Contact Angle Hysteresis on Rough Surfaces Part I: Mechanical Energy Balance Framework

Abstract: Using as a starting point conservation of momentum, a multiphase mechanical energy balance equation is derived that accounts for multiple material phases and interfaces present within a moving control volume. This balance is applied to a control volume that is anchored to a three phase contact line as it advances over the surface of a rough and chemically homogeneous solid. Using semi-quantitative models for the material behaviour occurring within the control volume, an order-of-magnitude analysis is performed to find what terms within the balance are significant, producing an equation that can be used to predict contact angle hysteresis from a knowledge of interface dynamics occurring around the three phase contact line. In addition to this equation, the theory also answers several questions that have been discussed within the wetting literature: Namely that (static) contact angle hysteresis is a function of conditions around the three phase contact line, as opposed to the surrounding flow system; That contact angle hysteresis results from interface `jumps' that dissipate energy, rather than directly from contact line deformation; That interfacial dynamics is required to predict contact angle hysteresis, but that these dynamics should be interpreted via energy conservation, and; That dynamic contact angles depend on kinetic energy transport around the three phase contact line, as well as local energy dissipation. The framework has been derived using assumptions of incompressible Newtonian fluids, reversible interface formation and zero-strain solids -- future work could relax these assumptions to make the theory more generally applicable.