Circle swimming in biological and synthetic active matter. Near surface dynamics show swimming in circular trajectories for E. coli cells (A) and V. cholerae cells (B). (A) Superimposed microscopy images and (B) tracked trajectories. Inset in (B) shows bright-field image of V. cholerae. (A) Reprinted with permission from [130]. Copyright©2006 Elsevier. (B) Reprinted with permission from [132]. Copyright©2014 Springer Nature. (C) Janus colloids coated with a Ni/Ti cap and a protective SiO₂ layer and sandwiched between two coverslips are energised with an AC vertical electric field and perform circular trajectories with tunable radius R resulting from an externally applied in plane rotating magnetic field. The image shows reconstructed trajectories. The inset shows an experimental image revealing that the particles spontaneously orient in opposite directions along to the magnetic field. Reprinted with permission from [135]. Copyright©2017 National Academy of Sciences. (D) Asymmetric Zn/Au rods (inset) show self-electrophoresis exhibiting four different modes (ballistic, linear, circular, helical). By controlling UV light intensity and fuel concentration, the rods can be transformed from ballistic motion to continuous rotating motion, and by adjusting the angle of incident light, these rods can be switched from circular motion to spiral, and eventually to linear motion. The image shows circular motion mode. Reprinted with permission from [139]. Copyright©2020 American Chemical Society. (E) Asymmetric L-shaped colloids exhibit self-phoretic circular motion where the radius of the trajectory depends only on the shape of the object, but is unaffected by the propulsion strength. Reprinted figure with permission from [129]. Copyright©2013 the American Physical Society.