Thermocapillary Convection in Thin Liquid Films on Walls With Microgrooves

Thermocapillary induced convection in thin liquid films on a horizontal wall with microgrooves is studied experimentally and numerically. To this end, we carry out experiments with silicon oil on a heated copper wall with parallel groves. The flow is visualized by tracking glass spheres seeded within the liquid film. The results of velocity measurements are reported. A numerical model for a liquid film on a structured wall is proposed. The full incompressible Navier-Stokes equations and the energy equation are integrated by a finite difference algorithm, whereas the mobile gas-liquid interface is tracked by the volume-of-fluid method. The numerical model is verified by comparison with the experimental data showing good agreement. The model is used to study flow patterns and film rupture caused by thermocapillary forces. We demonstrate that at any Marangoni number, either positive or negative, thermocapillary convection characterized by rolls develops within the film. In the experiments, two rolls in each groove are observed. The numerical solutions predict that at certain conditions the rolls are doubled under the influence of the wall structure guiding the flow. It is also found that an abrupt increase in wall temperature may rupture the liquid film near the structure crest. The results of this study may be applied to the design of microfluidic mixers and heat exchangers.