Thermocapillary instability of a surfactant-laden shear-imposed film flow

Abstract

We examine the linear thermocapillary instability of a two-dimensional gravity-driven shear-imposed incompressible viscous film flowing over a uniformly heated inclined wall when the film surface is covered by an insoluble surfactant. The aim is to expand the prior research [Wei, Phys. Fluids 17, 012103 (2005)] to the case of a nonisothermal viscous film. As a result, the energy equation is incorporated into the governing equations along with the mass conservation and momentum equations. In the present study, we have found two additional thermocapillary S- and P-modes in the low to moderate Reynolds number regime, along with the known H-mode (surface mode) and surfactant mode. The long-wave analysis predicts that the surfactant Marangoni number, which measures the surface tension gradient due to a change in insoluble surfactant concentration, has a stabilizing impact on the H-mode, but the thermal Marangoni number, which measures the surface tension gradient due to a change in temperature, has a destabilizing impact. These opposing effects produce an analytical relationship between them for which the critical Reynolds number for the H-mode instability of the nonisothermal film flow coincides with that of the isothermal film flow. On the other hand, the numerical result exhibits that the surfactant Marangoni number has a stabilizing influence on the thermocapillary S-mode and P-mode. More specifically, these thermocapillary instabilities diminish with an increase in the value of the surfactant Marangoni number. However, these thermocapillary instabilities can be made stronger by increasing the value of the thermal Marangoni number. Furthermore, the thermal Marangoni number destabilizes the surfactant mode instability, but the onset of instability is not affected in the presence of the thermal Marangoni number, which is in contrast to the influence of the surfactant Marangoni number on the onset of surfactant mode instability. Interestingly, the Biot number, which measures the ratio of heat convection and heat conduction, shows a dual role in the surfactant mode instability, even though the threshold of instability remains the same. In the high Reynolds number regime, the shear mode appears and is stabilized by the surfactant Marangoni number but destabilized by the thermal Marangoni number. Moreover, the comparison of results with inertia and without inertia exhibits a stabilizing role of inertia in the surfactant mode.