Partial coalescence of a droplet with a pool of different viscosity*

The partial coalescence of a drop with a liquid surface is a critical factor during the coarsening of emulsions, as this process can produce smaller droplets. It is well-known that increased liquid viscosity or reduced drop size will eventually stop the satellite pinch-off at a critical Ohnesorge number. On the other hand, similar satellites can be pinched off during the rapid spreading of a drop on a solid surface, if it is strongly hydrophilic. Herein we study this process when the substrate transitions from low to high viscosity liquid, modeled to approach the boundary conditions of a solid. We use miscible silicone oils, with a low-viscosity drop and a large range of pool viscosities up to a million times that of the drop. We observe non-monotonic behavior and as the viscosity increases we see a propensity for second-stage pinch-off1. We connect the observed critical Ohnesorge numbers to the spreading behavior. 1. Zhang, Li {\&} Thoroddsen, Phys. Rev. Lett. 102,104502 (2009).