Jacco Snoeijer

Coalescence of floating drops

A basic feature of liquid drops is that they can merge upon contact to form a larger drop. Here, we experimentally and theoretically discuss the dynamics of drop coalescence on a thick layer of a low-viscosity liquid — like oil drops floating on top of a soup. It is shown that these so-called ``liquid lenses" merge by the self-similar vertical growth of a bridge connecting the two lenses. Using a slender analysis, we derive similarity solutions corresponding to both the viscous and inertial limits. Excellent agreement is found with the experiments without any adjustable parameters, capturing the spatiotemporal structure of the flow during coalescence. Special attention is given to the viscous limit, for which the coalescence velocity exhibits a slow convergence that involves logarithmic corrections.