Microswimmers That Change Their Stroke
Bacteria push and algae pull, as shown in the figure. And for decades, every artificial “microswimmer” built in a laboratory has been locked into one such “swimming style” the moment it was fabricated. Now, physicists at Leipzig University, together with colleagues at Charles University in Prague, have created the first synthetic microswimmer whose hydrodynamic identity can be reprogrammed on the fly, while the particle keeps swimming. The results were published in Nature Communications.
Microswimmers are tiny objects, typically only a few micrometers in size, that move through fluids. They include living microorganisms as well as engineered particles. As they move, they stir up characteristic flow patterns in the liquid around them, and these flows govern how swimmers interact with walls, with currents and with each other. Physicists classify swimmers by these flow signatures: “pushers”, such as E. coli bacteria, push fluid backwards as they swim; “pullers”, such as the green alga Chlamydomonas, draw fluid in from the front; neutral swimmers generate only a weak flow far from the particle. Until now, changing from one type to another meant building a different particle.
The Leipzig team, led by Prof. Frank Cichos, achieved dynamic control over this identity using light. Their swimmers are micrometer-sized transparent spheres sparsely decorated with 10-nanometre gold particles. Illuminated by a laser whose beam is shaped with a spatial light modulator, the gold nanoparticles act as tiny, individually addressable heat sources. By projecting tailored intensity patterns onto the particle surface and updating them in real time as the particle moves, the researchers sculpt the temperature landscape on the sphere. This temperature pattern drives the particle forward due to thermophoresis and, at the same time, determines the flow field around it. Changing the light pattern changes the flow. The same particle can be a pusher one moment and a puller the next. Measurements of the flow around the particles confirmed that the generated flow fields match hydrodynamic theory across the full range of swimming modes.
Which swimming style is the most efficient?
Being able to dial in any swimming style also opens a new way to ask which style is best. Because the optical power absorbed by the swimmer can be measured directly, the team could determine the true energetic cost of each mode, something that has rarely been possible for artificial swimmers.
“Textbook theory predicts that the neutral swimmer should be the most efficient, since it wastes the least energy stirring the surrounding fluid. However, we managed to show that, at equal swimming speed, the pusher requires the least power. The reason is a simple constraint: a laser can heat the particle surface but never cool it, and this restriction changes which temperature pattern is the most economical. The best swimming strategy, it turns out, depends not only on hydrodynamics but on the physical mechanism that drives the swimmer,” says Viktor Holubec of Charles University, who contributed to the theoretical derivation.
From controlling swimmers to designing flows
The bigger promise of the work, however, lies in what becomes possible when a swimmer's character is no longer fixed. Because the illumination is updated continuously, the swimming style could be adjusted by feedback or by machine learning, so that a particle adapts its stroke to crowding, confinement, or a changing environment. Addressing several swimmers independently would allow mixtures of pushers and pullers with tunable composition, an experimental playground for the collective behaviour of active matter, from bacterial turbulence to swarming, that is difficult to realize with conventional, fixed swimmers.
Looking further ahead, groups of independently controlled particles could be used not just to move but to design the surrounding flow field itself, specifying its structure in space and time on demand. Such programmable microscale flows are relevant wherever mixing, transport, or chemical reactions in tiny volumes need to be controlled, and could one day contribute to adaptive microreactors whose flow patterns adjust to the reaction conditions. In this sense, the work shifts microswimmer research from studying predefined flow fields towards actively engineering hydrodynamic interactions with light.
Original
Study:
Lisa Rohde, Gordei Anchutkin, Viktor Holubec & Frank
Cichos, Programmable hydrodynamics of active particles, Nature Communications
17, 9406 (2026). DOI
10.1038/s41467–026–77281-x
