How a "pinch of salt" can tune chaotic mixing of colloidal suspensions

TL;DR

This paper shows that adding trace salt can significantly enhance or inhibit chaotic mixing of colloids by triggering diffusiophoresis, a chemically-driven transport mechanism that outperforms Brownian diffusion.

Contribution

It reveals that trace salt influences colloidal mixing through diffusiophoresis, providing a new way to control mixing processes beyond flow and diffusion conditions.

Findings

Salt addition can boost or inhibit mixing.

Diffusiophoresis dominates over Brownian diffusion.

Small-scale effects impact large-scale mixing.

Abstract

Efficient mixing of colloids, particles or molecules is a central issue in many processes. It results from the complex interplay between flow deformations and molecular diffusion, which is generally assumed to control the homogenization processes. In this work we demonstrate on the contrary that despite fixed flow and self-diffusion conditions, the chaotic mixing of colloidal suspensions can be either boosted or inhibited by the sole addition of trace amount of salt as a co-mixing species. Indeed, this shows that local saline gradients can trigger a chemically-driven transport phenomenon, diffusiophoresis, which controls the rate and direction of molecular transport far more efficiently than usual Brownian diffusion. A simple model combining the elementary ingredients of chaotic mixing with diffusiophoretic transport of the colloids allows to rationalize our observations and highlights how small-scale out-of-equilibrium transport bridges to mixing at much larger scales in a very effective way. Considering chaotic mixing as a prototypal building block for turbulent mixing, this suggests that these phenomena, occurring whenever the chemical environment is inhomogeneous, might bring interesting perspective from micro-systems up to large-scale situations, with examples ranging from ecosystems to industrial contexts.