In general, when a liquid drop sits on a smooth planar surface, a three-phase contact line is formed where the gas, the liquid and the solid are in equilibrium. The contact angle (θ0) is the angle between the gas–liquid and the solid–liquid interface; it is a thermodynamic property of the system. However, if the contact line coincides with a sharp edge rather than a plane, the angle at which the meniscus meets the edge is in general not equal to the contact angle (Rusanov and Prokhorov, 1996). In the case of a cubical particle, the angle between the tangent of the gas–liquid surface and the horizontal axis can vary between (θ0 − π/2) and θ0 (Princen, 1969, Oliver et al., 1977, Orr et al., 1977, Kurogi et al., 2008). The angle of contact (α) adjusts itself until the surface tension force balances the hydrostatic and gravitational forces as shown in Fig. 1. The range of possible values of α depends on the shape of the particle and the equilibrium contact angle. However, at each point of the three-phase contact line, α must be less than the advancing contact angle. Otherwise, the liquid will spread over the surface of the solid particle, and a particle at a free surface in a container will sink in the liquid (Hesla and Joseph, 2004).
Fig. 1. Pinning of three-phase contact line for cubical particle, where θ0 is the equilibrium contact angle and α is the angle of contact which can vary from (θ0 − π/2) to θ0.