous dissipation allow us to derive a relation with which we can rescale all experimentally measured front velocities, obtained for various pattern geometries and materials, on one single curve.