Wetting of Heterogeneous Nanopatterned Inorganic Surfaces

The wetting of heterogeneous nanopatterned surfaces composed of an ultrathin TiO₂ layer homogeneously perforated by nanocraters through which the surface of the substrate (SiO₂ or Au) is accessible has been investigated by water contact angle measurements. The evaporation-induced (surfactant) self-assembly (EISA) conditions used to prepare such TiO₂ nanoperforated films allowed the crater dimension to be adjusted to 11 and 30 nm. The hydrophilic−hydrophobic contrast of the films was varied through selective functionalization of the TiO₂ portion of the film with Zonyl functions. Detailed structural characterization of the nanoperforated layers using AFM, SEM, ellipsometry, and electrochemistry allowed the results to be quantitatively evaluated on the basis of existing models for wetting of heterogeneous and rough surfaces. Three different types of wetting were observed, depending on the hydrophilic−hydrophobic contrast and the volume and means of deposition of the water droplet. For hydrophilic nanoperforated layers, full wetting of the films as a result of 3D capillary wetting was observed, regardless of the absolute value of the contact angle of the substrate. When 2 µL water droplets were used, hydrophobically functionalized films with hydrophilic craters showed advancing contact angles exceeding 120°, regardless of the hydrophilicity of the substrate. In this case, the contact angles could be described within a few degrees by the Cassie−Baxter formalism, which assumes that a composite solid−vapor surface exists below the droplet. However, when droplets with larger volumes were dropped onto the surface, the contact angles of the composite films decreased and could be fairly well described by a combination of the Cassie and Wenzel equations, which assume full contact of the water droplet with the film. The large contact-angle hysteresis observed suggests that an intermediate state between the Cassie−Baxter and Cassie−Wenzel models possibly exists. Also, the geometry of the nanopatterned surfaces contributes to a large hysteresis, since a long, continuous contact line can form.