School of Engineering and Applied Sciences and Department of Physics, Harvard University, Cambridge, Massachusetts, USA.
Lab Chip. 2012 Apr 21;12(8):1516-21. doi: 10.1039/c2lc21263c. Epub 2012 Mar 9.
At low capillary number, drop formation in a T-junction is dominated by interfacial effects: as the dispersed fluid flows into the drop maker nozzle, it blocks the path of the continuous fluid; this leads to a pressure rise in the continuous fluid that, in turn, squeezes on the dispersed fluid, inducing pinch-off of a drop. While the resulting drop volume predicted by this "squeezing" mechanism has been validated for a range of systems, as of yet, the pressure rise responsible for the actual pinch-off has not been observed experimentally. This is due to the challenge of measuring the pressures in a T-junction with the requisite speed, accuracy, and localization. Here, we present an empirical study of the pressures in a T-junction during drop formation. Using Laplace sensors, pressure probes we have developed, we confirm the central ideas of the squeezing mechanism; however, we also uncover other findings, including that the pressure of the dispersed fluid is not constant but rather oscillates in anti-phase with that of the continuous fluid. In addition, even at the highest capillary number for which monodisperse drops can be formed, pressure oscillations persist, indicating that drop formation in confined geometries does not transition to an entirely shear-driven mechanism, but to a mechanism combining squeezing and shearing.
在低毛细数的情况下，T 型分叉处的液滴形成主要受界面效应的控制：当分散相流体流入滴头喷嘴时，它会阻塞连续相流体的通道；这会导致连续相流体中的压力上升，从而挤压分散相流体，导致液滴的断裂。虽然这种“挤压”机制所预测的液滴体积已经在一系列系统中得到了验证，但导致实际断裂的压力上升尚未在实验中观察到。这是由于在 T 型分叉处测量所需速度、精度和定位的压力存在挑战。在这里，我们对 T 型分叉处形成液滴过程中的压力进行了实证研究。使用我们开发的拉普拉斯传感器和压力探头，我们验证了挤压机制的核心思想；然而，我们也发现了其他发现，包括分散相流体的压力不是恒定的，而是与连续相流体相反相振荡。此外，即使在可以形成单分散液滴的最高毛细数下，压力振荡仍然存在，这表明受限几何形状中的液滴形成不会过渡到完全由剪切驱动的机制，而是过渡到结合挤压和剪切的机制。
