Electric-field-induced oscillations in ionic fluids: a unified formulation of modified Poisson-Nernst-Planck models and its relevance to correlation function analysis

We theoretically investigate an electric-field-driven system of charged spheres as a primitive model of concentrated electrolytes under an applied electric field. First, we provide a unified formulation for the stochastic charge and density dynamics of the electric-field-driven primitive model using the stochastic density functional theory (DFT). The stochastic DFT integrates various frameworks of the equilibrium and dynamic DFTs, the liquid state theory, and the field-theoretic approach, which allows us to justify in a unified manner various modifications previously made for the Poisson-Nernst-Planck model. Next, we consider stationary density-density and charge-charge correlation functions of the primitive model with a static electric field. We focus on an electric-field-induced synchronization between the emergence of density and charge oscillations, or the crossover from monotonic to oscillatory decay of density-density and charge-charge correlations. The correlation function analysis demonstrates the appearance of stripe states formed by segregation bands perpendicular to the external field. We also predict the following: (i) the electric-field-induced crossover occurs prior to the conventional Kirkwood crossover without an applied electric field, and (ii) the ion concentration dependence of the decay lengths at the electric-field-induced crossovers bears a similarity to the underscreening behavior found by simulation and theoretical studies on the oscillatory decay length in equilibrium.