Poiseuille flow past a nanoscale cylinder in a slit channel: Lubrication theory versus molecular dynamics analysis

TL;DR

This study compares lubrication theory and molecular dynamics simulations to analyze flow past a nanoscale cylinder in a slit channel, revealing how confinement and thermal motion affect drag and flow characteristics.

Contribution

It provides a detailed comparison between lubrication theory and molecular dynamics for nanoscale flow, including effects of thermal motion and confinement asymmetry.

Findings

Lubrication theory closely matches molecular dynamics results for thin gaps.

Maximum drag occurs at symmetric confinement, decreasing with off-center displacement.

Thermal motion reduces average drag and hydraulic resistance.

Abstract

Plane Poiseuille flow past a nanoscale cylinder that is arbitrarily confined (i.e., symmetrically or asymmetrically confined) in a slit channel is studied via hydrodynamic lubrication theory and molecular dynamics simulations, considering cases where the cylinder remains static or undergoes thermal motion. Lubrication theory predictions for the drag force and volumetric flow rate are in close agreement with molecular dynamics simulations of flows having molecularly thin lubrication gaps, despite the presence of significant structural forces induced by the crystalline structure of the modeled solid. While the maximum drag force is observed in symmetric confinement, i.e., when the cylinder is equidistant from both channel walls, the drag decays significantly as the cylinder moves away from the channel centerline and approaches a wall. Hence, significant reductions in the mean drag force on the cylinder and hydraulic resistance of the channel can be observed when thermal motion induces random off-center displacements. Analytical expressions and numerical results in this work provide useful insights into the hydrodynamics of colloidal solids and macromolecules in confinement.