Shape and Stability of Liquids That Partially Wet Solids

The shape at rest of a liquid that partially wets a solid depends on the contact-angle at the three-phase common line.  Moreover, in the absence of other forces, the relative affinities of the triplet of materials (liquid, solid and gas) predicts the entire shape of the liquid at equilibrium and its stability depends on the mobility of the contact-line, as is well-known.  What seems not as well-appreciated is that, for mobile contact-lines, fully three-dimensional shapes can be more stable than axisymmetric shapes.  For example, a mobile axisymmetric sessile drop on a planar substrate can be unstable to a non-axisymmetric shape.   These observations have a number of fundamental implications for the inviscid spreading behavior of drops and rivulets on solids.   We shall illustrate these in this talk.