Competition between thermocapillary and solutocapillary flows in thin liquid films

We investigate the thermocapillary flow in a thin liquid film which is subjected to local heating, in the presence of insoluble surfactants. While surfactant molecules are first advected from warmer to cooler regions, the resulting concentration gradient drives a solutal counterflow in the opposite direction. This competition is theoretically addressed within the lubrication approximation. Assuming small deviations with respect to the mean surfactant concentration, we derive the time evolution equation governing the shape of the interface. Our study reveals that both interfacial deformations and velocities are progressively suppressed as the solutal Marangoni number increases. Our versatile model, adaptable to a range of experimental setups, offers a quantitative tool for understanding the effect of surfactants in thermocapillary-driven systems.