How Swimming Microorganisms Reach Their Targets Without Control

Spermatozoa, bacteria and other biological swimmers in nature can be modeled by using self-propelling, micron-sized particles. Researchers using this approach have so far focused on spherical and rod-like particles, but new experiments with L-shaped particles may give better insight into the many biological microorganisms that lack symmetry about their propulsion axis. As Felix Kümmel from the University of Stuttgart, Germany, and colleagues report in Physical Review Letters, the experiments show that such particles move in circles in open liquid, but follow a straighter path near walls.

Kümmel et al. carved the L-shaped particles from polymer and then selectively deposited gold on the underside of each object’s short arm. When a particle was placed in a solvent mixture and illuminated with light, the gold activated a local demixing of the solvent into its two components (water and lutidine), creating a tiny chemical gradient that propelled the particle in one direction. But unlike symmetric particles, which typically move in a random walk, the L-shaped particles rotated in either a clockwise or counterclockwise circle. Near a wall, however, the particles were seen to either bounce or surf along in one direction, depending on their angle of incidence.

The scientists explain that their experimental observations can be understood by considering the "viscous forces the fluid exerts on the particles: The particle experiences a velocity-dependent torque that leads to the circular motion, but near a boundary, such as a channel wall, the circular motion is disturbed by the steric particle-wall interaction." This system could be used to provide insight into how odd-shaped microorganisms move through narrow blood vessels or plant veins. -- Katherine Thomas

Paper:

Felix Kümmel, et al.: Circular Motion of Asymmetric Self-Propelling Particles, Phys. Rev. Lett. 110, 198302 (2013).