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In contrast to advancements in the fabrication of new superhydrophobic materials, the characterization of their water repellency and quality is often coarse and unsatisfactory. In view of the problems and inaccuracies, particularly in the measurement of very high contact angles, we developed alternative methods for the characterization of superhydrophobic surfaces. It was found that adhering water remnants after immersion are a useful criterion in determining the repellency quality. In this study, we introduce microscopy methods to detect traces of water-resembling test liquids on superhydrophobic surfaces by scanning electron microscopy (SEM) or fluorescence light microscopy (FLM). Diverse plant surfaces and some artificial superhydrophobic samples were examined. Instead of pure water, we used aqueous solutions containing a detectable stain and glycerol in order to prevent immediate evaporation of the microdroplets. For the SEM examinations, aqueous solutions of lead acetate were used, which could be detected in a frozen state at -90 °C with high sensitivity using a backscattered electron detector. For fluorescence microscopy, aqueous solutions of auramine were used. On different species of superhydrophobic plants, varying patterns of remaining microdroplets were found on their leaves. On some species, drop remnants occurred only on surface defects such as damaged epicuticular waxes. On others, microdroplets regularly decorated the locations of increased adhesion, particularly on hierarchically structured surfaces. Furthermore, it is demonstrated that the method is suitable for testing the limits of repellency under harsh conditions, such as drop impact or long-enduring contact. The supplementation of the visualization method by the measurement of the pull-off force between a water drop and the sample allowed us to determine the adhesive properties of superhydrophobic surfaces quantitatively. The results were in good agreement with former studies of the water repellency and contact angles. In contrast to contact angle measurements, the acqusition of SEM images with high resolution and wide depth of sharpness gives better insight into the wetting behavior and susceptibility of the structural elements of the superhydrophobic surfaces.

The contact area between liquids and solid surfaces plays the crucial role in the wetting and self-cleaning properties of surfaces. In this study, we have developed a cryo-preparation method to visualize the contact area between liquids and superhydrophobic biological surfaces by scanning electron microscopy. Aqueous liquids that do not crystallize during freezing, such as glycerol and phosphoric acid, were used. First, the samples in contact with the liquid droplets were cooled with liquid nitrogen. After this, the droplets were separated and the contact areas on the frozen droplets were visualized by scanning electron microscopy. The contact areas of droplets on various biological and artificial surfaces with microstructure, nanostructure, and hierarchical structures are shown in detail. It could be shown that spaces between nanostructures were not penetrated by the droplet, which rested only on top of the structures. Measurements of the contact areas showed the largest reduction in the solid-liquid contact area on hierarchically structured leaf surfaces. On these surfaces, the droplets are in the "Cassie state" at both levels of surface structuring. On plant surfaces, the varying height of the epidermal cells and the surface relief caused considerable variations in the contact between droplet and surface. The examples demonstrate that this new approach provides detailed insights into the wetting behavior of surfaces in the Cassie state with partial contact with the liquid.

The stability of superhydrophobic properties of eight plants and four technical surfaces in respect to water condensation has been compared. Contact and sliding angles were measured after application of water drops of ambient temperature (20 degrees C) onto cooled surfaces. Water evaporating from the drops condensed, due to the temperature difference between the drops and the surface, on the cooled samples, forming "satellite droplets" in the vicinity of the drops. Surface cooling to 15, 10, and 5 degrees C showed a gradual decrease of superhydrophobicity. The decrease was dependent on the specific surface architecture of the sample. The least decrease was found on hierarchically structured surfaces with a combination of a coarse microstructure and submicrometer-sized structures, similar to that of the Lotus leaf. Control experiments with glycerol droplets, which show no evaporation, and thus no condensation, were carried out to verify that the effects with water were caused by condensation from the drop (secondary condensation). Furthermore, the superhydrophobic properties after condensation on cooled surfaces from a humid environment for 10 min were examined. After this period, the surfaces were covered with spherical water droplets, but most samples retained their superhydrophobicity. Again, the best stability of the water-repellent properties was found on hierarchically structured surfaces similar to that of the Lotus leaf.