Europhysics Letters PREPRINT Colloidal hydrodynamic coupling in concentric optical vor- tices

Abstract. – Optical vortex traps created from helical modes of light can drive fluid-borne colloidal particles in circular trajectories. Concentric circulating rings of particles formed by coaxial optical vortices form a microscopic Couette cell, in which the amount of hydrodynamic drag experienced by the spheres depends on the relative sense of the rings ’ circulation. Tracking the particles ’ motions makes possible measurements of the hydrodynamic coupling between the circular particle trains and addresses recently proposed hydrodynamic instabilities for collective colloidal motions on optical vortices. A beam of light with helical wavefronts [1] focuses to a ring-like optical trap known as an optical vortex [11, 14, 24], which not only traps micrometer-scale objects, but also exerts torque on them [10,13,25]. Since their introduction nearly a decade ago, optical vortices hav