New frontiers in microfluidics: light channels to guide particles in liquids
Is it possible to precisely control the motion of particles and microscopic objects in liquid environments? Answering this question is one of the central challenges of microfluidics, the discipline that studies the behaviour of fluids at the micrometre scale, with key applications in the biomedical, diagnostic, and laboratory fields.
A new study published in Nature Communications, resulting from a collaboration between Politecnico di Torino, the Norwegian University of Science and Technology (NTNU), the University of Gothenburg, and the University of Münster, has demonstrated that it is indeed possible to precisely control the motion of particles on a liquid surface. This can be achieved using light alone, without the need to fabricate physical channels or apply hydrodynamic pressure to generate flows.
From an application perspective, this discovery could have a significant impact. In the future, the technique could be used, for example, to transport and position cells, bacteria or micro-particles in diagnostic tests; to mix or separate substances in laboratory-on-a-chip devices without pumps or valves; to assemble microscopic structures in a controlled manner; and to simplify biomedical and research devices by reducing the size, cost and complexity of traditional microfluidic systems.
The approach introduced by the authors of the study departs radically from the traditional methods currently used in microfluidics. The innovative idea lies in “designing” flow channels on a liquid surface directly under the microscope, using light as the only working tool. Specifically, the phenomenon relies on the use of tiny suspended particles - colloidal particles - made from special photoactive polymer materials (azopolymers). When illuminated, these particles change shape and set the surrounding fluid in motion, generating controllable flows. In this way, both the particles themselves and passive objects - such as microparticles, cells, and bacteria - can be transported along controlled paths, without the need for microfabricated structures such as pumps or valves, which often limit experimental flexibility and increase costs and space requirements in the laboratory.
The distinctive feature of the discovery is the precise control of flow direction, achieved not only by shaping the light beam, but also by exploiting a fundamental property of light: polarisation, which determines the direction of oscillation of the electric field of the light wave. It is precisely this characteristic that makes it possible to create inherently directional flows, setting the method apart from other optical manipulation techniques.
An additional advantage of the new approach is its natural integration with optical systems: since many experiments and analyses are already conducted under a microscope, controlling flows with light can be implemented directly within the same instrument, thus avoiding the use of separate and often bulky fluidic setups.
The research is the result of nearly three years of interdisciplinary work and stems from a collaboration initiated within the framework of the Geilo School, a historic international school of condensed matter physics. What began as an exploratory study driven by scientific curiosity led to an unexpected discovery, opening new lines of research in the fields of soft matter, colloid science, and optofluidics, with potential technological and biomedical implications.
“This work is a fine example of serendipity in research,” comments Emiliano Descrovi, Professor at the Department of Applied Science and Technology-DISAT and one of the study’s authors. “Our initial intention was to study the photo-deformations of individual azopolymer particles dispersed on a liquid surface. (Un)fortunately, it proved very difficult to isolate the particles individually, and we ended up illuminating many particles simultaneously with the laser under the microscope. We then realised that, under certain particle density conditions, a collective motion would emerge, generating a flow whose direction was controlled by the polarisation of light. The effect was very surprising because it had never been observed before and led us to abandon our original plan in order to understand the mechanisms more deeply. In the future, we hope to apply this new fluid transport system in biological contexts, in synergy with other well-known systems such as optical tweezers.”