The effect of surfactant on the motion of a buoyancy-driven drop at intermediate Reynolds numbers: A numerical approach

A numerical investigation of the flow field inside and around a deformed dr op translating in another quiescent liquid contaminated by surfactant solub le in continuous phase but not soluble in dispersed phase is presented in t his paper. The finite difference method was used to solve numerically the c oupled Navier-Stokes and convective-diffusion equations in a body-fitted or thogonal coordinate system. The present numerical simulation of a two-fluid free-boundary problem was validated by comparison of the simulation result s of a drop contaminated by surfactant at creeping flow with the analytical solution and the predicted rise velocity of a bubble contaminated by SDS w ith experimental data at the moderate Reynolds number. To evaluate the effe ct of surfactant on the drag coefficient, the steady motion of a drop of th e same volume in the pure and contaminated systems was simulated. The behav ior of the recirculating wake behind a contaminated drop was also examined and found that it becomes more closely attached to the rear stagnancy of th e drop when the surfactant concentration is increase gradually. Finally, th e stagnant-cap model was revisited and its defects and limitations were ana lyzed against numerical simulation. (C) 2001 Academic Press.