Microphase Separation in Scalar Active Matter

Continuous consumption of energy in systems of active particles generate novel collective behaviours and structures that are generically not observed in systems at thermal equilibrium. Motile active matter has been studied extensively, however non-motile (and so truly scalar in a sense) minimal models of active matter are crucially lacking in the literature and present a wide range of non-equilibrium collective phenomena.

We present a microscopic model inspired by the bacteria Neisseria Meningitidis in which diffusive agents feel intermittent attractive forces determined by an internal variable that switches stochastically. We solve the microscopic equations numerically to study the emergence of phase separation and demixing in the system. Through a formal coarse-graining procedure, we then derive an evolution equation for the particle density and show that our model pertains to the Active Model B+ class, confirming the presence of microphase separation by solving the kinetic equations numerically. We conclude on good agreement between the known results on the field theory and the current system. 

This is the first truly scalar model of active matter whose dynamics have been coarse-grained to produce both time-reversal-symmetry breaking terms predicted by this field theory.