Sliding behaviour of water-ethanol mixture droplets on inclined low-surface-energy solid

Abstract

The timing of the onset of droplet sliding behaviour on an inclined solid surface is mainly characterised by the onset of the rear contact line movement. The critical inclined angle is predicted from contact angle hysteresis, which is the difference between the advancing and receding contact angles of the droplets. However, at present, it is difficult to explain the contact angle hysteresis using the properties of liquids and solids. In this study, the behaviours of water–ethanol mixture droplets sliding down a low-surface-energy solid, inclined with constant angular velocities, are experimentally investigated. The front and bottom views of the droplets are observed to understand the detailed variations of the droplet length, width, and contact area during the inclination of the solid. From the experimental results, it was found that the adhesion forces related to the onset of the advancing and receding contact lines exhibit proportional relation with respect to the surface energy density of the liquid.