Interaction forces between hydrophobic surfaces evaluated by AFM - The role of nanoscopic bubbles in the interactions

The interaction forces between a hydrophobic silicon plate and a silica particle in an aqueous solution were investigated with an atomic force microscope (AFM). The surfaces were hydrophobised by a silane coupling agent and the hydrophobicity (contact angle θ) was controlled by varying the immersion time of the surfaces into the silane coupling agent solution. The interactions were long-ranged at θ = 105° and 92°, and the range and magnitude of the interaction were decreased with decreasing θ. In both cases, a discontinuous step appeared in the approaching and separating force curves respectively. On the other hand, the interactions at θ = 80° were unstable and no long-range attraction was observed. When the gas phase on the surfaces was removed by flushing organic solvents between the surfaces, the interactions became short-ranged at θ = 105° and 92°, and die interaction was described DLVO theory at large distances at θ = 80°. A large number of nano-size domain structures were found on the surfaces by tapping mode AFM. These results imply that the bridging of the nanobubble is the main origin of the long-range attractive force between them. The tapping mode AFM images of the hydrophobic surfaces revealed that the diameter of the nanobubble on the surface became small with decreasing θ, indicating that the size of the bubble has critical effect on die range and magnitude of the attractive force. The short-range interactions without bubbles were found to consist of a repulsive force well described by DLVO theory at larger distances and an attractive force at smaller distance. The attractive force was sufficiently longer-ranged than the van der Waals force, suggesting that the force is 'genuine' hydrophobic attraction.