Active Torque Dipoles Create a Nonequilibrium Cholesteric Phase in Wet and Dry Active Matter*

Despite the fact that most biological active components are intrinsically chiral (e.g. actomyosin, swimming bacteria), many active models still disregard chiral processes and describe active units simply as force dipoles.
We address the effects of chirality by modeling a collection of active torque dipoles generating chiral active stresses. Combining linear stability analysis and numerical simulations (Lattice Boltzmann and finite differences) we study quasi-1D and 2D systems of active torque dipoles both in dry active matter and suspended in a Newtonian fluid. In both cases, the increase of activity leads to a spontaneous breaking of chiral symmetry giving rise to a nonequilibrium transition into a cholesteric-like phase. Such chiral phase self-assembles in the absence of any thermodynamic interactions favoring cholesteric order. We characterize the transition by using a twist parameter that measures the length scale of twisting (i.e. pitch) and highlight the differences between the dry and wet cases.
Finally, we discuss the implications of our results to biologically-relevant active systems.