Helical opto-thermoviscous flows drive out-of-plane rotation and particle spinning in a highly viscous micro-environment

Contact-free and object-agnostic three-dimensional (3D) rotation remains a challenge at both the micro and nanoscale, with broad relevance to advanced imaging, biology, microrobotics, and materials science. Specifically, precise 3D rotation is desirable in diffusion-suppressing environments, where conventional micromanipulation methods fail. Here we introduce an opto-thermoviscous strategy that scans a focused laser spot within a two-dimensional plane to robustly generate 3D helical thermoviscous flows (TVFs) within highly viscous media. We further report on the discovery of opto-hydrodynamic focusing that converges a spiral motion to a defined particle height. By exploiting symmetry relations, we use rational design to decouple out-of-plane rotation from lateral displacements, leading to stable spinning with positional fluctuations below 200 nm, and demonstrate compatibility with a broad range of microstructures, from nano-printed tiles to stained biological cells, and even perfectly round homogenous spheres. Finally, leveraging the kinematic nature of thermoviscous manipulations, we demonstrate how stepwise rotation, alternated with 3D volumetric microscopy, can be combined with established multi-view image fusion strategies to increase resolution in biological imaging. ... mehrConceptually, this elevates TVFs from planar transport to symmetry-engineered volumetric actuation, delivering robust, material-agnostic, out-of-plane rotational control and sheathless opto-hydrodynamic focusing for all-optical micromanipulations and augmented microscopy.