Hydrophobic pore array surfaces: Wetting and interaction forces in water/ethanol mixtures

[Display omitted] ► Forces between hydrophobic pore array surfaces in water/ethanol mixes were studied. ► Theories like Wenzel and Cassie–Baxter models could not describe the surface wetting. ► Confocal Raman microscopy showed water and ethanol penetration into the pores. ► In contrast to water measurements, no air cavitation was seen in presence of ethanol. ► Capillary condensation of ethanol was concluded to be the main mechanism behind. Interactions between and wetting behavior of structured hydrophobic surfaces using different concentrations of water/ethanol mixtures have been investigated. Silica surfaces consisting of pore arrays with different pore spacings and pore depths were made hydrophobic by silanization. Their static and dynamic contact angles were found to be independent of the pore depth while fewer pores on the surface, i.e. a closer resemblance to a flat surface, gave a lower contact angle. As expected, a higher amount of ethanol facilitated wetting on all the surfaces tested. Confocal Raman microscopy measurements proved both water and ethanol to penetrate into the pores. AFM colloidal probe force measurements clearly showed that formation of air cavitation was hindered between the hydrophobic surfaces in presence of ethanol, and an increase in ethanol concentration was followed by a smaller jump-in distance and a weaker adhesion force. On separation, an immediate jump-out of contact occurred. The measured forces were interpreted as being due to capillary condensation of ethanol between the surfaces giving rise to very unstable cavities immediately rupturing on surface separation.