Interaction forces between hydrophobic surfaces. Attractive jump as an indication of formation of "stable" submicrocavities

We study the interaction between hydrophobized silica surfaces of defined contact angle in water using an AFM-related force measuring device. In all cases the attraction between surfaces leads to a jump into contact from separations larger than expected if only van der Waals forces were acting. The jump distance was found to vary dramatically for different pairs of interacting surfaces. For a given pair of interacting surfaces the jump-in distance is smallest for the first approach, then increases and reaches a steady-state value after measuring 3-10 force curves. This confirms that hydrophobic attraction is closely related to the formation of long-life submicrocavities in the gap between surfaces. These submicrocavities are formed due to the first contacts (separations) of surfaces. Their "stability" is probably a consequence of confinement between hydrophobic solids.