1. Simulations of Active Nematics
G. Redner, A. Baskaran, M. F. Hagan
Brandeis MRSEC ( )
Simulations of a model for microtubule(MT)-based active nematics capture experimentally observed defect dynamics. The image on the right shows three sequential images from experimental system in which +½ and -½ defects are created through a bending stability and subsequently separate. The image on the left shows similar defect behaviors observed in dynamical simulations of a coarse grain model for the MT-based active nematics.
We simulate a generalized coarse-grained representation of an active nematic confined to a 2D plane. In order to simulate experimentally relevant length- and time-scales, the elemental ‘particle’ in our simulation represents an extensile bundle, which is modeled as a spherocylindrical rod with an aspect ratio of Two nearby rods interact by excluded volume alone, modeled by the WCA potential acting between the point of closest approach of the two particles. The simulation employs Brownian dynamics to propagate trajectories forwards in time. Currently the simulations do not consider hydrodynamic interactions (HI) between particles, since these are short ranged due to the presence of the planar surface. We will consider the effects of HI in later simulations. In the absence of activity this system is identical to an equilibrium nematic system. Activity enters the model through the evolution of the rods themselves: to model the extensile nature of bundles, each particle’s length is increased by a fixed amount at each simulation timestep, at an activity-dependent rate (which roughly maps to the experimental ATP concentration). When a rod’s length reaches a fixed limit, it is broken in half and replaced by two rods each of half its length. To model bundle merging events, and also to maintain a fixed particle count and a nearly-fixed area fraction for the system, when a splitting event takes place we also eliminate a rod from elsewhere in the system.