Vortical flows of rotating particles describe interactions ranging from molecular machines to atmospheric dynamics. Yet to date, direct observation of the hydrodynamic coupling between artificial micro-rotors has been restricted by the details of the chosen drive, either through synchronization (using external magnetic fields) or confinement (using optical tweezers). Here we present a new active system that illuminates the interplay of rotation and translation in free rotors. We develop a non-tweezing circularly polarized beam that simultaneously rotates hundreds of silica-coated birefringent colloids. The particles rotate asynchronously in the optical torque field while freely diffusing in the plane. We observe that neighboring particles orbit each other with an angular velocity that depends on their spins. We derive an analytical model in the Stokes limit for pairs of spheres that quantitatively explains the observed dynamics. We then find that the geometrical nature of the low Reynolds fluid flow results in a universal hydrodynamic spin-orbit coupling. Our findings are of significance for the understanding and development of far-from-equilibrium materials.

旋转粒子的涡流描述了从分子机器到大气动力学的相互作用。然而迄今为止，对人造微转子之间流体动力耦合的直接观察受到所选驱动器细节的限制，无论是通过同步（使用外部磁场）还是限制（使用光镊）。在这里，我们提出了一种新的主​​动系统，它阐明了自由转子中旋转和平移的相互作用。我们开发了一种非镊子圆偏振光束，可同时旋转数百个涂有二氧化硅的双折射胶体。粒子在光学扭矩场中异步旋转，同时在平面中自由扩散。我们观察到相邻粒子以取决于它们自旋的角速度相互绕转。我们推导出球体对斯托克斯极限下的分析模型，定量解释了观察到的动力学。然后我们发现低雷诺流体流动的几何性质导致了通用的流体动力自旋轨道耦合。我们的发现对于理解和开发非平衡材料具有重要意义。