title: Superhydrophobicity: cavity growth and wetting transition.
abstract: We show by using AFM colloidal probe microscopy (combinations of hydrophobic/superhydrophobic as probe/surface) that superhydrophobicity displays a set of specific events when compared with hydrophobicity. Both attraction (due to capillary and wetting forces) and repulsion (most likely due to repelling air/vapor layers or micro-/nanobubbles) occur upon approach and when surfaces are pulled apart both shorter range (50-100 nm or more) and longer range (several micrometers) attractive forces are displayed. The interaction is explained by forces generated through the formation of air and water vapor cavities, in the shorter-range (>50 nm) case maintaining a constant volume of the cavity, in agreement with calculation of capillary forces, and in the longer-range (>1 μm) case through access of air to the cavity, in agreement with thermodynamics of cavity growth. An added sodium dodecyl sulphate surfactant gave a partially reversible wetting transition and reduced the longer-range interaction to shorter-range, suggesting a transfer from the Cassie-Baxter to the Wenzel wetting regime. The findings would be of interest in development of practical applications, such as for anti-soiling, anti-icing, protection of electrical components and for extreme water-repellency in paper and textiles.
authors.affiliation: SP Technical Research Institute of Sweden - Chemistry, Materials and Surfaces, Box 5607, SE-114 86 Stockholm, Sweden; KTH Royal Institute of Technology, Department of Fibre and Polymer Technology, SE-100 44 Stockholm, Sweden.
authors.forename: Martin
authors.initials: M
authors.lastname: Wåhlander
authors.affiliation: SP Technical Research Institute of Sweden - Chemistry, Materials and Surfaces, Box 5607, SE-114 86 Stockholm, Sweden.
authors.forename: Petra M
authors.initials: PM
authors.lastname: Hansson-Mille
authors.affiliation: SP Technical Research Institute of Sweden - Chemistry, Materials and Surfaces, Box 5607, SE-114 86 Stockholm, Sweden; KTH Royal Institute of Technology, Department of Chemistry, Surface and Corrosion Science, SE-100 44 Stockholm, Sweden. Electronic address: agne.swerin@sp.se.
authors.forename: Agne
authors.initials: A
authors.lastname: Swerin
keywords: Atomic force microscopy; Cavitation; Superhydrophobicity; Surface forces; Surfactant; Wetting