Zigzagging Diffusion and Non-Standard Transport in Particle-laden Nanopores Under Extreme Confinement

TL;DR

This study reveals how molecular layering and structural commensurability in nanopores cause non-monotonic, zig-zag variations in transport properties of confined liquids and particles, challenging classical hydrodynamic models.

Contribution

It demonstrates the impact of layering and commensurability on transport in nanopores, showing deviations from continuum hydrodynamics even with nanoparticle presence.

Findings

Transport coefficients oscillate with pore width due to layering.

Flow velocity and permeability show non-monotonic dependence on confinement.

Nanoparticles do not eliminate layering effects, maintaining zig-zag diffusivity patterns.

Abstract

Understanding transport subject to molecular-scale confinement is key to advancing nanofluidics, yet classical hydrodynamic laws often fail at these scales. Here, we study a model system: transport of toluene as a solvent and small fullerenes as model particles confined within alumina slit nanopores using molecular dynamics simulations. We find that toluene organizes into discrete layers whose commensurability with the pore width leads to a striking, non-monotonic, zig-zag dependence of transport coefficients on confinement. This layering drives oscillations not only in solvent diffusivity but also in flow velocity and permeability under pressure-driven conditions, breaking the expected scaling relations between diffusion, viscosity, and flow. Surprisingly, introducing a nanoparticle does not wash out these effects - although the fullerene perturbs local layering, the nanoparticle diffusivity retains a zig-zag dependence on pore width. Our results demonstrate how structural commensurability and interfacial effects dominate transport in nanoconfined liquids, and lead to important deviations from continuum expectations. These findings establish a microscopic basis for size-dependent transport in nanopores and highlight the need for beyond-hydrodynamic models in confined soft matter systems.