Wettability of Microstructured Hydrophobic Sol-Gel Coatings

Abstract

The formation of appropriate surface patterns on hydrophobic surfaces leads to a general change in their wettability and the contact angle increases substantially. Such coatings are of great technical interest, especially if aqueous media are concerned as in the prevention of ice-adhesion. For this reason various fluorine containing nanocomposite coatings have been developed by sol-gel processing.

The morphology of these hydrophobic surfaces has been controlled by varying the content of silica particles regarding size, degree of aggregation, and concentration. The wettability is characterized by the measurement of dynamic contact angles against water. The complete range of different wettability regimes is accessible, i.e. smooth surfaces (both low advancing contact angle and hysteresis between advancing and receding contact angle), surfaces within the Wenzel regime (high advancing contact angle and hysteresis), and superhydrophobic surfaces (high advancing contact angle and low hysteresis). The wettability is correlated with the surface roughness as determined using a profilometer or AFM.

The wettability of superhydrophobic surfaces is greatly dependent on the surface tension of the liquid. By comparison of the tiltangle θ_t of a smooth and a superhydrophobic surface, a critical surface tension γ_c is identified, where θ_t (smooth surface) = θ_t (microstructured surface). The microstructured surface provides a better run-off of liquids γ_lg > γ_c ≈ 55 mN·m⁻¹.