Nanometre-scale features with special shapes impart a broad spectrum of unique properties to the surface of insects. These properties are essential for the animal's survival, and include the low light reflectance of moth eyes, the oil repellency of springtail carapaces and the ultra-adhesive nature of palmtree bugs. Antireflective mosquito eyes and cicada wings are also known to exhibit some antifogging and self-cleaning properties. In all cases, the combination of small feature size and optimal shape provides exceptional surface properties. In this work, we investigate the underlying antifogging mechanism in model materials designed to mimic natural systems, and explain the importance of the texture's feature size and shape. While exposure to fog strongly compromises the water-repellency of hydrophobic structures, this failure can be minimized by scaling the texture down to nanosize. This undesired effect even becomes non-measurable if the hydrophobic surface consists of nanocones, which generate antifogging efficiency close to unity and water departure of droplets smaller than 2 μm.
We have measured the contact angle of microsized and nanosized alkane droplets partially wetting a model substrate using true noncontact atomic force microscopy. The large range of droplet sizes accessible using this technique allowed us to determine the contact line curvature dependence of the contact angle with unprecedented accuracy. Whereas previous studies aimed at explaining such a dependence by a line tension effect, our results and calculations on a model system exclude such an effect and point to an extreme sensitivity to weak substrate heterogeneities confirmed by numerical simulations.
Robust, large area, superhydrophobic surfaces with feature sizes approaching 10 nm are fabricated by block-copolymer-based thin-film patterning. We show that tuning the nanostructure shape and aspect ratio dramatically influences the surface wetting properties, with proper control crucial for achieving superhydrophobicity.
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