Delaying dynamic wetting failure using thermal Marangoni flow

Coating processes are limited by the onset of dynamic wetting failure beyond a critical substrate speed. In this work, we study the influence of thermal Marangoni flow on dynamic wetting with the objective of delaying wetting failure to higher substrate speeds. A two-dimensional hydrodynamic model is developed to examine steady-state dynamic wetting of a Newtonian liquid in a parallel-plate geometry where a temperature gradient between the plates generates thermal Marangoni flow. The dynamics of the air displaced by the liquid are accounted for, and the Galerkin finite element method is used to calculate steady-state solutions and find the critical substrate (bottom plate) speed at which wetting failure occurs. It is found that thermal Marangoni flow directed toward the dynamic contact line at the substrate delays wetting failure to a higher speed, whereas flow away from the contact line causes wetting failure at a lower speed. Flow toward the contact line reduces the bending of the air-liquid interface and increases the thickness of the air film near the contact line. This lowers the magnitude of pressure gradients in the air phase and facilitates removal of air from the contact line, delaying the entrainment of air and consequently wetting failure to higher speeds. In contrast, flow away from the contact line increases interface bending at a given speed, leading to thinning of the air film near the contact line and causing wetting failure at lower speeds. The findings presented in this study suggest a novel strategy for designing faster coating processes through the application of thermal Marangoni flow.