Defect Unbinding in Active Nematics

Suraj Shankar, Sriram Ramaswamy, M Cristina Marchetti, Mark John Bowick

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Abstract

We formulate the statistical dynamics of topological defects in the active nematic phase, formed in two dimensions by a collection of self-driven particles on a substrate. An important consequence of the nonequilibrium drive is the spontaneous motility of strength +1/2 disclinations. Starting from the hydrodynamic equations of active nematics, we derive an interacting particle description of defects that includes active torques. We show that activity, within perturbation theory, lowers the defect-unbinding transition temperature, determining a critical line in the temperature-activity plane that separates the quasi-long-range ordered (nematic) and disordered (isotropic) phases. Below a critical activity, defects remain bound as rotational noise decorrelates the directed dynamics of +1/2 defects, stabilizing the quasi-long-range ordered nematic state. This activity threshold vanishes at low temperature, leading to a reentrant transition. At large enough activity, active forces always exceed thermal ones and the perturbative result fails, suggesting that in this regime activity will always disorder the system. Crucially, rotational diffusion being a two-dimensional phenomenon, defect unbinding cannot be described by a simplified one-dimensional model.

Original languageEnglish (US)
Article number108002
JournalPhysical Review Letters
Volume121
Issue number10
DOIs
StatePublished - Sep 7 2018

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ASJC Scopus subject areas

  • Physics and Astronomy(all)

Cite this

Shankar, S., Ramaswamy, S., Marchetti, M. C., & Bowick, M. J. (2018). Defect Unbinding in Active Nematics. Physical Review Letters, 121(10), [108002]. https://doi.org/10.1103/PhysRevLett.121.108002