Spreading and rupture dynamics of soluble surfactant-laden thin film flow down a slippery incline in presence of external shear

Abstract

The spreading and rupture of local distribution of surfactant on a slippery inclined thin film flow in the presence of external shear is explored in this article. The surfactant can be adsorbed at the free surface or can be dissolved in the bulk. The surfactant concentrations are governed by advection and diffusion equations for bulk as well as the interface. Moreover, the adsorption-desorption rates of the bulk and interface surfactants are regulated by sorption kinetics rates. The van der Waals forces are considered for rupture dynamics. The lubrication approximation method is used to derive the evolution equations of film thickness and interface surfactant concentration. Two different scenarios are considered for sorption kinetics rates: (i) rapid and (ii) slow in the case of the spreading phenomenon. Then, two cases are considered related to the distribution of the surfactant in case of rapid sorption kinetics. The slippery bottom helps more fluid to flow and the capillary ridge to gain height for rapid sorption kinetics, and the external shear force amplifies the thinning of the film. However, in the case of slow sorption kinetics, the adsorption-desorption takes place at a slow rate, and the transient period results in a reduced Marangoni gradient at the interface. This leads to a pulse-type character in the film thickness profile. The external shear force reduces the pulse height, whereas a slippery surface at the bottom increases the pulse height. On the other hand, van der Waals forces are considered to be the major factor behind the rupture mechanism. The linear stability analysis depicts that external shear force destabilizes the flow, but the slip parameter displays a dual effect based on the Bond number, capillary number, and Hamaker constant.