Diffusiophoretic transport of colloids in porous media.

Chemical gradients are ubiquitous in porous media flows, from tidal salt gradients in aquifers to irrigation-driven gradients in soils and ionic gradients from metabolic activity in tissues. Although chemical gradients are known to drive diffusiophoretic migration of colloids, these nonequilibrium forces have largely been ignored in porous media flows. Under typical subsurface conditions, flow velocities within preferential pathways exceed phoretic velocities by orders of magnitude, suggesting that diffusiophoresis would be limited to stagnant pockets. Here, using microfluidic experiments, numerical simulations, and theoretical modeling, we show that even moderate solute gradients, typical of natural mixing, can markedly alter colloid transport. We uncover a previously overlooked effect: cross-streamline phoretic migration within preferential flow pathways, which changes macroscopic dispersion by orders of magnitude and suppresses the impact of geometric disorder on transport. Our findings challenge classical models of colloid transport, highlighting the broad implications of solute gradients for technological, biomedical, and environmental applications.