Title: Droplet-on-demand using a positive pressure pulse Show Chinese title

Title: 按需液滴生成使用正压力脉冲

Abstract: Droplet generation under steady conditions is a common microfluidic method for producing biphasic systems. However, this process works only over a limited range of imposed pressure: beyond a critical value, a stable liquid jet can instead form. Furthermore, for a given geometry the pressure conditions set both the generation rate of droplets and their volume. Here, we report on-demand droplet production using a positive pressure pulse to the dispersed-phase inlet of a flow-focusing geometry. This strategy enables confined droplet generation within and beyond the pressure range observed under steady conditions, and decouples volume and production rate. In particular, elongated plugs not possible under steady conditions may be formed when the maximal pressure during the pulse reaches the jet regime. The measured volume of droplets-on-demand, as well as the onset of droplet generation are both captured with a simple model that considers hydraulic resistances. This work provides a strategy and design rules for processes that require individual droplets or elongated plugs in a simple microfluidic chip design. Show Chinese abstract

Abstract: 在稳态条件下生成液滴是一种常见的微流控方法，用于生产双相体系。然而，该过程仅在有限的施加压力范围内有效：超过某一临界值时，稳定的液体射流反而会形成。此外，对于特定的几何结构，压力条件既决定了液滴的生成速率，也决定了液滴的体积。在此，我们报告了一种利用正压脉冲控制分散相入口的方法，以实现按需生成液滴。这种方法能够在稳态条件下观察到的压力范围内外实现受限的液滴生成，并且能够解耦液滴体积和生成速率之间的关系。特别是，在脉冲期间的最大压力达到喷射状态时，可以形成在稳态条件下不可能出现的拉长型栓塞。通过一个考虑液压阻力的简单模型，我们可以捕捉到按需生成液滴的体积以及液滴生成的起始点。本研究为需要在简单微流控芯片设计中产生单个液滴或拉长栓塞的过程提供了一种策略和设计规则。