Convective Instability Driven by Diffusiophoresis of Colloids in Binary Liquid Mixtures

TL;DR

This study demonstrates that colloidal suspensions in binary mixtures can become convectively unstable due to diffusiophoresis, leading to transient convection patterns driven by solute concentration gradients.

Contribution

It reveals the microscopic mechanism of convective instability caused by diffusiophoresis in colloidal suspensions within binary liquids.

Findings

Colloidal suspensions exhibit transient convective patterns.

Diffusiophoresis causes an unstable density layer to form.

Convection arises to reduce gravitational potential energy.

Abstract

In a binary fluid mixture, the concentration gradient of a heavier molecular solute leads to a diffusive flux of solvent and solute to achieve thermodynamic equilibrium. If the solute concentration decreases with height, the system is always in a condition of stable mechanical equilibrium against gravity. We show experimentally that this mechanical equilibrium becomes unstable in case colloidal particles are dispersed uniformly within the mixture, and that the resulting colloidal suspension undergoes a transient convective instability with the onset of convection patterns. By means of a numerical analysis, we clarify the microscopic mechanism from which the observed destabilisation process originates. The solute concentration gradient drives an upward diffusiophoretic migration of colloids, in turn causing the development of a mechanically unstable layer within the sample, where the density of the suspension increases with height. Convective motions arise to minimize this localized rise in gravitational potential energy.