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

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 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.

Emergent phenomena in chiral active matter

“…Examples (see figure 1a-e) are rotating motor proteins in membranes [28] such as ATP synthase [29], circular swimming algae [30], co-rotating bacteria [31] and starfish embryos [32], and bound states of Volvox colonies [33]. Significant effort has been put in designing synthetic particles with spinning motion (see figure 1f-j), which include magnetically driven synchronously spinning colloidal [34][35][36] or larger particles [37,38] with a ferromagnetic moment, light-driven asynchronously spinning colloids [39], shaking grains [40] and vibrating robots [41], expanding nearly five decades of length scales. Such rotating particles allow for the systematic involvement of the rotational degrees of freedom as a continuation of active dynamics that exclusively utilise the translational degrees of freedom.…”

“…h Vaterite colloidal particles (σ ≈ 2 − 12µm) asynchronously rotate in circularly polarised light resulting from birefringence, leading to hydrodynamic spin-orbit coupling. Image taken from [39], licensed under CC BY. i 3D-printed granular gear-like rotors (D1 = 16mm, D2 = 21mm) with tilted bristles at the bottom can be brought into a state of active rotation powered by vertical vibration.…”

Hydrodynamically induced aggregation of two dimensional oriented active particles

“…4 Active membrane proteins act as either rotary proteins, such as ATP synthase, 5,6 or as shakers—particles that are not self-propelled but apply active forces on the membrane due to conformational changes, polymerization, or reorganization. 7 Artificial 2D systems are also widespread, such as active colloids driven by a chemical reaction, 8–10 by light 11,12 or by an external magnetic field. 13 Particle dynamics can be dominated by complex interactions ( e.g.…”