Spontaneous chiralization of polar active colloids*

The development of engineered active colloids that harness the chemical energy of the environment to move has shown promising applications such as bioremediation, micro-mixing, micro-machinery and drug delivery. To mimic the intrinsic asymmetry of flagellates and other motile microorganisms, active colloids are designed with fore-aft asymmetric chemical activity, which define their polar axis and their preferential direction of motion. Recent experiments showed that Janus active particles can spontaneously break their polar symmetry and transition from a persistent Brownian motion to chiral trajectories. Using numerical simulations, we show that such spontaneous chiralization is a generic feature of polar active colloids. As observed in the experiments, the transition from polar to chiral symmetry results in the onset of active particle rotation and the emergence of circular trajectories. We find that the symmetry breaking instability is driven by the advection of a solute that interacts differently with the two portions of the particle surface and it occurs as through supercritical pitchfork bifurcation.