Surfactant effects on buoyancy-driven viscous interactions of deformable drops

Buoyancy-driven interactions of two deformable drops with a bulk-insoluble, nonionic surfactant are studied with a curvatureless boundary-integral algorithm. At moderate drop-to-medium viscosity ratios with a linear equation of state, the surfactant causes breakup of the larger drop to become dominant over breakup of the smaller drop at O (0.1) values of the dimensionless gas constant β for a surface Péclet number P e_s of 100. For P e_s ≤ 10, the surfactant has little effect on breakup for β ≤ 0.2. For P e_s = 1000, critical horizontal offsets for breakup of the larger drop exceed those for breakup of the smaller drop at β = O (0.01). The influence of surfactant on the deformation and breakup of the smaller drop increases when it becomes closer in size to that of the larger drop. Surfactant effects are less significant when the drop-to-medium viscosity ratio is greater than unity. For the case of bubbles, at P e_s = 100, the surfactant is swept to the trailing edges early in the trajectories and leads to cusp formation at β = O (0.01).