Temperature guided behavioral transitions in confined helium: Gas-wall interaction effects on dynamics and transport in the cryogenic limit

Abstract

The low-temperature dynamics and transport of a real gas He, in confined condition are studied employing molecular dynamics simulations. Characterization of the gaseous properties at varying temperatures (T) in the range 30 - 150 K, is carried out employing velocity autocorrelation function (VACF), mean squared displacement (MSD), and the transport properties such as, self-diffusion coefficient (D) and thermal conductivity (λ) are determined. In the confined system, gas-wall interactions are found to dictate the above properties, and near-wall accumulation of molecules is observed. Further, depending upon T, the effect of wall-mediated forces is found on the distant gas particles. At very short timescales (t* < 0.1), gas-wall interactions result in faster decay of VACF and shorter ballistic phase in MSD along the confining direction, which are most prominent at higher T, compared to those for the unconfined directions. Whereas, at short timescales (t*∼ 0.5) non-thermal gas-wall collisions are observed, leading to a minimum in VACF along the confining direction depending upon T. At longer timescales (t* > 1), gas-wall particle collisions lead to the sub-diffusive behavior of the confined gas along the confinement, which is most prominent at higher T. The results are compared with those for the bulk He to quantify the confining effect on the gas. Reasonably good agreement of the bulk transport properties with numerically calculated quantum mechanical results for LJ potential and existing results from literature (experimental and quantum mechanical) has been found. In the confined system, the parallel self-diffusion coefficient and thermal conductivity (D∥ and λ∥) are found to be close to the bulk values. Additionally, λ is estimated from direct heat flux measurements using the Green-Kubo (G-K) formalism and non-equilibrium method for the confined system, and the results are compared. The findings of this work have far-reaching consequences in investigating complex systems.