Irregular, nanostructured superhydrophobic surfaces: Local wetting and slippage monitored by fluorescence correlation spectroscopy

Superhydrophobic surfaces used in various applications to enhance flow and reduce drag typically consist of irregular nanostructures. However, many theoretical models and most laboratory microscale experiments dealing with these phenomena are limited to structures consisting of regular microarrays and cannot explain the macroscopic flow enhancement observed in applications. Here, we investigated microscopically the wetting and flow over fluorinated silicon nanofilaments as an example for an application-relevant, irregularly nanostructured, superhydrophobic surface. Using fluorescence correlation spectroscopy with an improved evaluation method, we found that velocity profiles are still nonlinear at distances below 1 μm to the surface. Furthermore, we observed that the air layer in between and on the nanofilaments is not continuous on a micrometer length scale. First, there are regions with homogeneous wetting, where the air-water interface regularly touches all uppermost fibers. These regions possess a low slip length (<5μm). Both the wetting and the slip length match with expectations from microarray or homogeneous porous surfaces. Second, there are large patches with air inclusions, which present two orders of magnitude higher slip lengths. Our results contribute to the understanding of the drag reduction observed in applications and can help in designing new, optimized surfaces.