Normal capillary forces.

A liquid meniscus between two lyophilic solid surfaces causes an attractive force, the capillary force. The meniscus can form by capillary condensation or by accumulation of adsorbed liquid. Under ambient conditions and between hydrophilic surfaces, capillary forces usually dominate over other surface forces. They are relevant in many processes occurring in nature and technical applications, for example the flow of granular materials and friction between surfaces. Here we review normal capillary forces, focusing on a quantitative description with continuum theory. After introducing the capillary force between spherical surfaces, we extend the discussion to other regular and irregular surfaces. The influence of surface roughness is considered. In addition to capillary forces at equilibrium, we also describe the process of meniscus formation. Assumptions, limits, and perspectives for future work are discussed.