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Fluctuating hydrodynamics of electrolytes at electroneutral scales

Aleksandar Donev1,*, Andrew J. Nonaka2, Changho Kim2,3, Alejandro L. Garcia4, and John B. Bell2

  • 1Courant Institute of Mathematical Sciences, New York University, New York, New York 10012, USA
  • 2Center for Computational Science and Engineering, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
  • 3Department of Applied Mathematics, University of California, Merced, Merced, California 95343, USA
  • 4Department of Physics, San Jose State University, San Jose, California 95192, USA

  • *donev@courant.nyu.edu
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Phys. Rev. Fluids 4, 043701 – Published 5 April, 2019

DOI: https://doi.org/10.1103/PhysRevFluids.4.043701

Abstract

At mesoscopic scales electrolyte solutions are modeled by the fluctuating generalized Poisson-Nernst-Planck (PNP) equations [J.-P. Péraud et al., Phys. Rev. Fluids 1, 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 nonideality 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.

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References (27)

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Outline

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Phys. Rev. Fluids 4, 043701– Published 5 April, 2019
  • Received 25 September 2018
Article part of CHORUS
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