Surfactant-covered drops between parallel plates

We use a numerical model to study the deformation of surfactant-covered drops in a confined situation. A boundary-integral method with modified Green’s functions, which exactly satisfy the no-slip condition at the wall, is used to solve the flow equations. The resulting velocities are used in the convection–diffusion equation for the surfactant concentration which is handled with a finite volume method. The local surfactant concentration is coupled to the local surface tension via a Langmuir isotherm. The model presented here is three dimensional, but only suited for equi-viscous drops and insoluble surfactants, but can be easily extended. Several examples of the influence of surfactants are shown: for drops in shear flow, surface dilution is of more significance due to the larger deformation of drops in confined situations. For pressure-driven flows, the non-uniform surfactant distribution speeds up the migration towards the center line, while the migration velocity in the velocity direction is lower.