Derivation and Analysis of Amplitude Equation for Generalized AMB+ in Presence of Chemical Reaction

TL;DR

This paper derives and analyzes an amplitude equation for pattern formation in a generalized active model with chemical reactions, revealing how parameters influence the transition type and stability of patterns.

Contribution

The paper introduces a generalized amplitude equation for active matter with chemical reactions, extending previous models and analyzing parameter-dependent pattern transition behaviors.

Findings

Transition is always supercritical for g=0.

Transition can be supercritical or subcritical for g≠0 depending on parameters.

Eckhaus instability condition is independent of g.

Abstract

We derive and analyze the amplitude equation for the roll patterns in case of generalized Active Model B+ (AMB+) in the presence of chemical reactions. The generalized AMB+ differs from the original AMB+ introduced by Tjhung \textit{et al.} [E. Tjhung \textit{et al.}, Phys. Rev. X \textbf{8}, 031080 (2018)] by the addition of a quadratic term, $g\phi^2$, in the expression for the equilibrium part of the current. Also, the model includes a rotation-free active current of strength $\lambda$ and a rotational current of strength $\xi$. The inclusion of a chemical reaction with rate $\Gamma$ removes the conservation constraint and introduces a preferred wavenumber that governs the pattern formation below a critical reaction rate $\Gamma_c$. We argue for the analytical form of the amplitude equation based on symmetry considerations and explicitly derived it using multiscale analysis. By taking different limits of $g$, $\lambda$, and $\xi$, we recover amplitude equations for several well-known physical models as special cases and determine the nature transitions close to the onset of instability. We find that for $g = 0$, the transition is always supercritical, whereas for $g \ne 0$, the transition between the supercritical and subcritical regimes depends sensitively on the model parameters. Further, we derive the condition for the \textit{Eckhaus instability} from the stability analysis of the amplitude equation as well as from the phase diffusion equation, and find that it is independent of $g$.