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We experimentally show that, unlike the solid-solid frictional force, the solid-liquid retention force is determined by interactions at the triple line rather than over the solid-liquid contact area, as predicted by theory. We have prepared drops whose triple line enclosed a uniform surface, a hydrophobic island, and topographical inhomogeneities, and measured the same retention force for all three cases. We have also studied the retention force on drops whose initial triple line was non-circumferential and measured different retention forces for different shapes, also as predicted by theory. The experiments with non-circumferential drops provide (1) another way to show that the retention force is not proportional to the contact area, and (2) a manner to falsify a recent theory on drop depinning.

We calculate the normal capillary retention force that anchors a drop to a solid surface in the direction perpendicular to the surface, and study the relationship between such force and the Young-Dupre work of adhesion. We also calculate the work necessary to create or destroy a patch of solid-liquid interface by moving the triple line on a solid substrate. We argue that when the capillary number is small and a drop is sliding on a surface at constant speed, the lateral retention force is the major source of energy dissipation, whereas viscous dissipation plays a minor role.

We address the issues raised in [R. Tadmor et al., Langmuir 2020, 36, 475-476]. In particular, we explain why we did not use Tadmor's theory to explain our results.

We compare the lateral retention forces on sessile drops (which are drops that are placed on top of a solid surface), pendant drops (which are drops that are placed on the underside of the surface), and inverted sessile drops (which are drops that are first placed on top and then on the underside of the surface by flipping the surface). We have found experimentally that the retention force on a truly pendant drop is always smaller than that on a sessile drop. However, the retention force on an inverted sessile drop is comparable to, and usually larger than, that on a sessile drop. Thus, the retention force on a drop depends not only on whether it is placed on top or on bottom of a surface, but also on the history of drop deposition, since such history affects the width, the shape and the contact angles of the drop.