Defect interactions and dynamics control ordering in self-propelled rod active nematics.

Myxococcus xanthus is a soil bacteria which lives in groups of hundreds of thousands to millions of cells and are able change their colony morphology in response to environmental cues. Individuals can tune their own motility properties such as speed and reversal rate which in turn drives the colony morphology changes seen on a scale much larger than the individual cells. When nutrients are prevalent they spread out into a thin layer of densely packed cells on the surface of a gel, forming an active nematic liquid crystal. In order to understand the detailed effects of tuning reversal rate and speed on this material we developed an agent-based simulation for M. xanthus as a system of self-propelled flexible rods. We then identify and study topological defects (locations where cell alignment is undefined) in the simulations and experiments which are signatures of active nematic materials. We find that many of the defect properties and interactions measured in the experiments are captured by this kind of modeling with wild-type cell motility parameters. Then by varying motility parameters we found that the order in the system is driven by the balance between creation and decoupling rate of defect pairs due to activity, and defect annihilation rate due to noise and elasticity.