Hydrodynamic theory of chemically active emulsions

TL;DR

This paper develops a hydrodynamic theory for chemically active emulsions, incorporating active chemical reactions and gradient terms, predicting microphase formation, bubbly separation, and active filaments, with implications for entropy production.

Contribution

It extends Active Model B+ to include higher order gradient terms for chemically active emulsions, providing a comprehensive theoretical framework and numerical predictions.

Findings

Prediction of microphase formation when interfacial energy becomes negative

Identification of bubbly phase separation with noise inclusion

Discovery of a dynamic active filament phase

Abstract

We present a systematic theory of chemically active emulsions in the hydrodynamic limit by constructing a thermodynamically consistent framework in which the equilibrium is broken by chemo-stating of fuel molecules. For ternary solutions with active chemical reactions, we obtain an effective dynamics of the conserved field dynamics at long length and time scales. The effective dynamics takes into account the broken time reversal symmetry that manifests itself by the emergence of gradient terms akin to those of Active Model B+, which is a generic theory of active phase separation. In addition to the active coefficients modifying the interfacial energy coefficient, the theory contains higher order terms in the gradient expansion that are necessary to correctly describe the dynamics of chemically active emulsions, extending thus Active Model B+. We study numerically a Flory-Huggins model with active chemical reactions. Our theory predicts the formation of microphases when the effective interfacial energy coefficient becomes negative. Moreover, including noise, we show the existence of bubbly phase separation. We also identify a new type of phase behavior, a dynamic active filament phase. Finally, we discuss the steady state entropy production rate in the system resulting from the active chemical reactions. We observe that the total entropy production rate increases with the driving chemical potential and exhibits a kink-like singularity at the transition to the dynamic active filament phase. Our work shows that the generic behaviors of active phase separation can emerge in chemically active emulsions.