Competition between Electrostatic and Capillary Forces Acting on a Single Particle

Using an AFM, interactions between a conducting particle and SiO 2 /Si substrates with different hydrophilicities have been investigated with voltage applied in air. Attractive forces observed on hydrophobic substrates showed simple V 2 dependence at any distance, and was purely of electrostatic interaction. Forces measured for hydrophilic substrates, on the other hand, exhibited the complicated dependence on V and distance. V dependence of the force for hydrophilic substrates showed that the interaction changed continuously from electrostatic at long distances to capillary condensation at short distances including the contact region. This interaction change could be explained by electrification and deformation of the water layer adsorbing onto hydrophilic surfaces, the sudden formation of the water bridge, and the enlargement of the cross section area of the water bridge.