Surfactant effects on deformable drop motion in a slit
Journal Article
Overview
abstract
The effect of insoluble surfactants on drop motion and deformation in a pressure-driven flow between two parallel plates at low Reynolds numbers is simulated by a combination of three-dimensional boundary-integral and flow-biased least squares methods to solve the coupled hydrodynamic and surfactant transport equations for the deforming drop surface. The external flow induces droplet motion and deformation that result in subsequent surfactant redistribution along the interface, which ultimately gives rise to tangential Marangoni stresses. In general, drop steady-state velocity decreases and drop deformation increases with increasing drop diameter, though increasing velocity with increasing diameter occurs for small drop viscosities. In the convection-dominated regime, i.e., in the limit of high surface Péclet number, Marangoni stresses partially immobilize the surface of the drop, leading to a significant decrease in the steady-state velocity of a low-viscosity drop. The accumulation of surfactants at the rear tips of the drop lowers the local interfacial tension, thereby inducing larger deformations to satisfy the normal stress balance. For an initially off-centered drop position, a surfactant-laden drop is observed to migrate to the channel centerplane either faster or slower than a clean drop does, depending on the drop-to-bulk viscosity ratio and the capillary number. Moreover, it is found that higher surfactant concentration values on the drop surface delay the cross-stream migration of nearly spherical droplets, while for deformable droplets, higher surfactant concentration facilitates faster migration.
