Two-phase liquid-liquid systems are prevalent in a range of commercial and environmental applications. Understanding the behavior of liquid-liquid systems under various processing conditions requires the study of droplet dynamics under precisely controlled flow fields. Here we trap and control the position of droplets in a microfluidic trap 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. Using the hydrodynamic trap, confined droplet dynamics in response to perturbation are studied 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 in the moderately confined regime shows a strong dependence on droplet radius, and a weaker dependence on the ratio of dispersed to continuous phase viscosity. An empirical scaling relationship is developed, and relaxation times from the experiments are compared to theoretical relaxation times for the limiting regime of unconfined droplets. The droplet response in the moderately confined regime differs from both limiting regimes of unconfined and highly confined droplets with regards to the radius scaling. 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 droplet coalescence in planar extensional flow is presented. The microfluidic four-channel hydrodynamic trap can thus be applied for studying the fundamental physics of droplet deformation and droplet-droplet interactions on the microscale.

中文翻译：

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四通道微流动力阱中的液滴形状松弛

两相液-液系统普遍存在于一系列商业和环境应用中。要了解液-液系统在各种处理条件下的行为，需要研究在精确控制的流场下的液滴动力学。在这里，我们捕获并控制微流阱中液滴的位置，以单独使用流体动力而不使用外部场来研究其动力学。通过并入按需滴注系统以在同一微流控芯片上以T形结几何形状产生液滴，从先前实现的“斯托克斯陷阱”改编了流体动力陷阱。使用流体动力阱，通过施加毫秒压力脉冲使捕获的液滴变形，研究了响应于扰动的受限液滴动力学。压力脉冲停止后，液滴形状松弛随指数衰减。从形状衰减曲线获得特征性的液滴形状弛豫时间，对于在分散相中具有不同粘度的含水甘油液滴，在连续相中具有轻质和重质矿物油。选择的系统可提供相似的平衡界面张力（5-10 mN / m），且粘度比变化很大。发现在中等限制状态下的液滴形状松弛显示出对液滴半径的强依赖性，而对分散相与连续相粘度之比的依赖性较弱。建立了经验标度关系，并将实验的弛豫时间与无约束液滴的限制机制的理论弛豫时间进行了比较。就半径缩放而言，中等限制状态下的液滴响应与非限制液滴和高度限制状态下的液滴的限制状态都不同。当在各种加工条件下经受瞬时流动时，液滴形状的松弛时间可用于告知乳液中液滴的响应。最后，提出了该平台在平面延伸流中直接可视化液滴聚结的应用。因此，微流体四通道流体动力阱可用于研究微尺度上液滴变形和液滴与液滴相互作用的基本物理学。当在各种加工条件下经受瞬时流动时，液滴形状的松弛时间可用于告知乳液中液滴的响应。最后，提出了该平台在平面延伸流中直接可视化液滴聚结的应用。因此，微流体四通道流体动力阱可用于研究微尺度上液滴变形和液滴与液滴相互作用的基本物理学。当在各种加工条件下经受瞬时流动时，液滴形状的松弛时间可用于告知乳液中液滴的响应。最后，提出了该平台在平面延伸流中直接可视化液滴聚结的应用。因此，微流体四通道流体动力阱可用于研究微尺度上液滴变形和液滴与液滴相互作用的基本物理学。