Experimental and numerical investigation of binary coalescence: Liquid bridge building and internal flow fields

In this work, we present the results of experimental and numerical investigation of the liquid bridge building and internal flows during a binary droplet-droplet coalescence event. A system consisting of two rising toluene droplets with different droplet radii that coalesce in surrounding water is considered. The growth of the liquid bridge between the two droplets during the coalescence event is found to be in the inertial regime, whereas the bridge radius grows in proportion to the square root of time. Further, the internal mixing of the two droplets is investigated. It is observed that when two droplets with the same radius coalesce, there is nearly no internal mixing within the first 40 ms after the coalescence start. In contrast, when two droplets with different radii coalesce, a liquid jet directed from the smaller to the larger droplet appears during the coalescence event. A good agreement between the experimental and numerical data is established.