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

Area of Science:

• Soft Matter Physics
• Active Matter Systems
• Hydrodynamics

Background:

• Vortical flows of rotating particles are crucial in diverse systems, from molecular machines to atmospheric dynamics.
• Previous studies on artificial micro-rotors were limited by synchronization or confinement methods, hindering direct observation of hydrodynamic coupling.

Purpose of the Study:

• To develop a novel active system for observing the interplay of rotation and translation in free rotors.
• To investigate the hydrodynamic interactions between freely diffusing micro-rotors without external confinement or synchronization.

Main Methods:

• Utilizing a non-tweezing circularly polarized laser beam to simultaneously rotate hundreds of silica-coated birefringent colloids.
• Observing asynchronous rotation and free diffusion of particles in a 2D plane.
• Deriving an analytical model in the Stokes limit for pairs of spheres to explain observed dynamics.

Main Results:

• Hundreds of micro-rotors were simultaneously rotated asynchronously by a circularly polarized light beam.
• Neighboring particles were observed to orbit each other with angular velocities dependent on their spins.
• A universal hydrodynamic spin-orbit coupling was identified, stemming from the geometry of low Reynolds number fluid flow.

Conclusions:

• The developed system enables direct observation of rotation-translation coupling in free rotors.
• The findings demonstrate a universal hydrodynamic spin-orbit coupling in micro-rotor systems.
• This research contributes to understanding far-from-equilibrium materials and micro-scale fluid dynamics.