Ion-mediated interactions between net-neutral slabs: Weak and strong disorder effects

TL;DR

This paper explores how charge disorder on neutral dielectric slabs in an ionic fluid significantly alters their effective interactions, revealing complex behaviors including non-monotonic and strongly attractive forces due to combined electrostatic effects.

Contribution

It demonstrates the impact of surface charge disorder and multivalent ions on slab interactions, extending beyond traditional van der Waals and image depletion models.

Findings

Weak disorder leads to non-monotonic interactions with both repulsive and attractive regions.

Strong disorder results in dominant, long-range attractive interactions.

Disorder strength critically influences the nature and magnitude of the effective forces.

Abstract

We investigate the effective interaction between two randomly charged but otherwise net-neutral, planar dielectric slabs immersed in an asymmetric Coulomb fluid containing a mixture of mobile monovalent and multivalent ions. The presence of charge disorder on the apposed bounding surfaces of the slabs leads to substantial qualitative changes in the way they interact, as compared with the standard picture provided by the van der Waals and image-induced, ion-depletion interactions. While, the latter predict purely attractive interactions between strictly neutral slabs, we show that the combined effects from surface charge disorder, image depletion, Debye (or salt) screening and also, in particular, their coupling with multivalent ions, give rise to a more diverse behavior for the effective interaction between net-neutral slabs. Disorder effects show large variation depending on the properly quantified strength of disorder, leading either to non-monotonic effective interaction with both repulsive and attractive branches when the surface charges are weakly disordered (small disorder variance) or to a dominating attractive interaction that is larger both in its range and magnitude than what is predicted from the van der Waals and image-induced, ion-depletion interactions, when the surfaces are strongly disordered (large disorder variance).