Fluctuation-enhanced electric conductivity in electrolyte solutions

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 114; no. 41; pp. 10829 - 10833
• Main Authors: Péraud, Jean-Philippe, Nonaka, Andrew J., Bell, John B., Donev, Aleksandar, Garcia, Alejandro L.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 10.10.2017; Proceedings of the National Academy of Sciences
• Subjects: Anions; Cations; Charge transport; Diffusion; Diffusion coefficient; Dilution; Electric fields; Electrical resistivity; Electrodiffusion; electrohydrodynamics; Electrolytes; fluctuating hydrodynamics; Fluctuations; Fluid dynamics; Fluid flow; Fluid mechanics; Heat conductivity; Hydrodynamic equations; Hydrodynamics; Macroscopic equations; Mass transport; Mathematical analysis; Mathematical models; MATHEMATICS AND COMPUTING; Momentum equation; multicomponent diffusion; Navier–Stokes equations; Nernst–Plank equations; Physical Sciences; Velocity; electrohydrodynamics; Navier–Stokes equations; Nernst–Plank equations; fluctuating hydrodynamics; multicomponent diffusion
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1714464114

We analyze the effects of an externally applied electric field on thermal fluctuations for a binary electrolyte fluid. We show that the fluctuating Poisson–Nernst–Planck (PNP) equations for charged multispecies diffusion coupled with the fluctuating fluid momentum equation result in enhanced charge transport via a mechanism distinct from the well-known enhancement of mass transport that accompanies giant fluctuations. Although the mass and charge transport occurs by advection by thermal velocity fluctuations, it can macroscopically be represented as electrodiffusion with renormalized electric conductivity and a nonzero cation–anion diffusion coefficient. Specifically, we predict a nonzero cation–anion Maxwell–Stefan coefficient proportional to the square root of the salt concentration, a prediction that agrees quantitatively with experimental measurements. The renormalized or effective macroscopic equations are different from the starting PNP equations, which contain no cross-diffusion terms, even for rather dilute binary electrolytes. At the same time, for infinitely dilute solutions the renormalized electric conductivity and renormalized diffusion coefficients are consistent and the classical PNP equations with renormalized coefficients are recovered, demonstrating the self-consistency of the fluctuating hydrodynamics equations. Our calculations show that the fluctuating hydrodynamics approach recovers the electrophoretic and relaxation corrections obtained by Debye–Huckel–Onsager theory, while elucidating the physical origins of these corrections and generalizing straightforwardly to more complex multispecies electrolytes. Finally, we show that strong applied electric fields result in anisotropically enhanced “giant” velocity fluctuations and reduced fluctuations of salt concentration.