Silica nano-particle super-hydrophobic surfaces: The effects of surface morphology and trapped air pockets on hydrodynamic drainage forces

Abstract

We used atomic force microscopy to study dynamic forces between a rigid silica sphere (radius ∼45 μm) and a silica nano-particle super-hydrophobic surface (SNP-SHS) in aqueous electrolyte, in the presence and absence of surfactant. Characterization of the SNP-SHS surface in air showed a surface roughness of up to two microns. When in contact with an aqueous phase, the SNP-SHS traps large, soft and stable air pockets in the surface interstices. The inherent roughness of the SNP-SHS together with the trapped air pockets are responsible for the superior hydrophobic properties of SNP-SHS such as high equilibrium contact angle (>140°) of water sessile drops on these surfaces and low hydrodynamic ..