The wide adoption of inertial microfluidics in biomedical research and clinical settings, such as rare cell isolation, has prompted the inquiry of its underlying mechanism. Although tremendous improvement has been made, the mechanism of inertial migration remains to be further elucidated. Contradicting observations are not fully reconciled by the existing theory, and details of the inertial migration within channel cross sections are missing in the literature. In this work, for the first time, we mapped the inertial migration pathways within channel cross section using high-speed imaging at the single-particle level. This is in contrast to the conventional method of particle streak velocimetry (PSV), which provides collective information. We also applied smoothed particle hydrodynamics (SPH) to simulate the transient motion of particles in 3D and obtained cross-sectional migration trajectories that are in agreement with the high-speed imaging results. We found two opposing pathways that explain the contradicting observations in rectangular microchannels, and the force analysis of these pathways revealed two metastable positions near the short walls that can transition into stable positions depending on the flow condition and particle size. These new findings significantly improve our understanding of the inertial migration physics, and enhance our ability to precisely control particle and cell behaviors within microchannels for a broad range of applications.

惯性微流体在生物医学研究和临床环境中的广泛采用，例如稀有细胞分离，促使对其潜在机制的探究。尽管已经取得了巨大的进步，但惯性迁移的机制仍有待进一步阐明。现有理论并未完全协调矛盾的观察结果，文献中缺少通道横截面内惯性迁移的细节。在这项工作中，我们首次使用单粒子水平的高速成像绘制了通道横截面内的惯性迁移路径。这与提供集体信息的传统粒子条纹测速 (PSV) 方法形成对比。我们还应用平滑粒子流体动力学 (SPH) 来模拟 3D 中粒子的瞬态运动，并获得与高速成像结果一致的横截面迁移轨迹。我们发现了两条相反的通路，可以解释矩形微通道中相互矛盾的观察结果，并且这些通路的力分析揭示了短壁附近的两个亚稳态位置，可以根据流动条件和颗粒大小转变为稳定位置。这些新发现显着提高了我们对惯性迁移物理学的理解，并增强了我们精确控制微通道内粒子和细胞行为的能力，以实现广泛的应用。我们发现了两条相反的通路，可以解释矩形微通道中相互矛盾的观察结果，并且这些通路的力分析揭示了短壁附近的两个亚稳态位置，可以根据流动条件和颗粒大小转变为稳定位置。这些新发现显着提高了我们对惯性迁移物理学的理解，并增强了我们精确控制微通道内粒子和细胞行为的能力，以实现广泛的应用。我们发现了两条相反的通路，可以解释矩形微通道中相互矛盾的观察结果，并且这些通路的力分析揭示了短壁附近的两个亚稳态位置，可以根据流动条件和颗粒大小转变为稳定位置。这些新发现显着提高了我们对惯性迁移物理学的理解，并增强了我们精确控制微通道内粒子和细胞行为的能力，以实现广泛的应用。