&Bullet; physics 14, p68

A self-propelled particle trapped in a potential source defined by energy availability has a unique swimming pattern that results from hidden currents in the fluid in which it is swimming.

Birds, bison, and bacteria all take energy from their environment to move around. When these living beings – known to physicists as “active” systems – come together in swarms, herds and swarms, they show behaviors that researchers want to recreate in reconfigurable materials, for example. However, to take full advantage of this behavior, physicists need to understand the basic rules that govern idealized versions of these systems. Now Frank Cichos from the University of Leipzig and colleagues add another rule to the list of known ones by revealing how a single active particle behaves when it is included in an activity-potential wave. [1] . The team says their results could be used to create active ratchets.

In their experiments, the team used a gold-coated colloidal particle that “floated” when exposed to laser radiation (see Synopsis: Particles Propelled Like Rockets). They placed the particle in a glass-walled, water-filled 2D channel. Then they limited its movement by turning off the laser when the particle hit a predefined “red line” that acted like the wall of a potential source but did not exert any force on the particle. They didn’t turn the laser back on until the passive motion of the particle drove it back into the well.

The team found that the particle’s swimming direction was linked to its position in the borehole, so it randomly moved in the center of the borehole, but in a preferred direction near the red line “wall” of the borehole. The researchers predict that this behavior – which they call polarization – results from hidden particle-induced currents in the water. Now that they understand how a single active particle moves in an activity, they plan to study how multiple particles behave under similar conditions, says Cichos.

–Katherine Wright

Katherine Wright is assistant editor of physics.


  1. NA Söker et al., “How activity landscapes polarize micro-swimmers without alignment forces”, Phys. Rev. Lett.126, 228001 (2021).

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