Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology (KAIST) , Daejeon 34141, Republic of Korea.
Department of Bioengineering, Mechanical and Aerospace Engineering, and California NanoSystems Institute, University of California , Los Angeles, California 90095, United States.
Anal Chem. 2018 Feb 20;90(4):2902-2911. doi: 10.1021/acs.analchem.7b05143. Epub 2018 Feb 8.
Inertial microfluidics has drawn much attention not only for its diverse applications but also for counterintuitive new fluid dynamic behaviors. Inertial focusing positions are determined by two lift forces, that is, shear gradient and wall-induced lift forces, that are generally known to be opposite in direction in the flow through a channel. However, the direction of shear gradient lift force can be reversed if velocity profiles are shaped properly. We used coflows of two liquids with different viscosities to produce complex velocity profiles that lead to inflection point focusing and alteration of inertial focusing positions; the number and the locations of focusing positions could be actively controlled by tuning flow rates and viscosities of the liquids. Interestingly, 3-inlet coflow systems showed focusing mode switching between inflection point focusing and channel face focusing depending on Reynolds number and particle size. The focusing mode switching occurred at a specific size threshold, which was easily adjustable with the viscosity ratio of the coflows. This property led to different-sized particles focusing at completely different focusing positions and resulted in highly efficient particle separation of which the separation threshold was tunable. Passive separation techniques, including inertial microfluidics, generally have a limitation in the control of separation parameters. Coflow systems can provide a simple and versatile platform for active tuning of velocity profiles and subsequent inertial focusing characteristics, which was demonstrated by active control of the focusing mode using viscosity ratio tuning and temperature changes of the coflows.
惯性微流控不仅因其多样化的应用，还因其反直觉的新流体动力学行为而备受关注。惯性聚焦位置由两种升力决定，即剪切梯度和壁面诱导升力，这两种力在通道中的流向通常是相反的。然而，如果适当调整速度分布，剪切梯度升力的方向可以反转。我们使用两种不同粘度的液体的共流来产生复杂的速度分布，从而导致拐点聚焦和惯性聚焦位置的改变；通过调整液体的流速和粘度，可以主动控制聚焦位置的数量和位置。有趣的是，3 入口共流系统根据雷诺数和颗粒尺寸在拐点聚焦和通道面聚焦之间表现出聚焦模式切换。聚焦模式切换发生在一个特定的尺寸阈值，这个阈值可以很容易地通过共流的粘度比进行调节。这种特性导致不同尺寸的颗粒在完全不同的聚焦位置聚焦，从而实现高效的颗粒分离，并且分离阈值是可调的。被动分离技术，包括惯性微流控，在分离参数的控制方面通常存在局限性。共流系统为主动调整速度分布和随后的惯性聚焦特性提供了一个简单而通用的平台，这通过使用粘度比调节和共流的温度变化来主动控制聚焦模式得到了证明。
