Effect of soluble surfactants on dynamic wetting of flexible substrates: A finite element study

Dynamic wetting plays an important role in coating processes. This paper presents a finite element study of the effect of surfactants under the sorption-controlled regime on the location of the dynamic contact line for flexible solids at steady state. The bulk concentration of the surfactant is assumed to be in excess such that the bulk phase diffusion is neglected and the surface equation of state based on the Frumkin adsorption framework is used to predict the distribution of surface tension as a function of surface surfactant concentration. The finite element model solves for the fluid-structural interactions between an elastic solid and a viscous liquid along with the surfactant distribution at the liquid/vapor free surface. To predict the shape of the free surface, the model uses arbitrary Lagrangian Eulerian mesh motion in both fluid and solid phases. The variation in dynamic contact line position with respect to various parameters such as downstream pressure, Peclet number, surfactant elasticity number, Biot number, interaction parameter, and initial surface coverage are presented for the finite element model. It can be concluded that the presence of surfactant has a significant effect on the location of the dynamic contact line for flexible solids at steady state and the stability of the liquid-vapor free surface.