Polarization of active Janus particles
  We study the collective motion of Janus particles in a temperature or concentration gradient. Because of the torque exerted by an external or self-generated field, the particles align their axis on this gradient. In a swarm of self-driven particles, this polarization enhances the interactiondriven confinement. Self-polarization in a non-uniform laser beam could be used for guiding hot particles along a given trajectory. 
Thomas Bickel, Guillermo Zecua, and Alois W\"urger
cond-mat.soft