We report experimental evidence of circulation reversal in evaporating drops. The internal flow patterns and particle distribution inside the evaporating water droplet with suspended polystyrene particles was studied using the Particle Image Velocimetry (PIV) technique. Droplet placed on externally heated substrate showed two symmetric and counter rotating Marangoni convection cells at the beginning of evaporation process. Towards the final stages of evaporation, the two counter rotating Marangoni convection cells reversed the direction of circulation. Interestingly, it was observed that between these two stages four symmetric and counter rotating convection cells appeared inside the evaporating droplet. Our results are in support of recent theoretical investigations on circulation reversal in evaporating drops. The flow patterns were driven by the surface tension gradient caused by the surface temperature distribution along the droplet-air interface. The surface temperature distribution is not only dependent on the non-uniform evaporation flux but also on the conduction path from the substrate to the droplet surface. Towards the end of evaporation, the fluid flow becomes radially outward from the center to the pinned edge of the droplet. We have also investigated the evaporating droplets placed on a curved substrate. Unlike the case of flat substrate, the evaporation from curved surface resulted monotonic temperature profile. The particle distribution and resulting deposition patterns was strongly influenced by the internal fluid flow pattern and was observed to be different from that of a flat substrate.

我们报告蒸发液滴的循环逆转的实验证据。使用粒子图像测速（PIV）技术研究了悬浮聚苯乙烯颗粒的蒸发水滴内部的内部流型和颗粒分布。放置在外部加热的基材上的液滴在蒸发过程开始时显示出两个对称且反向旋转的Marangoni对流单元。在蒸发的最后阶段，两个反向旋转的Marangoni对流单元反转了循环方向。有趣的是，观察到在这两个阶段之间，在蒸发的液滴内部出现了四个对称和反向旋转的对流单元。我们的研究结果支持了有关蒸发液滴循环逆转的最新理论研究。流动模式由沿液滴-空气界面的表面温度分布引起的表面张力梯度驱动。表面温度分布不仅取决于不均匀的蒸发通量，还取决于从基板到液滴表面的传导路径。在蒸发即将结束时，流体流从液滴的中心到钉扎边缘径向向外。我们还研究了放置在弯曲基板上的蒸发液滴。与平坦基板的情况不同，弯曲表面的蒸发导致单调温度分布。颗粒的分布和所产生的沉积模式受内部流体流动模式的强烈影响，并且观察到与平面基板不同。