Dipolar thermocapillary motor and swimmer

The study of thermocapillary driven flows is typically restricted to "open" systems, i.e., ones where a liquid film is bounded on one side solely by another fluid. However, a large number of natural and engineered fluidic systems are composed of solid boundaries with only small open regions exposed to the surrounding. In this work we study the flow generated by the thermocapillary effect in a liquid film overlaid by a discontinuous solid surface. If the openings in the solid are subjected to a temperature gradient, the resulting thermocapillary flow will lead to a nonuniform pressure distribution in the film, driving flow in the rest of the system. For an infinite solid surface containing circular openings, we show that the resulting pressure distribution yields dipole flows which can be superposed to create complex flow patterns, and demonstrate how a confined dipole can act as a thermocapillary motor for driving fluids in closed microfluidic circuits. For a mobile, finite-size surface, we show that an inner temperature gradient, which can be activated by simple illumination, results in the propulsion of the surface, creating a thermocapillary surface swimmer.