3D Numerical Modeling of Soluble Surfactant at Fluidic Interfaces Based on the Volume-of-Fluid Method

We present a computational approach based on the Volume-of-Fluid (VOF) method for simulating the influence of a soluble surfactant on the behavior of two-phase systems with deformable interface. Our approach is applicable to diffusion controlled processes, where the relation between the area-specific excess surfactant concentration on the interface and the volume-specific concentration adjacent to the interface is given by an adsorption isotherm. Main issues of the numerical model are an extended surface transport theorem used for describing the interfacial flux and an iso-surface of the VOF-variable used as a connected approximation for the interface. 3D-simulations of a bubble moving through a surfactant solution show the formation of a monotone concentration profile along the bubble surface with a surfactant-rich zone at the bubble's rear end. This is accompanied by regions of depleted and increased surfactant concentration in the bulk phase due to adsorption and desorption, respectively. The rise velocity reflects the retardation effect known from experiments.