Diffusivity and hydrodynamic drag of nanoparticles at a vapor-liquid interface

Measurements of the surface diffusivity of colloidal spheres translating along a vapor-liquid interface 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. We perform molecular dynamics simulations of the diffusivity and force experiments for a nanoparticle with a small surface roughness at a vapor-liquid interface to examine the effect of contact line fluctuations. The drag calculated from both calculations 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. We observe weak contact line fluctuations and transient pinning events, but these do not give rise to an anomalous increase in drag in this system.