近日，美国芝加哥大学的William T. M. Irvine及其研究小组与美国纽约熨斗研究所的Michael J. Shelley等人合作并取得一项新进展。经过不懈努力，他们对中等雷诺数自旋粒子的自推进、聚集和手性活性相进行研究。相关研究成果已于2024年10月8日在国际知名学术期刊《自然—物理学》上发表。

该研究团队通过悬浮近似圆柱形的粒子，并利用磁力驱动它们在中等雷诺数下旋转，从而从内部为流动提供动力。研究发现，单个粒子会在其周围产生一个局部的三维涡量区域——研究人员称之为涡子——它能引发一系列显著的行为。粒子形状的轻微不对称会扭曲涡子，并导致粒子自推进。涡子之间的相互作用同样丰富，能产生约束的动力学状态。

当大量涡子相互作用时，它们会自发形成集体移动的群。这些群在推进、分裂和合并时保持一致性。如果添加足够多的粒子以饱和流动室，则会形成一个均匀的三维活性手性涡子流体，该流体可通过重力或流动室边界进行操控，从而产生生动的集体动力学。

这一研究发现展示了合成活性物质的一个惯性状态，为非感知系统中三维集群行为的定量研究提供了一个受控物理系统，并为三维活性手性流体的研究建立了平台。

据悉，涡量，作为流体元局部旋转速率的度量，是不可压缩流动的驱动力。在黏性流体中，驱动整体流动需要不断从边界注入涡量，以抵消黏性的扩散效应。

附：英文原文

Title: Self-propulsion, flocking and chiral active phases from particles spinning at intermediate Reynolds numbers

Author: Chen, Panyu, Weady, Scott, Atis, Severine, Matsuzawa, Takumi, Shelley, Michael J., Irvine, William T. M.

Issue&Volume: 2024-10-08

Abstract: Vorticity, a measure of the local rate of rotation of a fluid element, is the driver of incompressible flow. In viscous fluids, powering bulk flows requires the continuous injection of vorticity from boundaries to counteract the diffusive effects of viscosity. Here we power a flow from within by suspending approximately cylindrical particles and magnetically driving them to rotate at Reynolds numbers in the intermediate range. We find that a single particle generates a localized three-dimensional region of vorticity around it—which we call a vortlet—that drives a number of remarkable behaviours. Slight asymmetries in the particle shape can deform the vortlet and cause the particle to self-propel. Interactions between vortlets are similarly rich, generating bound dynamical states. When a large number of vortlets interact, they spontaneously form collectively moving flocks. These flocks remain coherent while propelling, splitting and merging. If enough particles are added so as to saturate the flow chamber, a homogeneous three-dimensional active chiral fluid of vortlets is formed, which can be manipulated with gravity or flow chamber boundaries, leading to lively collective dynamics. Our findings demonstrate an inertial regime for synthetic active matter, provide a controlled physical system for the quantitative study of three-dimensional flocking in non-sentient systems and establish a platform for the study of three-dimensional active chiral fluids.

DOI: 10.1038/s41567-024-02651-5

Source: https://www.nature.com/articles/s41567-024-02651-5