The concept of droplet microfluidics and droplet reactors is becoming popular in the chemical processing industry, especially to handle expensive specialty chemicals. The convective mixing within a single droplet or two coalescing droplets plays an important role during the reaction involving two-phase systems, which requires a good understanding of the internal fluid motion within the droplet. In this work, we study the flow field inside a liquid droplet falling through an immiscible liquid in a narrow-width rectangular vertical channel using particle image velocimetry (PIV). The droplet can be considered a representation of a midplane cutting through a spherical droplet. The effect of the size and release position of the droplet on the velocity field is examined. When the droplet is released at the center of the channel, a pair of symmetric counter-rotating vortices is observed inside the droplet. The vortices are positioned in the bottom half of the droplet and move toward its horizontal centerline as the diameter increases. However, when the droplet is released at an off-center position, the vortices are not symmetric. The terminal velocity of the droplet is observed to increase with an increase in droplet diameter, and therefore, coalescence occurs when a bigger droplet is released following a smaller one. The growth of the neck of the combined droplet with time and the velocity field at the neck are studied during coalescence.

中文翻译：

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落在矩形通道内液体介质中的液滴内的速度场


液滴微流体和液滴反应器的概念在化学加工行业中越来越流行，特别是在处理昂贵的特种化学品方面。单个液滴或两个聚结液滴内的对流混合在涉及两相系统的反应中起着重要作用，这需要对液滴内的内部流体运动有很好的了解。在这项工作中，我们使用粒子图像测速（PIV）研究了在窄宽度矩形垂直通道中穿过不混溶液体的液滴内部的流场。液滴可以被认为是穿过球形液滴的中平面的表示。研究了液滴尺寸和释放位置对速度场的影响。当液滴在通道中心释放时，在液滴内部观察到一对对称的反向旋转涡流。涡流位于液滴的下半部分，并随着直径的增加而向其水平中心线移动。然而，当液滴在偏离中心的位置释放时，涡流不对称。观察到液滴的终端速度随着液滴直径的增加而增加，因此，当较大的液滴跟随较小的液滴释放时，会发生聚结。研究了合并过程中合并液滴颈部随时间的增长以及颈部的速度场。