Precise manipulation of microscopic objects is essential in many areas of science and technology, including advanced imaging, cellular biology, microfluidics, microrobotics, and materials assembly. Among these capabilities, achieving contact-free and object-agnostic three-dimensional (3D) rotation at the micro- and nanoscale remains especially challenging. Conventional micromanipulation methods often struggle in highly viscous environments, where diffusion is suppressed and force-based approaches become less effective. In addition, many existing techniques require specific material properties, engineered particle geometries, or complex external fields, which limits their versatility and practical use.
In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Moritz Kreysing and Dr. Fan Nan from Institute of Biological and Chemical Systems, Karlsruhe Institute of Technology, Germany, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, UK, and co-workers have introduced an opto-thermoviscous strategy that enables robust 3D rotation of microscopic objects by simply scanning a focused laser spot within a two-dimensional plane. By doing so, they generate three-dimensional helical thermoviscous flows in highly viscous media, allowing stable out-of-plane rotation, spinning, transport, and trapping of a wide variety of micro-objects. More importantly, the researchers discovered an opto-hydrodynamic focusing effect that drives particles toward a defined height, making the manipulation process more stable and controllable. Their method is compatible with a broad range of structures, including nano-printed tiles, stained biological cells, self-assembled particle clusters, and even perfectly round homogeneous spheres, demonstrating an unusual degree of material and shape independence.
The new method is centered on thermoviscous flows generated by a mildly heating infrared laser beam that is rapidly scanned across a fluid sample. Unlike traditional optical trapping systems, which often rely on particles to act as handles or to possess specific optical properties, this approach works through the flow field induced in the surrounding viscous medium. By carefully designing the scan geometry, direction, and timing, the scientists were able to tune the balance between in-plane transport and out-of-plane rotational components, thereby producing fully 3D helical flows. Through symmetry-based scan design, they further decoupled rotation from unwanted lateral drift, achieving stable spinning with positional fluctuations below 200 nm.
These scientists summarize the operational principle of their method:
"We introduce an opto-thermoviscous strategy that scans a focused laser spot within a two-dimensional plane to robustly generate 3D helical thermoviscous flows within highly viscous media."
They further emphasize one of the key discoveries of the work:
"We further report on the discovery of opto-hydrodynamic focusing that converges a spiral motion to a defined particle height."
By exploiting these helical flows, the team demonstrated not only continuous and stepwise rotational control, but also the ability to manipulate multiple types of microscopic objects in a highly reconfigurable manner. Particularly striking is that the technique remains effective even for perfectly spherical particles, which are normally difficult to rotate in a controlled way because they lack geometric asymmetry. The researchers also showed that the method can rotate suspended assemblies, release and reorient nano-fabricated microstructures, and expose hidden biological features, such as a yeast cell bud that would otherwise remain outside the imaging plane.
One of the most promising applications of the technique lies in microscopy. Because conventional 3D optical imaging suffers from much lower axial resolution than lateral resolution, rotating a sample into different orientations can reveal structural details that are otherwise obscured. Leveraging the kinematic nature of thermoviscous manipulation, the researchers combined stepwise particle rotation with volumetric microscopy and multi-view image fusion. In proof-of-concept experiments on HCT116 cells, this approach resolved two distinct nuclei that could not be separated in a conventional single-view confocal dataset, thereby improving the effective 3D imaging resolution.
The scientists highlight the broader significance of their findings:
"Conceptually, this elevates thermoviscous flows from planar transport to symmetry-engineered volumetric actuation, delivering robust, material-agnostic, out-of-plane rotational control and sheathless hydrodynamic focusing for all-optical micromanipulations and augmented microscopy."
"The presented advances establish a new paradigm in microscale flow control for micromanipulation in the life and engineering sciences, and constitute a significant step toward a future of generally applicable 3D microrobotics."
This work opens new possibilities for all-optical micromanipulation in environments where conventional approaches perform poorly. By turning the usual difficulty of high viscosity into an advantage, the reported strategy provides a powerful new route for precise 3D control of arbitrary micro-objects. The technique may find future applications in biological imaging but also next-generation microrobotic systems and advanced microscopy workflow.
