Systematically altering the hydrophobic nanobubble bridging capillary force from attractive to repulsive.

Atomic force microscopy (AFM) was used to examine how ethanol/water concentration affects the nanobubble bridging capillary force between a hydrophobic silica colloidal probe and a hydrophobic silica wafer. Nanobubbles were produced on the solid surfaces by a previously utilised method which uses solvent-exchange and surface scanning. In pure water a strong, long range attractive force ( approximately 230 nm) with a single jump in step was measured, typical of an interaction between two nanobubbles attached to the hydrophobic surfaces. An increase in the ethanol concentration had little effect on the range of the force but dramatically reduced its magnitude. At an ethanol concentration of 40% by mass, the force became repulsive after the initial attractive jump in. Above an ethanol concentration of 40% by mass, the capillary force disappeared. The change in the force with ethanol concentration was explained using a capillary force model with constant volume and contact angle. The bridge geometry, contact angle, volume and rupture distance were determined for different ethanol concentrations.