[Paper Review] Diffusiophoretic transport of colloids in porous media

Mobin Alipour, Yiran Li+2 more|arXiv (Cornell University)|Nov 22, 2024

Electrostatics and Colloid InteractionsChemistry3 citations

TL;DR

This study demonstrates that solute gradients induce significant diffusiophoretic migration of colloids in porous media, altering their macroscopic transit time and dispersion by up to an order of magnitude compared to control cases without gradients. Using microfluidic experiments, numerical simulations, and theoretical modeling, the authors show that diffusiophoresis modulates the influence of geometric disorder on colloid transport, challenging classical models that neglect non-equilibrium chemical gradients.

ABSTRACT

Understanding how colloids move in crowded environments is key for gaining control over their transport in applications such as drug delivery, filtration, contaminant/microplastic remediation and agriculture. The classical models of colloid transport in porous media rely on geometric characteristics of the medium, and hydrodynamic/non-hydrodynamic equilibrium interactions to predict their behavior. However, chemical gradients are ubiquitous in these environments and can lead to the non-equilibrium diffusiophoretic migration of colloids. Here, combining microfluidic experiments, numerical simulations, and theoretical modeling we demonstrate that diffusiophoresis leads to significant macroscopic changes in the dispersion of colloids in porous media. We displace a suspension of colloids dispersed in a background salt solution with a higher/lower salinity solution and monitor the removal of the colloids from the medium. While mixing weakens the solute gradients, leading to the diffusiophoretic velocities that are orders of magnitude weaker than the background fluid flow, we show that the cross-streamline migration of colloids changes their macroscopic transit time and dispersion through the medium by an order of magnitude compared to the control case with no salinity gradients. Our observations demonstrate that solute gradients modulate the influence of geometric disorder on the transport, pointing to the need for revisiting the classical models of colloid transport in porous media to obtain predictive models for technological, medical, and environmental applications.

Motivation & Objective

• To investigate how solute gradients and geometric disorder jointly influence colloid transport in porous media.
• To challenge classical colloid transport models that ignore non-equilibrium diffusiophoretic effects.
• To quantify the macroscopic impact of diffusiophoresis on colloid dispersion and transit time in disordered porous networks.
• To establish a framework for predicting colloid behavior in environments with chemical gradients, relevant to drug delivery, filtration, and environmental remediation.

Proposed method

• Microfluidic chips with ordered and disordered obstacle arrays were fabricated using photolithography, with disorder controlled by a perturbation parameter β.
• Colloids (1 μm diameter, carboxylate-coated, zeta potential ≈ -70 mV) were imaged at 64 fps to track trajectories in response to solute gradients.
• Solute fronts were visualized using fluorescent sodium salt (0.01 mM) as a proxy for LiCl, with diffusivity mismatch acknowledged.
• A custom OpenFOAM solver simulated 3D transient transport, incorporating diffusiophoretic mobility using the thin Debye layer approximation.
• Theoretical modeling used the diffusiophoretic mobility equation: Γₚ = (ε/η)(kBT/ze)²[βₛ(zeζₚ/kBT) + 4ln cosh(zeζₚ/4kBT)], with βₛ derived from cation/anion diffusivities.
• Experiments compared colloid removal dynamics under 'attractive' (c₁/c₀ ≈ 100), 'control' (c₁ = c₀), and 'repulsive' (c₁/c₀ ≈ 0.01) solute gradient conditions.

Figure 1: Flow disorder and solute gradients shape the macroscopic transport of colloids in porous media. (a) Schematic of the microfluidic chips patterned with obstacles. The field of view (FOV) of the experiments is marked with the white rectangle. We introduce geometric disorder into an ordered l

Experimental results

Research questions

• RQ1 How do solute gradients affect the macroscopic dispersion and transit time of colloids in disordered porous media?
• RQ2 To what extent does diffusiophoresis alter colloid transport compared to classical models that neglect non-equilibrium chemical gradients?
• RQ3 How does geometric disorder interact with diffusiophoretic forces to shape colloid migration pathways?
• RQ4 Can diffusiophoresis significantly modulate the influence of velocity heterogeneity in porous networks on colloid transport?

Key findings

• Diffusiophoresis alters colloid transit time and dispersion by up to an order of magnitude compared to the control case with no solute gradient.
• In the 'attractive' case (c₁/c₀ ≈ 100), colloid removal is significantly accelerated due to diffusiophoretic migration toward the solute front.
• In the 'repulsive' case (c₁/c₀ ≈ 0.01), colloid removal is slowed, indicating diffusiophoretic repulsion from the solute front.
• The transverse velocity distribution deviates from Gaussian due to geometric disorder, and solute gradients further broaden this distribution, enhancing cross-streamline migration.
• The diffusiophoretic mobility of the colloids was estimated at approximately 800 μm²/s under the experimental conditions.
• Numerical simulations confirmed that diffusiophoresis dominates over hydrodynamic effects in altering colloid dispersion, even when diffusiophoretic velocities are orders of magnitude weaker than the background flow.

Figure 2: Diffusiophoretic migration of colloids across the flow streamlines leaves a lasting fingerprint on the velocity distribution experienced by the colloids. (a) The temporal evolution of the normalized mean colloid velocity and density fields. In the control case, we observe a monotonic decre