Tunable 3D optohydrodynamic torques from optical phase gradient-driven colloidal assemblies.

Optohydrodynamic manipulation offers a versatile, noninvasive, and reconfigurable approach for controlling microscopic objects. Here, we present a strategy for generating tunable three-dimensional optohydrodynamic torques through phase gradient-driven nanoparticle assemblies. Using programmable optical ring vortices (Laguerre-Gaussian beams), we assemble and rotate colloidal clusters with certain particle numbers, whose induced hydrodynamic flows apply switchable in-plane and out-of-plane torques on target particles. Torque control is achieved via two mechanisms: (i) reversing the handedness of circular polarization to break rotational symmetry and (ii) displacing optical ring vortices and modulating cluster rotation speed. These complementary controls provide robust, high-resolution torques tuned in arbitrary directions. As a proof of concept, we demonstrate full three-dimensional orientation control of a single cell. This framework greatly expands the capabilities of optohydrodynamic systems by explicitly incorporating light-driven interparticle interactions and establishes a foundation for advanced applications in biophysics, microrobotics, and biomedical engineering.