Coalescence and breakup of viscoelastic drops

The breakup and coalescence of drops are elementary topological transitions in interfacial flows. The breakup of a drop changes dramatically when polymers are added to the fluid. With the strong elongation of the polymers during the process, long threads connecting the two droplets appear prior to their eventual pinch-off. In this talk we will start by presenting some new results on pinchoff using an Oldroyd-B fluid. Subsequently, we experimentally demonstrate how elasticity affects drop coalescence, the complement of the much studied drop pinch-off. We reveal the emergence of an elastic singularity, characterised by a diverging interface curvature at the point of coalescence. Our central finding is that the polymers dictate the spatial features of coalescence, but they hardly affect the temporal evolution of the bridge. These results are explained using a novel viscoelastic similarity analysis.