An Investigation of Repulsive Capillary Forces
In ambient conditions, capillary condensation allows for the formation of capillary bridges. Since most surfaces are naturally hydrophilic, the capillary bridges result in an attractive capillary force, increasing the normal load. When two hydrophilic surfaces slide against each other, the attractive capillary force can negatively affect their friction and wear behavior. In this work, the mechanics and effects of repulsive capillary forces are investigated. On the theoretical side, a mathematical model for the case of a capillary bridge between two planar solids with equal contact angles has been developed. The model can be used to calculate the capillary forces given the volume of the capillary bridge, the contact angle, and the separation distance between the two surfaces. For a microscopic water droplet, repulsive forces in the order of 1 N, and stiffness values in the order of 108 N⋅m-1 can be expected. On the experimental side, a procedure has been established for the coating of silicon wafers and silicon dioxide colloidal probes with octyltrichlorosilane, rendering the surfaces hydrophobic. The contact angle of water on the coated silicon wafer is measured at (106±1)°, which is in agreement with the literature. Moreover, atomic force microscopy has been employed to measure the adhesion force between modified colloidal probes and silicon wafers. A reduction in the adhesion force of 68 to 90% is observed for a hydrophobic probe and wafer compared to a hydrophilic probe and wafer. In addition, the friction force has been measured between a combination of hydrophilic and hydrophobic wafers using a tribometer. For two hydrophilic wafers, the strong attractive capillary force resulted in a significant friction force. For two hydrophobic wafers, however, a 99% decrease in friction is observed, which can be attributed to the repulsive capillary force caused by the capillary bridges.