Effect of insoluble surfactants on a thermocapillary flow

International audience; The thermocapillary effect, arising flow due to a temperature gradient along a fluid interface, is the dominant effect in some industrial and microfluidics processes and must be studied in order to optimise them. In this work, we analyse how insoluble surfactants adsorbed at the interface can affect such a flow. In particular, we analyse the case where the thermocapillary flow is induced at the air-water interface by locally heating it with an infrared laser, setup that is used to manipulate floating particles through the generated flow. Since water is a polar fluid, the air-water interface is easily polluted by surfactants. We developed a numerical model considering the uncontrolled presence of surfactants, which evidences that the effect of the surface contamination cannot be neglected, even for small surfactants concentration. The results of this numerical model were compared with different experimental measurements: particle tracking velocimetry, convection cell radius measurements and thermography of the surface. All the experimental observations agree with the numerical model with the initial surface contamination being a fitting parameter. The model was then validated comparing its results with measurements for which a known quantity of surfactant was added to the interface. Finally, an analytical model was developed to explain the effects of the governing parameters, which agrees with the simulations and the experimental results. The developed models give us insights towards the miniaturization of the manipulation platform