Peng Yi, Lai Lipeng, Tai Yi-Shu, Zhang Kechun, Xu Xinliang, Cheng Xiang
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Beijing Computational Science Research Center, Beijing 100193, China.
Phys Rev Lett. 2016 Feb 12;116(6):068303. doi: 10.1103/PhysRevLett.116.068303. Epub 2016 Feb 10.
Active fluids such as swarming bacteria and motile colloids exhibit exotic properties different from conventional equilibrium materials. As a peculiar example, a spherical tracer immersed inside active fluids shows an enhanced translational diffusion, orders of magnitude stronger than its intrinsic Brownian motion. Here, rather than spherical tracers, we investigate the diffusion of isolated ellipsoids in a quasi-two-dimensional bacterial bath. Our study shows a nonlinear enhancement of both translational and rotational diffusions of ellipsoids. More importantly, we uncover an anomalous coupling between particles' translation and rotation that is strictly prohibited in Brownian diffusion. The coupling reveals a counterintuitive anisotropic particle diffusion, where an ellipsoid diffuses fastest along its minor axis in its body frame. Combining experiments with theoretical modeling, we show that such an anomalous diffusive behavior arises from the generic straining flow of swimming bacteria. Our work illustrates an unexpected feature of active fluids and deepens our understanding of transport processes in microbiological systems.
诸如群体细菌和能动胶体等活性流体展现出与传统平衡态物质不同的奇异特性。作为一个特殊例子，浸没在活性流体中的球形示踪剂表现出增强的平动扩散，比其固有的布朗运动强几个数量级。在此，我们研究的不是球形示踪剂，而是孤立椭球体在准二维细菌浴中的扩散。我们的研究表明椭球体的平动和转动扩散都有非线性增强。更重要的是，我们发现了粒子平动和转动之间一种在布朗扩散中严格被禁止的反常耦合。这种耦合揭示了一种违反直觉的各向异性粒子扩散，即椭球体在其自身坐标系中沿短轴扩散最快。通过将实验与理论建模相结合，我们表明这种反常扩散行为源于游泳细菌的一般应变流。我们的工作阐明了活性流体一个意想不到的特征，并加深了我们对微生物系统中输运过程的理解。
