Droplet Adhesion Revisited: Experiments and Theory

Sessile and pendant droplets are commonplace in natural and applied contexts, such as dew or rain drops adorning plants and windowpanes. These drops can be removed by either sliding and overcoming lateral adhesion or via perpendicular detachment by overcoming normal liquid–solid adhesion. While the sliding motion has been studied extensively, the normal detachment motion has received less attention; in fact, the normal component is often related to the liquid–solid work of adhesion that is described by the Young–Dupre equation. In this contribution, we combine experiment and theory to pinpoint the normal force needed to detach a liquid droplet from a flat surface with intrinsic contact angle varying from 40°–100°. We employed the Centrifugal Adhesion Balance (CAB) to measure the detachment force of a liquid droplet. Analysis of droplet shapes at for a wide range of body forces revealed that the body force is balanced by the difference between the forces due to the surface tension and the Laplace pressure, i.e., the force due to the work of adhesion has no bearing on the normal droplet detachment. The experimental findings are in excellent agreement the predictions of the lattice-Boltzmann (LB) simulations and the numerical solution of a modified Young–Laplace equation. Specifically, as the body force reaches a critical value the droplet cannot maintain a stable shape and detaches; with the increasing intrinsic contact angle, the non-dimensional normal critical force decreases monotonically. These findings challenge the conventional wisdom on the normal droplet adhesion's dependence on the work of adhesion.