Equilibrium solutions of a sessile drop partially covered by another liquid

Abstract

We study the equilibrium solutions of a sessile drop on top of a horizontal substrate when it is partially covered by another inmiscible liquid, so that part of the drop is in contact with a third fluid (typically, air). The shapes of the interfaces are obtained by numerical integration of the differential equations resulting from the application of the Neumann's and Young's laws at the three fluids junction (triple point) and the two fluids--solid contact line (at the substrate), as well as no--flow boundary conditions at the wall of the container under axial symmetry. The formulation shows that the solutions are parametrized by four dimensionless variables, which stand for: i) the volume of the external liquid in the container, ii) two ratios of interfacial tensions, and iii) the contact angle of the liquid drop at the substrate. In this parameters space, we find solutions that consist of three interfaces: one which connects the substrate with the triple point, and two that connect this point with the symmetry axis and the wall. We find that the shape of the first interface strongly depend on the height of the external liquid, while the contact angle and the two ratios of surface tensions do not introduce qualitative changes. In particular, the solution can show one or more necks, or even no neck at all, depending on the value of the effective height of the external fluid. An energetic analysis allows to predict the breakups of the necks, which could give place to separated systems formed by a sessile drop and a floating lens, plus eventual spherical droplets in between (depending on the number of breakups).