The periodic motion of active nematic fluids and the role of particle shape.

Active nematic materials are non-equilibrium fluids composed of rod-like subunits that convert local internal (e.g. chemical) energy into large-scale mechanical motion, which generates a self-stirring fluid. One prominent example of an active nematic utilizes rod-like microtubules—components of the cytoskeleton—and kinesin molecular motors. The microtubules are densely packed in a 2D layer and form an ordered nematic phase. The kinesin motors cross-link the microtubules and cause them to slide relative to one another, creating an extensile flow. The nematic phase contains point-like topological defects that move around one another in a typically chaotic fashion. These defects can be viewed as virtual rods that are responsible for stirring the fluid. The geometric shape of the nematic subunits plays a crucial role in determining whether the defects move periodically. To understand the importance of the geometric shape of these particles and its role in defect behavior, we conducted simulations of the nematohydrodynamic equations, varying the particle aspect ratio.  We observed that when the particles are sufficiently elongated there is a transition from chaotic motion of defects to periodic motion; i.e., if the particles are sufficiently circular, the periodic behavior of the defects disappears. Thus, when modelling active nematics, it is important to use the correct aspect ratio to achieve the proper qualitative behavior.