Elucidating the mechanism of helium evaporation from liquid water

TL;DR

This study combines molecular dynamics simulations and theory to elucidate the mechanism of helium evaporation from water, revealing a super-Maxwellian energy distribution influenced by interface friction and predicting temperature effects.

Contribution

It introduces a continuum theory based on a Fokker-Planck equation to explain helium evaporation, highlighting the role of interface friction and deformation.

Findings

Helium evaporation exhibits a super-Maxwellian kinetic energy distribution.

Friction near the interface is anomalously small due to interface deformability.

The model predicts temperature dependence of excess kinetic energy, awaiting experimental validation.

Abstract

We investigate the evaporation of trace amounts of helium solvated in liquid water using molecular dynamics simulations and theory. Consistent with experimental observations, we find a super-Maxwellian distribution of kinetic energies of evaporated helium. This excess of kinetic energy over typical thermal expectations is explained by an effective continuum theory of evaporation based on a Fokker-Planck equation, parameterized molecularly by a potential of mean force and position-dependent friction. Using this description, we find that helium evaporation is strongly influenced by the friction near the interface, which is anomalously small near the Gibbs dividing surface due to the ability of the liquid-vapor interface to deform around the gas particle. Our reduced description provides a mechanistic interpretation of trace gas evaporation as the motion of an underdamped particle in a potential subject to a viscous environment that varies rapidly across the air-water interface. From it we predict the temperature dependence of the excess kinetic energy of evaporation, which is yet to be measured.