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 drop translating in another quiescent liquid contaminated by surfactant soluble in continuous phase but not soluble in dispersed phase is presented in this paper. The finite difference method was used to solve numerically the coupled Navier-Stokes and convective-diffusion equations in a body-fitted orthogonal coordinate system. The present numerical simulation of a two-fluid free-boundary problem was validated by comparison of the simulation results of a drop contaminated by surfactant at creeping flow with the analytical solution and the predicted rise velocity of a bubble contaminated by SDS with experimental data at the moderate Reynolds number. To evaluate the effect of surfactant on the drag coefficient, the steady motion of a drop of the same volume in the pure and contaminated systems was simulated. The behavior of the recirculating wake behind a contaminated drop was also examined and found that it becomes more closely attached to the rear stagnancy of the drop when the surfactant concentration is increased gradually. Finally, the stagnant-cap model was revisited and its defects and limitations were analyzed against numerical simulation. Copyright 2001 Academic Press.