[论文解读] Diffusiophoretic transport of colloids in porous media

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

Electrostatics and Colloid InteractionsChemistry被引用 3

一句话总结

本研究证明，溶质梯度会显著诱导多孔介质中胶体的扩散泳迁移，与无梯度的对照情况相比，其宏观迁移时间和分散度可改变一个数量级。通过微流体实验、数值模拟和理论建模，作者表明扩散泳改变了几何无序对胶体迁移的影响，挑战了忽略非平衡化学梯度的经典模型。

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.

研究动机与目标

• 研究溶质梯度与几何无序如何共同影响多孔介质中的胶体迁移。
• 挑战忽略非平衡扩散泳效应的经典胶体迁移模型。
• 量化扩散泳对无序多孔网络中胶体分散与迁移时间的宏观影响。
• 建立预测化学梯度环境中胶体行为的框架，相关应用包括药物输送、过滤及环境修复。

提出的方法

• 利用光刻技术制备具有有序与无序障碍物阵列的微流体芯片，通过扰动参数β控制无序度。
• 以64 fps的帧率对胶体（直径1 μm，羧基修饰，ζ电势≈ -70 mV）进行成像，追踪其在溶质梯度下的轨迹。
• 使用荧光钠盐（0.01 mM）作为LiCl的替代物，可视化溶质波前，承认扩散系数不匹配的问题。
• 采用自定义的OpenFOAM求解器模拟3D瞬态传输过程，基于薄德拜层近似引入扩散泳迁移率。
• 理论建模采用扩散泳迁移率方程：Γₚ = (ε/η)(kBT/ze)²[βₛ(zeζₚ/kBT) + 4ln cosh(zeζₚ/4kBT)]，其中βₛ由阳离子/阴离子扩散系数推导得出。
• 实验比较了在‘吸引’（c₁/c₀ ≈ 100）、‘对照’（c₁ = c₀）和‘排斥’（c₁/c₀ ≈ 0.01）溶质梯度条件下胶体的去除动力学。

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

实验结果

研究问题

• RQ1 溶质梯度如何影响无序多孔介质中胶体的宏观分散与迁移时间？
• RQ2 与忽略非平衡化学梯度的经典模型相比，扩散泳在多大程度上改变了胶体迁移行为？
• RQ3 几何无序如何与扩散泳力相互作用，从而塑造胶体迁移路径？
• RQ4 扩散泳能否显著调节多孔网络中速度异质性对胶体迁移的影响？

主要发现

• 与无溶质梯度的对照情况相比，扩散泳可使胶体迁移时间与分散度改变达一个数量级。
• 在‘吸引’情况（c₁/c₀ ≈ 100）下，由于胶体向溶质前缘迁移，胶体去除显著加速。
• 在‘排斥’情况（c₁/c₀ ≈ 0.01）下，胶体去除变慢，表明胶体受到溶质前缘的扩散泳排斥。
• 由于几何无序，横向速度分布偏离高斯分布，而溶质梯度进一步拓宽该分布，增强跨流线迁移。
• 在实验条件下，胶体的扩散泳迁移率估计约为800 μm²/s。
• 数值模拟证实，即使扩散泳速度比背景流速弱多个数量级，扩散泳仍主导于改变胶体分散性，而非水动力效应。

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