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Role of fluid-fluid interfacial topology in the transport of hydrophobic nanoparticles in partially saturated porous media
Summary
Scientists studying how tiny particles move through wet, air-and-water-filled soil-like materials found that simple math models can miss key details—like how trapped air pockets can delay particle movement without actually removing more particles from the water. This matters because it helps explain why some water treatment methods work better than others, and it could improve technologies for removing nanoplastics from water using oil-water separation (froth flotation) or cleaning up contaminated groundwater with engineered nanoparticles.
Identical macroscopic recoveries can correspond to different interfacial coverage values, as nanoparticle (NP) retention in unsaturated porous media is modulated by the non-wetting phase (NWP) topology and the accessibility of fluid-fluid interfacial area, rather than NWP saturation alone. In this work, experiments in partially saturated 2D glass microfluidic networks qualitatively validate prior pore-network simulation findings of transport in unsaturated 3D pore networks. Fluorescence microscopy reveals that NP retention via irreversible attachment at fluid-fluid interfaces takes place alongside reversible “trapping” in low advection or immobile zones, formed by large, well-connected NWP ganglia, which delay transport and lower recovery without increasing interfacial coverage. The latter effect is diminished when hydrodynamic dispersion is insufficient to deliver NPs to uncovered collectors. At low concentrations, attachment appears diffusion-controlled and the upscaled attachment rate coefficient, determined by a model that neglects this effect, is insensitive to residence time. For high NP input concentrations, determination of the attachment rate coefficient via continuum modeling faces parameter identifiability challenges, although delay in NP elution is evident through breakthrough curve (BTC) tailing. In experiments where oil is the NWP, similar coverage values emerge despite distinct BTCs, due to restricted accessibility of the oil-water interface. These findings expose the limitations of continuum advection-dispersion-retention models to predict transport without explicitly incorporating pore-scale geometry, accessible interfacial areas, and interface-scale adsorption kinetics, demonstrating the significant potential of pore-scale studies as a complement to macroscale upscaling approaches. Implications of this research extend to understanding hydrophobic NP interactions with fluid-fluid collectors, informing targeted nanoremediation of chlorinated solvents as well as nanoplastic elimination by froth flotation.