Relaxation dynamics of two interacting electrical double-layers in a 1D Coulomb system

TL;DR

This study investigates the relaxation dynamics of two interacting electrical double-layers in a one-dimensional Coulomb system, revealing different behaviors depending on the parity of the number of counterions and the influence of thermal effects.

Contribution

It provides a detailed analysis combining exact calculations and simulations, highlighting the role of ion number parity and length scales in relaxation processes.

Findings

For odd N, double-layers never decouple, exhibiting diffusive relaxation in L^2.

For even N, the Bjerrum length governs relaxation, with thermal effects either hindering or speeding up depending on N parity.

Mean-field theory accurately describes large N and remains valid down to N>3.

Abstract

We consider an out-of-equilibrium one-dimensional model for two electrical double-layers. With a combination of exact calculations and Brownian Dynamics simulations, we compute the relaxation time ($\tau$) for an electroneutral salt-free suspension, made up of two fixed colloids, with $N$ neutralizing mobile counterions. For $N$ odd, the two double-layers never decouple, irrespective of their separation $L$; this is the regime of like-charge attraction, where $\tau$ exhibits a diffusive scaling in $L^2$ for large $L$. On the other hand, for even $N$, $L$ no longer is the relevant length scale for setting the relaxation time; this role is played by the Bjerrum length. This leads to distinctly different dynamics: for $N$ even, thermal effects are detrimental to relaxation, increasing $\tau$, while they accelerate relaxation for $N$ odd. Finally, we also show that the mean-field theory is recovered for large $N$ and moreover, that it remains an operational treatment down to relatively small values of $N$ ($N>3$).