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 high-speed 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 α = 2π/3 = 120° for which droplets can form.

在许多自然和工业过程中，液滴的形成无处不在，并且随着微流体和微流体的出现，达到了前所未有的控制水平。尽管在过去的几十年中已经获得了对液滴形成机理的重要见解，但对于这种现象所基于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正确理解仍然缺乏，特别是对于逐步乳化的广泛应用的方法。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，我们明确地表明，最终的液滴夹断是由瑞利-普拉托型不稳定性触发的。但是，液滴的大小不是由瑞利-普拉多破裂决定的，而是由最初的润湿机制决定的，在该机制中，流体的接触角起着至关重要的作用。我们为润湿过程开发了一种物理理论，该理论密切描述了我们的实验测量结果，而无需调用任何自由拟合参数。我们的理论预言了瑞利高原破裂的开始以及从滴落到喷射的转变，这是流体接触角的函数。此外，该理论解决了为什么存在最小接触角的难题。流体的接触角起着至关重要的作用。我们为润湿过程开发了一种物理理论，该理论密切描述了我们的实验测量结果，而无需调用任何自由拟合参数。我们的理论预言了瑞利高原破裂的开始以及从滴落到喷射的转变，这是流体接触角的函数。此外，该理论解决了为什么存在最小接触角的难题。流体的接触角起着至关重要的作用。我们为润湿过程开发了一种物理理论，该理论密切描述了我们的实验测量结果，而无需调用任何自由拟合参数。我们的理论预言了瑞利高原破裂的开始以及从滴落到喷射的转变，这是流体接触角的函数。此外，该理论解决了为什么存在最小接触角的难题。α =2π/ 3 = 120°可能形成液滴。