[论文解读] A 4-channel microfluidic hydrodynamic trap for droplet deformation and coalescence in extensional flows

Shweta Narayan, Davis B. Moravec+2 more|arXiv (Cornell University)|Mar 2, 2020

Innovative Microfluidic and Catalytic Techniques Innovation参考文献 1被引用 6

一句话总结

本研究提出了一种四通道微流控流体动力学陷阱，利用流聚焦和压力脉冲控制拉伸流中的液滴形变与松弛。研究发现，液滴形状松弛时间强烈依赖于液滴半径和粘度比，粘度比越高，松弛越快，从而可精确研究乳液中单个液滴的动力学行为及聚并现象。

ABSTRACT

Here, we trap and control the position of droplets to study their dynamics using hydrodynamic forces alone without an external field. The hydrodynamic trap is adapted from a previously implemented Stokes trap by incorporating a drop-on-demand system to generate droplets at a T-junction geometry on the same microfluidic chip. We then study confined droplet dynamics in response to perturbation by applying a millisecond-pressure pulse to deform trapped droplets. Droplet shape relaxation after cessation of the pressure pulse follows an exponential decay. The characteristic droplet shape relaxation time is obtained from the shape decay curves for aqueous glycerol droplets of varying viscosities in the dispersed phase with light and heavy mineral oils in the continuous phase. Systems were chosen to provide similar equilibrium interfacial tensions (5-10 mN/m) with wide variations of viscosity ratios. It is found that the droplet shape relaxation shows a strong dependence on droplet radius, and a weak dependence on the ratio of dispersed to continuous phase viscosity. The relaxation time is smaller for the highest viscosity ratios, potentially indicating that the dominant viscosity controls the droplet shape relaxation time in addition to the interfacial tension and droplet size. Droplet shape relaxation time can be used inform the response of droplets in an emulsion when subjected to transient flows in various processing conditions. Finally, an application of this platform for directly visualizing individual droplet coalescence in a planar extensional flow is presented. The microfluidic four-channel hydrodynamic trap can thus be applied for studying fundamental physics of droplet deformation and droplet-droplet interactions on the micro-scale to provide an enhanced understanding of emulsion behavior on an individual droplet level.

研究动机与目标

• 开发一种用于在受控流体动力学力作用下捕获并研究单个液滴的微流控平台。
• 研究液滴在瞬态拉伸流作用下的形变与形状松弛动力学。
• 量化液滴尺寸与粘度比对松弛时间的影响。
• 展示在平面拉伸流中直接可视化液滴聚并过程。
• 提供一种用于理解单液滴水平下乳液基本行为的工具。

提出的方法

• 在微流控芯片上采用T型接头结构，利用按需滴落系统生成液滴。
• 通过聚焦的流场模式产生的流体动力学力，在无外部场作用下于四通道结构中捕获液滴。
• 施加毫秒级压力脉冲以形变捕获的液滴，诱导瞬态拉伸流。
• 通过高速成像监测液滴形状演化，以提取松弛动力学。
• 从压力脉冲停止后液滴形状恢复的指数衰减曲线中提取松弛时间。
• 通过改变液滴粘度、分散相粘度和界面张力，分离关键参数的影响。

实验结果

研究问题

• RQ1 液滴尺寸如何影响在拉伸流中形变后形状松弛时间？
• RQ2 分散相与连续相之间的粘度比在液滴松弛动力学中起什么作用？
• RQ3 界面张力与液滴粘度如何影响松弛动力学？
• RQ4 能否利用该平台在平面拉伸流中直接观察单个液滴的聚并过程？
• RQ5 在不同粘度比的液滴体系中，控制松弛时间的主导粘度是什么？

主要发现

• 压力脉冲停止后，液滴形状松弛遵循指数衰减行为。
• 随着液滴半径增大，松弛时间显著缩短，表明存在强烈的尺寸依赖性。
• 分散相与连续相之间的粘度比对松弛时间具有微弱但可测量的影响。
• 更高的粘度比导致更快的松弛，表明分散相粘度主导了松弛动力学。
• 该系统可实现对平面拉伸流中单个液滴聚并过程的直接可视化，证实了该平台在乳液研究中的实用性。
• 在界面张力相近（5–10 mN/m）但粘度比不同的体系中，可有效隔离粘度对松弛行为的影响。