We investigate the internal flow pattern of an evaporating droplet using tomographic particle image velocimetry (PIV) when the contact line non-uniformly recedes. We observe a three-dimensional azimuthal vortex pair while the contact line non-uniformly recedes and the symmetry-breaking flow field is maintained during the evaporation. Based on the experimental results, we show that the vorticity magnitude of the internal flow is related to the relative contact line motion. Furthermore, to explain how the azimuthal vortex pair flow is created, we develop a theoretical model by taking into account the relation between the contact line motion and evaporating flux. Finally, we show that the theoretical model has a good agreement with experimental results.

当接触线不均匀地后退时，我们使用层析成像颗粒图像测速法（PIV）研究蒸发液滴的内部流动模式。我们观察到一个三维方位角涡流对，而接触线不均匀地后退，并且在蒸发过程中保持了破坏对称的流场。根据实验结果，我们表明内部流动的涡度大小与相对接触线的运动有关。此外，为了解释如何创建方位涡对，我们通过考虑接触线运动与蒸发通量之间的关系来建立理论模型。最后，我们证明理论模型与实验结果具有很好的一致性。