Department of Bioengineering, University of California, 420 Westwood Plaza, 5121 Engineering V, P.O. Box 951600, Los Angeles, CA 90095, USA.
Proc Natl Acad Sci U S A. 2012 Jul 17;109(29):11593-8. doi: 10.1073/pnas.1207550109. Epub 2012 Jul 3.
In microfluidic systems at low Reynolds number, the flow field around a particle is assumed to maintain fore-aft symmetry, with fluid diverted by the presence of a particle, returning to its original streamline downstream. This current model considers particles as passive components of the system. However, we demonstrate that at finite Reynolds number, when inertia is taken into consideration, particles are not passive elements in the flow but significantly disturb and modify it. In response to the flow field, particles translate downstream while rotating. The combined effect of the flow of fluid around particles, particle rotation, channel confinement (i.e., particle dimensions approaching those of the channel), and finite fluid inertia creates a net recirculating flow perpendicular to the primary flow direction within straight channels that resembles the well-known Dean flow in curved channels. Significantly, the particle generating this flow remains laterally fixed as it translates downstream and only the fluid is laterally transferred. Therefore, as the particles remain inertially focused, operations can be performed around the particles in a way that is compatible with downstream assays such as flow cytometry. We apply this particle-induced transfer to perform fluid switching and mixing around rigid microparticles as well as deformable cells. This transport phenomenon, requiring only a simple channel geometry with no external forces to operate, offers a practical approach for fluid transfer at high flow rates with a wide range of applications, including sample preparation, flow reaction, and heat transfer.
在低雷诺数的微流控系统中，假设粒子周围的流场保持前后对称，流体在粒子的存在下被分流，然后在下游回到其原始流线。这个当前的模型将粒子视为系统的被动组件。然而，我们证明，在有限的雷诺数下，当考虑惯性时，粒子不是流场中的被动元件，而是显著地干扰和改变它。粒子在旋转的同时沿流向下游移动。流体围绕粒子的流动、粒子的旋转、通道限制（即粒子尺寸接近通道尺寸）以及有限的流体惯性的综合作用，在直通道内产生了一个垂直于主流方向的净循环流，类似于弯曲通道中众所周知的迪恩流。重要的是，产生这种流动的粒子在向下游平移时保持侧向固定，只有流体被侧向转移。因此，由于粒子保持惯性聚焦，可以以与下游分析（如流式细胞术）兼容的方式在粒子周围进行操作。我们将这种粒子诱导的转移应用于刚性微粒子以及可变形细胞周围的流体切换和混合。这种仅需要简单通道几何形状且无需外部力即可运行的输运现象，为在高流速下进行流体转移提供了一种实用方法，具有广泛的应用，包括样品制备、流动反应和传热。
