Interaction and coalescence of drops and bubbles rising through a tube

An experimental study of the interaction and coalescence of two drops (of the same fluid) or bubbles translating under the action of buoyancy in a cylindrical tube is performed. The close approach of two Newtonian drops or bubbles of different size in a Newtonian continuous phase is examined using image analysis, and measurements of the coalescence time are reported for various drop size ratios, Bond numbers, and drop to suspending fluid viscosity ratios. The time scale for coalescence in the non-axisymmetric configuration is found to be substantially larger than that for coalescence in the axisymmetric configuration. Experimental measurements of the radius of the liquid film between the two drops are used in conjunction with a simple film-drainage model to predict the dependence of the coalescence time on the drop size ratio. The agreement between the model predictions and the experimental measurements is satisfactory for axisymmetric coalescence in the low viscosity ratio systems. For the systems with O(1) viscosity ratio, on the other hand, model predictions are qualitatively different from experimental observations.