The hydrophobic force: the constant volume capillary approximation

Abstract

Long range attractive forces between hydrophobic surfaces, prepared by deposition of Langmuir–Blodgett monolayers and other techniques, follow the predictions of the equation for a capillary contracting force of a bridging meniscus of constant volume. By fitting experimental data to this simple equation, one arrives at reasonable estimates of the amount of the lipid-like material segregated between the surfaces, and of the wetting tension of the condensate. Droplets left on break-up of the bridge coalesce on approach. A small magnitude of the capillary attraction at the large coalescence distance accounts for apparent absence of condensation hysteresis. For surfaces on which water forms contact angles of over 90°, contacting of such protrusions triggers nucleation of a vapor cavity. A hydrophilisation of the surface under water by molecular reorientation and surfactant adsorption substitutes the attraction for a similarly ranged repulsion imposed by Laplace elasticity of the droplets. Limits of applicability of the capillary approximation are discussed in relation to different types of hydrophobic forces.