Stability of soluble surfactant-laden falling film over a hydrophobic incline in the presence of external shear

TL;DR

This study investigates the stability of a soluble surfactant-laden falling film over a hydrophobic incline under external shear, combining analytical and numerical methods to identify multiple flow instability modes.

Contribution

It extends previous work by analyzing the effects of external shear and slip conditions on flow stability using combined analytical and numerical approaches.

Findings

Identification of surface, surfactant, and shear instability modes.

Analytical longwave and numerical spectral methods confirm multiple modes.

Shear mode appears at high Reynolds number and low inclination.

Abstract

The hydrodynamic stability analysis of gravity-driven, soluble surfactant-laden fluid streaming down a slippery, slanted plane in the presence of external shear force is being explored in this article. The Navier-Stokes equations are considered for the fluid flow along with the appropriate advection-diffusion equations for the concentration of different surfactant species. The monomers considered here are anticipated to dissolve in the bulk flow and can be adsorbed at the interface of air-liquid as well. Also, the adsorption-desorption kinetics of the surfactants at the free space is taken into consideration. The motivation behind this work is to extend the work of Karapetsas and Bontozoglou[1] for flow over a slippery bottom and in the presence of externally imposed shear forces and observe their impact on the flow dynamics. The Orr-Sommerfeld eigensystem is obtained, then it is solved analytically using the longwave approximation method in the longwave regime ($k \ll 1$) and subsequently, the Chebyshev spectral collocation method is employed for numerical evaluation in the arbitrary wave regime. Using the analytical method, two longwave modes, viz, surface mode and surfactant mode, are detected. Alternatively, the numerical analysis substantiated the existence of temporal surface and surfactant modes. Moreover, another temporal mode named shear mode arises in the high modified Reynolds number region at a low inclination angle. Thereafter, the modified Reynolds-Orr energy equation is deduced under the normal mode conditions, and the behaviour of different energy components is investigated for various slip parameters and imposed shear force.