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.

涡度是流体元件局部旋转速率的量度，是不可压缩流的驱动因素。在粘性流体中，为整体流动提供动力需要从边界连续注入涡量，以抵消粘性的扩散效应。在这里，我们通过悬浮近似圆柱形的颗粒并通过磁力驱动它们以中间范围的雷诺数旋转来从内部驱动流动。我们发现单个粒子会在其周围产生局部三维涡旋区域（我们称之为涡小体），从而驱动许多显着的行为。颗粒形状的轻微不对称会使涡流变形并导致颗粒自我推进。 vortlet 之间的相互作用同样丰富，产生束缚动态状态。当大量小涡相互作用时，它们会自发地形成集体移动的群体。这些鸟群在推进、分裂和合并时保持一致。如果添加足够的粒子以使流动室饱和，则会形成均匀的三维活性手性流体小涡，可以通过重力或流动室边界对其进行操纵，从而产生活跃的集体动力学。我们的研究结果证明了合成活性物质的惯性状态，为无感知系统中三维植绒的定量研究提供了受控物理系统，并为三维活性手性流体的研究建立了平台。