Fluctuating hydrodynamics of electrolytes at electroneutral scales

TL;DR

This paper develops a fluctuating hydrodynamics model for electrolytes at electroneutral scales, enabling efficient simulations of multispecies charged fluids without assuming dilute solutions.

Contribution

It introduces a new electroneutral fluctuating hydrodynamics framework that accounts for all species equally and replaces the Poisson equation with an elliptic potential equation.

Findings

Demonstrates the model's ability to simulate gravitational fingering instability.

Shows the importance of treating ions individually due to different diffusion coefficients.

Provides a second-order numerical algorithm adaptable to charged or electroneutral equations.

Abstract

At mesoscopic scales electrolyte solutions are modeled by the fluctuating generalized Poisson-Nernst-Planck (PNP) equations [J.-P. P\'eraud et al., Phys. Rev. F, 1(7):074103, 2016]. However, at length and time scales larger than the Debye scales, electrolytes are effectively electroneutral, and the charged-fluid PNP equations become too stiff to solve numerically. Here we formulate the isothermal incompressible equations of fluctuating hydrodynamics for reactive multispecies mixtures involving charged species in the electroneutral limit, and design a numerical algorithm to solve these equations. Our model does not assume a dilute electrolyte solution but rather treats all species on an equal footing, accounting for cross-diffusion and non-ideality using Maxwell-Stefan theory. By enforcing local electroneutrality as a constraint, we obtain an elliptic equation for the electric potential that replaces the Poisson equation in the fluctuating PNP equations. We develop a second-order midpoint predictor-corrector algorithm to solve either the charged-fluid or electroneutral equations with only a change of the elliptic solver. We use the electroneutral algorithm to study a gravitational fingering instability, triggered by thermal fluctuations, at an interface where an acid and base react to neutralize each other. Our results demonstrate that, because the four ions diffuse with very different coefficients, one must treat each ion as an individual species, and cannot treat the acid, base, and salt as neutral species. This emphasizes the differences between electrodiffusion and classical Fickian diffusion, even at electroneutral scales.