Diffusivity and Hydrodynamic Drag of Nanoparticles at a Vapor-liquid Interface

TL;DR

This study investigates the surface diffusivity and hydrodynamic drag of nanoparticles at vapor-liquid interfaces, revealing that contact line fluctuations do not cause increased drag, contrary to previous hypotheses, and showing consistent drag behavior across experiments.

Contribution

Molecular dynamics simulations demonstrate that contact line fluctuations do not lead to anomalous drag increases, resolving a paradox in nanoparticle interfacial dynamics.

Findings

Drag decreases as particles move further into vapor.

Surface drag is less than bulk liquid drag due to partial submersion.

Contact line fluctuations do not cause increased drag.

Abstract

Measurements of the surface diffusivity of colloidal spheres translating along a vapor/liquid inter- face show an unexpected decrease in diffusivity, or increase in surface drag (from the Stokes-Einstein relation) when the particles situate further into the vapor phase. However, direct measurements of the surface drag from the colloid velocity due to an external force find the expected decrease with deeper immersion into the vapor. The paradoxical drag increase observed in diffusion experiments has been attributed to the attachment of the fluid interface to heterogeneities on the colloid surface, which causes the interface, in response to thermal fluctuations, to either jump or remain pinned, creating added drag. We have performed molecular dynamics simulations of the diffusivity and force experiments for a nanoparticle with a rough surface at a vapor/liquid interface to examine the effect of contact line fluctuations. The drag calculated from both experiments agree and decrease as the particle positions further into the vapor. The surface drag is smaller than the bulk liquid drag due to the partial submersion into the liquid, and the finite thickness of the interfacial zone relative to the nanoparticle size. Contact line fluctuations do not give rise to an anomalous increase in drag.