Hydrodynamic spin-orbit coupling in asynchronous optically driven micro-rotors

TL;DR

This paper demonstrates a novel optical method to study hydrodynamic interactions between micro-rotors, revealing universal spin-orbit coupling effects in low Reynolds number fluids.

Contribution

It introduces a new optical technique to observe asynchronous micro-rotors and develops an analytical model explaining their hydrodynamic coupling.

Findings

Neighboring particles orbit each other with spin-dependent angular velocity

Analytical model accurately predicts observed dynamics

Universal hydrodynamic spin-orbit coupling identified in low Reynolds fluids

Abstract

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 developed 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 results in a universal hydrodynamic spin-orbit coupling. Our findings are of significance for the understanding and development of far-from-equilibrium materials.