Buoyancy-driven interactions of viscous drops with deforming interfaces

Experiments were conducted on the interactions of two different-sized defor mable drops moving due to gravity in an immiscible viscous fluid at low Rey nolds number. As the drops come close to each other, several interactions a re possible: (i) separation of the drops, (ii) capture of the smaller drop behind the larger drop, (iii) breakup of the smaller drop into two or more drops, and (iv) pass-through of one drop through the other, with possible c ycle interaction or leap-frogging. The interactions depend on several syste m parameters, including the drop-to-medium viscosity ratio, the radius rati o of the two drops, the initial horizontal offset of the two drops at large vertical separation, and the gravitational Bond number (which represents t he ratio of buoyant forces to interfacial tension forces for the larger dro p and describes how much the drops will deform). Experimental analysis was conducted by videotaping trajectories of glycerol-water drops of various co mpositions falling in castor oil. The results show good agreement with avai lable theoretical results, both for inter-action maps and individual trajec tories. The results also provide data beyond the present limitations of the oretical algorithms and reveal the new pass-through phenomenon.