Diffusion in dense supercritical methane from quasi-elastic neutron scattering measurements

Abstract

Methane, the principal component of natural gas, is an important energy source and raw material for chemical reactions. It also plays a significant role in planetary physics, being one of the major constituents of giant planets. Here, we report measurements of the molecular self-diffusion coefficient of dense supercritical CH4 reaching the freezing pressure. We find that the high-pressure behaviour of the self-diffusion coefficient measured by quasi-elastic neutron scattering at 300 K departs from that expected for a dense fluid of hard spheres and suggests a density-dependent molecular diameter. Breakdown of the Stokes–Einstein–Sutherland relation is observed and the experimental results suggest the existence of another scaling between self-diffusion coefficient D and shear viscosity η, in such a way that Dη/ρ=constant at constant temperature, with ρ the density. These findings underpin the lack of a simple model for dense fluids including the pressure dependence of their transport properties.

Transfer-Free CVD Growth of High-Quality Wafer-Scale Graphene at 300 °C for Device Mass Fabrication

“…At a low chamber pressure, carbon atoms experience very little resistance during diffusion because the concentration of gas molecules and reactive radicals is low, and the scattering that occurs on the Cu surface is weak. This is consistent with the conclusion obtained by Ranieri et al and Du et al , They used quasi-elastic neutron scattering measurements and molecular dynamics simulations to confirm that the diffusion coefficient increases with decreasing pressure. Nevertheless, if the pressure is too low, for instance, dropped to 10 Pa, then the low concentration of carbon source in the chamber causes insufficient carbon atoms to be adsorbed on the Cu surface, and graphene cannot be synthesized (Figure S4).…”

Stokes–Einstein relation without hydrodynamic diameter in the TIP4P/Ice water model

“…A few recent examples confirming applicability of SE relation in the form of Eq. ( 2) to real liquid substances include liquid iron at conditions of planetary cores [19], dense supercritical methane (at least for the most state points investigated) [20,21], and silicon melt at high temperatures [22]. Several important non-spherical molecular liquids have been examined using numerical simulations in Ref.…”

Combining Molecular Dynamics and Machine Learning to Analyze Shear Thinning for Alkane and Globular Lubricants in the Low Shear Regime

“…We note that the SE relation itself is not applicable to nonequilibrium systems with shear, although the sliding distance is small, i.e., 0.01–10 Å at 1 ns. Moreover, the relation collapses for special cases, such as confined and highly dense liquids. , Because of the inhomogeneity of the systems, only the behavior in the streamwise direction was considered in this study. While MSD can be used in confined systems at equilibrium, , the usual MSD includes the effects of flow and molecular diffusivity.…”