Contact-angle-hysteresis effects on a drop sitting on an incline plane.

Abstract

We study the contact-angle hysteresis and morphology changes of a liquid drop sitting on a solid substrate inclined with respect to the horizontal at an angle α. This one is always smaller than the critical angle, α_{crit}, above which the drop would start to slide down. The hysteresis cycle is performed for positive and negative α's (|α|<α_{crit}), and a complete study of the changes in contact angles, free surface, and footprint shape is carried out. The drop shape is analyzed in terms of a solution of the equilibrium pressure equation within the long-wave model (lubrication approximation). We obtain a truncated analytical solution describing the static drop shapes that is successfully compared with experimental data. This solution is of practical interest since it allows for a complete description of all the drop features, such as its footprint shape or contact angle distribution around the drop periphery, starting from a very small set of relatively easy to measure drop parameters.