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 theta) of the surfaces was varied. The interactions were long-ranged at theta > 90 degrees with a discontinuous step appearing in the approaching and separating force curves respectively. The range and magnitude of the interaction were decreased with decreasing theta. On the other hand. the interactions at theta = 80 degrees 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 theta > 90 degrees, and the interaction was described DLVO theory at large distances at theta = 80 degrees. 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. (C) 2005 Elsevier Ltd. All rights reserved.