Simulations of Active Nematics
We developed a simulation model for active nematics, motivated by recent experiments in the lab of Zvonimir Dogic (Brandeis). They developed a minimal in vitro system that reproduces behaviors formally found only in biological organisms, such as streaming flows and cilia-like beating .
The system contains microtubules, a depletion agent which causes bundling, and motor protein complexes that push pairs of microtubules past one another. The action of motor proteins causes bundles to undergo longitudinal extension until they eventually break, while the depletion forces cause nearby bundles to merge laterally. In bulk, bundles reach a steady-state density and form an active gel, characterized by large scale flows . When the system is confined to two dimensions (on an oil-water interface), the bundles form a nematic phase (Fig. 1). However, due to bundle extensility, the uniform nematic state is unstable to the formation of topological defect pairs (Fig. 2). Defect pairs spontaneously appear, unbind, move through the sample, and annihilate, producing a seemingly chaotic steady-state. Since defect behaviors dictate the material properties, our group developed an algorithm to identify and track defects, both from experimental images and computational model output.
In the computational model hard spherocylinders which undergo extension from both ends at an ATP-concentration-dependent rate represent the extensile bundles observed in experiments. Using the defect tracking algorithm, we have measured lifetime, motility and interactions of defects in the active nematics experiments and computational trajectories. This data shows that, although that active nematics are inherently unstable against bend fluctuations , they retain long range order. In particular, motile +1/2 defect which have polar symmetry interact with each other and form a supra-molecular dynamical phase of highly-order defects. Such a defect-ordered phase has not been previously observed in active systems; its emergence in both the experimental and computational systems suggest that it may be a generic feature of active nematics. The work is described in 
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- DeCamp, S.J., G.S. Redner, A. Baskaran, M.F. Hagan, and Z. Dogic, Orientational order of motile defects in active nematics. arXiv:1501.06228, 2015.