Trochoidal trajectories of self-propelled Janus particles in a diverging   laser beam
  We describe colloidal Janus particles with metallic and dielectric faces that swim vigorously when illuminated by defocused optical tweezers without consuming any chemical fuel. Rather than wandering randomly, these optically-activated colloidal swimmers circulate back and forth through the beam of light, tracing out sinuous rosette patterns. We propose a model for this mode of light-activated transport that accounts for the observed behavior through a combination of self-thermophoresis and optically-induced torque. In the deterministic limit, this model yields trajectories that resemble rosette curves known as hypotrochoids. 
Henrique Moyses, Jeremie Palacci, Stefano Sacanna and David G. Grier
cond-mat.soft physics.optics