Interaction and coalescence of bubbles and drops moving through a tube

An experimental study of the interaction and coalescence of drops and bubbles moving through a cylindrical capillary tube under low Reynolds number conditions is presented. Both pressure- and buoyancy-driven motions of fluid particles are examined for a number of Newtonian and non-Newtonian continuous phases. The fluid particles used in the experiments consisted of immiscible Newtonian fluids or air bubbles. The motion of a single fluid particle and the interaction between two fluid particles are analyzed using image analysis. Observations of the shape and velocity of single fluid particles are reported for a wide range of the governing parameters such as the Bond number, the viscosity ratio, and the capillary number. Measurements of the film radius between the two fluid particles are used in conjunction with a simple coalescence model to predict the dependence of the coalescence time on the particle size ratio. It is found that fluid particles in a Newtonian suspending fluid are experienced larger deformations with increasing values of Bond number and capillary number. On the other hand, fluid particles in viscoelastic fluid exhibited several interesting phenomena such as cusped particle shapes, discontinuity in the terminal rise velocity and tail-streaming. The time scale for coalescence in the non-axisymmetric configuration is found to be substantially larger than that for coalescence in the axisymmetric configuration. The simple film drainage model captured the dependence of the coalescence time on particle size ratio for systems that exhibit axisymmetric coalescence.