  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.
