(52c) Widom Line, Dynamical and Structural Crossovers in Supercritical Oxygen Via Molecular Dynamics Simulations

Supercritical oxygen, a cryogenic fluid, is
widely used as an oxidizer in jet propulsion systems. Understanding and
characterizing the supercritical state is therefore of paramount importance in
providing physical insights into processes such as cryogenic jet break-up, seen
in combustion chambers of rockets, and jets. Recently, a ‘pseudo-boiling’
phenomenon was described for trancritical injection
processes, prompting the discussion on a through characterization of the
supercritical state of oxygen.

It is well known from existing literature that
the locus of the maxima of thermodynamic response functions converge to a
single line, referred to as ‘The Widom line’, on
approaching the critical point, for a number of substances such as argon, water,
nitrogen and others. In this study, we identify the Widom
line for oxygen from atomistic molecular dynamics simulations of supercritical
oxygen and compare them with experimental data of thermodynamic response
functions. Additionally, we also analyze the crossover in transport properties
from liquid-like to gas-like behavior with increasing temperature, which
appears to coincide with the Widom line. In addition
to the transition in shear viscosity and translational diffusion coefficient,
we also determine the crossover in the rotational diffusion constant by
analyzing the behavior of the rotational auto-correlation function of
supercritical oxygen, which shows a marked change from diffusion dominated
hindered rotation to a free rotation dominated state with increasing
temperature. Finally, we identify the structural crossover for supercritical
oxygen across the Widom line, by analyzing the pair
correlation function and the structure factor. Our results indicate that in
addition to the thermodynamic response functions, transport properties and
structural properties show a marked change in the vicinity of the critical
point, and that supercritical oxygen is far more complex than originally
perceived.

Fig 3. Rotational
correlation function C₂(t) of supercritical oxygen at different
thermodynamic states showing different rotational behavior, obtained via
atomistic MD simulations.