Solute dispersion boosts the phoretic removal of colloids from dead-end pores

TL;DR

This study demonstrates that solute dispersion enhances the removal of colloids from dead-end pores via diffusiophoresis, challenging the assumption that front sharpness is necessary for effective phoretic transport.

Contribution

The paper introduces an analytical model and experimental validation showing that diffuse solute fronts increase colloid clearance in dead-end pores, highlighting a counterintuitive effect.

Findings

Diffuse solute fronts extend the duration of phoretic forcing.

Solute dispersion increases cumulative colloid drift.

Enhanced removal efficiency compared to sharp fronts.

Abstract

Predicting and controlling the transport of colloids in porous media is essential for a broad range of applications, from drug delivery to contaminant remediation. Chemical gradients are ubiquitous in these environments, arising from reactions, precipitation/dissolution, or salinity contrasts, and can drive particle motion via diffusiophoresis. Yet our current understanding mostly comes from idealized settings with sharply imposed solute gradients, whereas in porous media, flow disorder enhances solute dispersion, and leads to diffuse solute fronts. This raises a central question: does front dispersion suppress diffusiophoretic migration of colloids in dead-end pores, rendering the effect negligible at larger scales? We address this question using an idealized one-dimensional dead-end geometry. We derive an analytical model for the spatiotemporal evolution of colloids subjected to slowly varying solute fronts and validate it with numerical simulations and microfluidic experiments. Counterintuitively, we find that diffuseness of solute front enhances removal from dead-end pores: although smoothing reduces instantaneous gradient magnitude, it extends the temporal extent of phoretic forcing, yielding a larger cumulative drift and higher clearance efficiency than sharp fronts. Our results highlight that solute dispersion does not weaken the phoretic migration of colloids from dead-end pores, pointing to the potential relevance of diffusiophoresis at larger scales, with implications for filtration, remediation, and targeted delivery in porous media.