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 c ontact angle in water using an AFM-related force measuring device. In all c ases the attraction between surfaces leads to a jump into contact from sepa rations larger than expected if only van der Waals forces were acting. The jump distance was found to vary dramatically for different pairs of interac ting surfaces. For a given pair of interacting surfaces the jump-in distanc e is smallest for the first approach, then increases and reaches a steady-s tate value after measuring 3-10 force curves. This confirms that hydrophobi c attraction is closely related to the formation of long-life submicrocavit ies in the gap between surfaces. These submicrocavities are formed due to t he first contacts (separations) of surfaces. Their "stability" is probably a consequence of confinement between hydrophobic solids.