Interaction forces between chemically modified 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 hydrophobized chemically by a silane coupling agent, and the hydrophobicity (contact angle θ) of the surfaces was varied. The interactions were long-ranged at θ > 90° with a discontinuous step appearing in the approaching and separating force curves respectively. The range and magnitude of the interaction were decreased with decreasing θ. On the other hand, the interactions at θ = 80° was 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 > 90°, and the 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 nanobubbles is the main origin of the long-range force between chemically hydrophobized surfaces and that the size of the bubble has critical effect on the range and magnitude of the attractive force. The short-range interactions without bubbles were found to consist of an electrostatic repulsive force at larger distances and an attractive force, which was sufficiently longer-ranged than the van der Waals force, at smaller distance.