Hugues Chate

Hugues Chaté, Saclay

Understanding dense active nematics from microscopic models

We introduce and study in depth a Vicsek-style model of self-propelled particles with velocity reversals
subjected to both alignment and repulsion interactions. The full phase diagram of this prototypical model for dense active nematics offers a comprehensive and unified account of previous partial results, but also reveals novel large-scale inhomogeneous `arch’ solutions which elucidate the heretofore intriguing nature of the `defect-ordered states’ described recently.

We then show that the Boltzmann-Ginzburg-Landau approach used successfully in the case of dilute active matter can be extended to deal with distance-dependent forces to derive, from our microscopic model, continuous ‘hydrodynamic’ equations for dense active nematics. These equations are essentially the same as those conjectured to describe these systems, with the crucial bonus that their many transport coefficients are now explicitly known as functions of the few important parameters of the microscopic model. This allows us to perform a comprehensive study of their solutions, yielding a phase diagram in qualitative agreement with that of the microscopic model, including the new inhomogeneous arches solutions.

We finally perform a comparative study of the dynamics and interactions of topological defects at the microscopic and hydrodynamic level with, again, satisfactory qualitative agreement. This body of work offers, for the first time, a detailed understanding of theoretical description of dense active nematics
directly rooted in their microscopic dynamics.