SURFACE TENSION AND CAPILLARY INTERACTION

Surface tension is a property of the surface of a liquid. Since the molecules at the surface don't have other like molecules on 'all sides' of them, the cohesive forces that hold them together to the molecules associated with the surface are stronger. This forms a surface 'film' which makes it more difficult to move an object through the surface than to move it when it is completely submersed, the deformation of the liquid surface (which is supposed to be flat) is the origin of lateral capillary forces. This forces cause the attraction of two similar particles floating on a liquid's surface (like cheerios floating in a bowl of milk). Two types of capillary forces can be identified: lateral flotation forces and lateral immersion forces. The former refers to particles that are freely floating over the surface of liquid (like the paper above) where the attraction of the particles appears because of the deformation of the liquid surface originated in the particle's weight. The latter refers to the attrction between particles that are partially immersed in the liquid (the cheerio) where the deformation of the liquid surface is related to the wetting properties of the particle surface, i.e. the position of the contact line and the contact angle.