The sediment of mixtures of charged colloids: segregation and inhomogeneous electric fields

TL;DR

This paper presents a theoretical analysis of sedimentation in charged colloid mixtures, revealing how inhomogeneous electric fields and charge separation lead to complex layering and nonmonotonic density profiles.

Contribution

It introduces a theoretical framework for understanding sedimentation equilibrium in charged colloid mixtures, emphasizing the role of inhomogeneous electric fields and charge-driven segregation.

Findings

Charge separation occurs spontaneously in low-salt conditions.

Electric fields can lift colloids against gravity, creating nonmonotonic density profiles.

Colloids can segregate into layers of equal mass-per-charge, with heavy particles floating above lighter ones.

Abstract

We theoretically study sedimentation-diffusion equilibrium of dilute binary, ternary, and polydisperse mixtures of colloidal particles with different buoyant masses and/or charges. We focus on the low-salt regime, where the entropy of the screening ions drives spontaneous charge separation and the formation of an inhomogeneous macroscopic electric field. The resulting electric force lifts the colloids against gravity, yielding highly nonbarometric and even nonmonotonic colloidal density profiles. The most profound effect is the phenomenon of segregation into layers of colloids with equal mass-per-charge, including the possibility that heavy colloidal species float onto lighter ones.