Numerical Study Of Surfactant To The Influence Of Drop-drop Interactions

In this paper,we numerically investigate the efects of surfactant on drop-dropinteractions in a2D extension flow using a coupled level-set and immersed interfaceapproach.We find that surfactant plays a critical and nontrivial role in drop-drop inter-actions.In particular,we find that the minimum distance between the drops is a non-monotone function of the surfactant coverage and Capillary number.This non-monotonic behavior,which does not occur for clean drops,is foundto be due to the presence of Maragoni forces along the drop interfaces.We alsofound that the rotation angle of surfactant on the two droplets have importantinfluence.This suggests that there are non-monotonic conditions for coalescence ofsurfactant-laden drops,as observed in recent experiments of Leal and co-workers.Although our study is two-dimensional,we believe that drop-drop interactionsin three-dimensional flows should be qualitatively similar as the Maragoni forces inthe near contact region in3D should have a similar efect.