The thermocapillary effect, arising flow due to a temperature gradient along a fluid interface, is the dominant effect in some industrial and microfluidic processes and must be studied in order to optimize them. In this work, we analyze how insoluble surfactants adsorbed at the interface can affect such a flow. In particular, we analyze 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 insight toward the miniaturization of the manipulation platform.

中文翻译：

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不溶性表面活性剂对热毛细管流动的影响

热毛细管效应，由于沿流体界面的温度梯度而引起的流动，是一些工业和微流体过程中的主要效应，必须对其进行研究以对其进行优化。在这项工作中，我们分析了吸附在界面上的不溶性表面活性剂如何影响这种流动。特别是，我们分析了通过用红外激光局部加热在空气 - 水界面处诱导热毛细管流的情况，该设置用于通过生成的流动操纵漂浮的颗粒。由于水是极性流体，空气-水界面很容易被表面活性剂污染。我们开发了一个考虑表面活性剂不受控制存在的数值模型，这证明即使表面活性剂浓度很小，表面污染的影响也不容忽视。该数值模型的结果与不同的实验测量进行了比较：粒子跟踪测速、对流单元半径测量和表面热成像。所有的实验观察都与数值模型一致，初始表面污染是一个拟合参数。然后将该模型的结果与将已知数量的表面活性剂添加到界面中的测量结果进行比较来验证。最后，开发了一个分析模型来解释控制参数的影响，这与模拟和实验结果一致。开发的模型让我们深入了解操纵平台的小型化。和表面的热成像。所有的实验观察都与数值模型一致，初始表面污染是一个拟合参数。然后将该模型的结果与将已知数量的表面活性剂添加到界面中的测量结果进行比较来验证。最后，开发了一个分析模型来解释控制参数的影响，这与模拟和实验结果一致。开发的模型让我们深入了解操纵平台的小型化。和表面的热成像。所有的实验观察都与数值模型一致，初始表面污染是一个拟合参数。然后将该模型的结果与将已知数量的表面活性剂添加到界面中的测量结果进行比较来验证。最后，开发了一个分析模型来解释控制参数的影响，这与模拟和实验结果一致。开发的模型让我们深入了解操纵平台的小型化。最后，开发了一个分析模型来解释控制参数的影响，这与模拟和实验结果一致。开发的模型让我们深入了解操纵平台的小型化。最后，开发了一个分析模型来解释控制参数的影响，这与模拟和实验结果一致。开发的模型让我们深入了解操纵平台的小型化。