Lifetime of an oil droplet coalescing with a water bath

The extent of partial coalescence of an oil droplet when it gently deposits on a pool of water is often the rate determining step in its complete merger with the bulk. For oil spills, it translates not only to a longer time before the droplet forms a slick but also markedly different spreading behavior. In this work, using high speed video imaging complemented by theoretical modeling, we show for the first time that this early time spreading behavior of the droplet can be oscillatory or reach a constant value, asymptotically, differing from the well documented, power law, spreading behavior for an oil film. We attribute this to the strength of the capillary waves and role of inertia which is diminished greatly at later times and is dependent on the droplet viscosity. Our results are rationalized using a phase diagram demarcating the different regimes, scaling laws governing the size of the daughter droplet, and the effect of droplet viscosity on the total coalescence time. We envision our results to greatly expand our knowledge and understanding of oil spills, in particular, and oil-water interactions in general.