Helical Flow Enables Opto-Thermoviscous Microrotation for Multiview 3D Microscopy

Micromanipulating tiny objects in 3D—especially in highly viscous environments—has long been a stubborn challenge. Force-based approaches often lose effectiveness as diffusion slows and motion becomes heavily damped. Optical trapping can help, but it typically depends on particle properties or engineered geometries. Now, researchers led by Prof. Moritz Kreysing and Dr. Fan Nan report an all-optical alternative that sidesteps contact and object-specific constraints.

Their strategy relies on an opto-thermoviscous effect: a mildly heating infrared laser spot is rapidly scanned across a two-dimensional plane within a microfluidic sample. Although the laser path is planar, the resulting flow field is three-dimensional. By carefully tuning scan geometry and timing, the team generates helical thermoviscous flows that simultaneously transport particles laterally and drive out-of-plane rotation.

Crucially, they uncover an opto-hydrodynamic focusing phenomenon. This mechanism converges particles toward a defined height, stabilizing their motion and making the manipulation more repeatable in viscous microenvironments. Using symmetry-based scan design, the researchers also decouple rotational control from unwanted lateral drift, achieving positional fluctuations below 200 nm.

To demonstrate robustness, the approach works not only for structured micro-objects but also for perfectly spherical, homogeneous particles—an especially notable feat because spheres lack geometric asymmetry typically needed for controlled spinning. The team shows spinning, transport, and trapping of nano-printed tiles, stained biological cells, self-assembled particle clusters, and suspended assemblies, including reorientation and release of nano-fabricated microstructures.

Beyond rotation, the method offers functional reconfiguration. For example, it can expose hidden biological features by moving cells relative to the imaging plane—such as revealing a yeast cell bud that would otherwise remain outside the visible focus. This kind of “spatial rescue” is particularly valuable when conventional imaging struggles with limited axial resolution.

The most exciting prospect may be microscopy. Because axial resolution in confocal imaging is poorer than lateral resolution, changing orientations can reveal structures that remain indistinct in a single viewpoint. The researchers combine stepwise thermoviscous rotation with volumetric microscopy and multi-view image fusion.

In proof-of-concept experiments on HCT116 cells, their workflow resolves two distinct nuclei that conventional single-view confocal datasets cannot separate. The result suggests a new paradigm: thermoviscous flows evolve from planar transport into symmetry-engineered volumetric actuation, enabling material-agnostic, out-of-plane rotational control with sheathless hydrodynamic focusing.

Overall, the work offers a viral promise for microscale engineering and life science tools: robust 3D micromanipulation driven purely by scan-defined laser heating, turning high viscosity from a limiting factor into an operational advantage.