Wetting controls of droplet formation in step emulsification
Wetting controls of droplet formation in step emulsification
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DOI：
10.1073/pnas.1803644115
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发表时间：
2018-09-18
影响因子：
11.1
通讯作者：
Studart, Andre R.
中科院分区：
文献类型：
--
作者：
Eggersdorfer, Maximilian L.;Seybold, Hansjorg;Studart, Andre R.
液滴的形成在许多自然和工业过程中普遍存在，并且随着毫流控和微流控技术的出现，液滴的控制达到了前所未有的水平。尽管在过去的几十年里人们对液滴形成的机制有了重要的了解，但仍然缺乏对这种现象背后的物理原理以及流体的流动和润湿特性对液滴尺寸和生产率的影响的充分理解，特别是对于广泛应用的分步乳化方法。在这项工作中，我们通过使用流体通道的润湿作为可调参数来探索广泛的乳化条件，阐明了逐步乳化中微滴形成的物理控制。借助高速测量，我们明确表明最终的液滴夹断是由瑞利高原型不稳定性触发的。然而，液滴尺寸并非由瑞利平台破裂决定，而是由初始润湿状态决定，其中流体的接触角起着至关重要的作用。我们开发了润湿过程的物理理论，它紧密地描述了我们的实验测量，而无需调用任何自由拟合参数。我们的理论预测瑞利高原破裂的开始以及从滴落到喷射的转变是流体接触角的函数。此外，该理论还解决了为什么液滴形成的最小接触角 alpha = 2 pi/3 = 120 度的难题。
The formation of droplets is ubiquitous in many natural and industrial processes and has reached an unprecedented level of control with the emergence of milli- and microfluidics. Although important insight into the mechanisms of droplet formation has been gained over the past decades, a sound understanding of the physics underlying this phenomenon and the effect of the fluid's flow and wetting properties on the droplet size and production rate is still missing, especially for the widely applied method of step emulsification. In this work, we elucidate the physical controls of microdroplet formation in step emulsification by using the wetting of fluidic channels as a tunable parameter to explore a broad set of emulsification conditions. With the help of highspeed measurements, we unequivocally show that the final droplet pinch-off is triggered by a Rayleigh-Plateau-type instability. The droplet size, however, is not determined by the Rayleigh-Plateau breakup, but by the initial wetting regime, where the fluid's contact angle plays a crucial role. We develop a physical theory for the wetting process, which closely describes our experimental measurements without invoking any free fit parameter. Our theory predicts the initiation of the Rayleigh-Plateau breakup and the transition from dripping to jetting as a function of the fluid's contact angle. Additionally, the theory solves the conundrum why there is a minimal contact angle of alpha = 2 pi/3 = 120 degrees for which droplets can form.