Diffusiophoretic dispersion of a colloidal blob in porous media

TL;DR

This study investigates how diffusiophoresis influences colloid dispersion in porous media, revealing counterintuitive effects where attraction enhances dispersion and repulsion suppresses it, with implications for environmental and biological transport.

Contribution

The paper combines experiments and simulations to uncover the reversal of expected diffusiophoretic effects on colloid dispersion in porous media, introducing a minimal two-layer flow model.

Findings

Longitudinal dispersion increases when colloids are attracted to solute-rich regions.

Dispersion decreases when colloids are repelled from solute-rich regions.

Geometric disorder modulates the diffusiophoretic exchange mechanism.

Abstract

Predicting and controlling the transport of colloids in porous media is essential for applications ranging from contaminant remediation to drug delivery. In these complex environments, solute gradients are ubiquitous and could drive diffusiophoretic particle migration, yet their impact on macroscopic colloid dispersion remains poorly understood. Here we combine experiments and simulations to quantify how diffusiophoresis alters the spreading of a colloidal blob in a 2D ordered/disordered porous medium. A joint blob of colloids and salt at high concentration is introduced into a medium filled with salt at low concentration and advected by a background flow. Intuition suggests that when colloids are attracted toward or repelled from the solute-rich blob, dispersion should be suppressed or enhanced, respectively. Instead, we observe the opposite trend: longitudinal dispersion is enhanced in the attractive case, whereas dispersion is suppressed in the repulsive case. Numerical simulations reveal that this striking reversal arises from diffusiophoretic exchange of particles between slow and fast streamlines, which we capture using a minimal two-layer model of coupled fast and slow plug flows. Finally, we probe how geometric disorder in the medium modulates this mechanism. Our results demonstrate that diffusiophoresis can strongly modulate macroscopic dispersion of colloids in porous media with implications for transport in subsurface and biological environments.