Abstract
When a low-viscosity drop coalesces with a pool surface of the same liquid, it often portrays partial coalescence, where it pinches off a daughter droplet from its top. Such partial coalescence can also occur for a drop spreading on a strongly hydrophilic solid surface. Herein, we investigate the partial coalescence of a low-viscosity drop with a pool surface, when the viscosity of the miscible pool is changed from low to very high, in other words, spanning the conditions from a pool to a solid surface. We find that above a certain pool viscosity, the partial coalescence transitions to second-stage coalescence with a much smaller satellite droplet. This occurs because higher pool viscosity prevents drainage from the drop into the pool, which, in turn, increases the axial curvature in the neck connecting the primary satellite to the drop, thereby preventing the first-stage pinch-off.