Flow induced intermittent transport shapes colloid filtration in complex media

TL;DR

This study reveals how intermittent particle transport within heterogeneous porous media influences colloid filtration, combining microfluidic experiments with stochastic modeling to better understand natural and industrial filtration processes.

Contribution

It introduces a multiscale microfluidic system and a CTRW-based stochastic model to capture the effects of pore heterogeneity on colloidal transport and filtration.

Findings

Particles alternate between flights and dives, affecting attachment.

Distributed flight sizes lead to anomalous filtration.

Model links pore structure to macroscopic filtration behavior.

Abstract

The macroscopic phenomenon of filtration is the separation between suspended and liquid phases and it takes place in natural environments (e.g. groundwater, soil, hyporheic zone) and industrial systems (e.g. filtration plants, pharmaceutical industry, hospital care). Porous materials represent excellent filters since they are characterized by a large solid surface to which flowing particles can attach and be retained. Colloidal filtration by porous media is governed by a complex interplay between transport dynamics through intricate pore structures and surface-mediated retention. Yet, classical approaches fail to capture key properties (such as filter spatial heterogeneity) and experimental observations--e.g. non-exponential deposition profiles. A key limitation of such approaches lies in the assumption that particle attachment to solid surfaces occurs at a constant rate over a given length scale, neglecting the intrinsic heterogeneity of the medium, i.e. pore size variability. Here, we develop a multiscale microfluidic model system to directly observe colloidal transport and deposition over more than three orders of magnitude in length--from tens of microns to a meter--within a porous architecture with controlled heterogeneity. By tracking individual colloidal particles within the pores, we reveal intermittent dynamics along each trajectory: particles alternate between long-range "flights" through pore channels and short-range localized "dives" near grain surfaces. During the dives, attachment can occur at a constant rate, but the distributed flight sizes, during which attachment cannot occur, produce anomalous filtration. This is quantified by deposition profiles and breakthrough curves. A stochastic model based on a continuous time random walk (CTRW), constrained by experimental data, captures this behavior and links pore structure with macroscopic filtration.