A numerical model of an immiscible surfactant drop spreading over thin liquid layers using CFD/VOF approach

Abstract

Numerical investigations of immiscible surfactant drops spreading over thin liquid substrates have been carried out. The CFD numerical model is based on a volume of fluid technique (VOF) coupled with piecewise linear interface calculations method (PLIC). This interface reconstruction is applied to simulate the time evolution of the dynamics of drop of surfactants spreading on thin water liquid substrates. In the model, the surfactant is considered as a separate phase instead of a solution of water containing the surfactant. This approach allows to avoid the parallel determination of the surfactant molecular diffusion coefficient, the maximum packing concentration, and the rate constants for adsorption and desorption prior to the simulations. Series of superspreader trisiloxane (M(D′EnOH)M surfactant are considered. The predictions of the CFD model agree remarkably well with the measurements from previously published experimental results. The simulations are used to predict and optimize the spreading behavior as a function of a range of well-defined parameters including the water/surfactant interfacial tension (28−35 mN/m), the volume of the drop of surfactant (0.61−38.79 mm3) and the thickness of the water layer (1−3 mm). The dewetting process of the thin water layer from the surface by the surfactant occurs for almost all the configurations. The formation of secondary drops and instable crown happen solely for the largest drop volume of 38.79 mm3. Only for the thicker films (h = 2 and 3 mm), a cavity on the liquid is generated after the impact. For all the thicknesses, instabilities appear at the surfactant/water interfaces and their numbers are larger for the thinner film (h = 1 mm). The kinetic analysis of the numerical data confirms the existence of two successive spreading regimes for all the drops of surfactants studied. The faster time evolution of the moving front proceeds for surfactant drops of low volume and high surface tension as well as a thick water underlayer.