Researchers at New York University (NYU) have devised a system of asynchronous, optically driven micro-rotors that could be used to study far-from-equilibrium phenomena such as turbulent weather and biosystems. The advancement could potentially be used to replicate natural phenomena in engineered systems. In vortical flows, which are found in both meteorological and biological systems, particles move into orbital motion in the flow generated by their own rotation, resulting in a range of complex interactions. To better understand these dynamics, the researchers sought to replicate vortical flows at their most basic level. They created a system to move micro-particles using micro-rotors and a laser beam. Direct observation of hydrodynamic coupling between artificial micro-rotors has been restricted by the details of the drive that is used, either through synchronization (using external magnetic fields) or confinement (using optical tweezers). The NYU system is enabled by a tweezing-free optical field. The researchers designed a force-free torque field using a collimated beam of circularly polarized light and developed a synthetic route for birefringent, silica-coated colloids to show the spinning of hundreds of micro-particles using photonic angular momentum. They systematically quantified the micro-rotors’ optical and hydrodynamic properties. Unlike previous synthetic micro-rotor systems, the particles rotated asynchronously in the optical torque field while freely diffusing in the plane. The researchers also found that the rotating particles affected each other’s orbital motion. Analysis of the particles’ spinning rates revealed that pairs of rotating particles mutually advected one another, and that their translation and rotation were coupled hydrodynamically. The coupling was geometric, indicating that it could potentially have general application in active systems, from living organisms to robotic systems. For example, the researchers found similarities in their system to the dynamics observed by other scientists in “dancing” algae, that is, in algae groupings that move in concert with each other. AI impression of orbiting particle spun by a rotating light beam. Courtesy of Matan Yah Ben Zion. “The spins of the synthetic particles reciprocate in the same fashion as that observed in algae — in contrast to previous work with artificial micro-rotors,” said Matan Yah Ben Zion, a doctoral student at the time of the work and now a researcher at Tel Aviv University. Synthetically, and on the micron scale, the researchers successfully reproduced an effect that is seen in living systems, he said. The NYU system could be used to investigate isotropic rotating ensembles with broken time-reversal symmetry and parity in order to shed light on new material properties theoretically predicted in active matter. These include odd viscosity and quantum hall fluids. Free optical rotors using nonspherical particles could also be used to study the effect of morphology and steric interactions in tandem with hydrodynamic coupling. Further, the NYU system of optical rotors, if combined with rotors driven by an external magnetic field, could enable the experimental study of ensembles of counter-rotating particles. In these mechanisms, optical rotors rotate independently from the magnetic rotors. Experimental investigation of an ensemble of counter-rotors could expand scientific understanding of far-from-equilibrium states of matter. “Collectively, these findings suggest that the ‘dance of algae’ can be reproduced in a synthetic system, better establishing our understanding of living matter,” Ben Zion said. “Living organisms are made of materials that actively pump energy through their molecules, which produce a range of movements on a larger cellular scale. “By engineering cellular-scale machines from the ground up, our work can offer new insights into the complexity of the natural world.” The research was published in Nature Communications (www.doi.org/10.1038/s41467-023-39582-3).