The Widom line and noise power spectral analysis of a supercritical fluid

TL;DR

This study uses molecular dynamics simulations to analyze noise power spectra of density and energy fluctuations in a supercritical fluid, revealing distinct 1/f^γ behaviors near the Widom line, which could serve as an alternative signature of this thermodynamic boundary.

Contribution

It demonstrates the presence of characteristic 1/f^γ noise behaviors in density fluctuations along the Widom line, providing new insights into supercritical fluid dynamics.

Findings

Density fluctuation spectra exhibit three 1/f^γ regimes with γ=0, 1.2, 2.

The 1/f^γ behavior with γ~1 appears near the Widom temperature.

Power spectra at low frequency peak on the Widom line, indicating a potential signature.

Abstract

We have performed extensive molecular dynamics simulations to study noise power spectra of density and potential energy fluctuations of a Lennard-Jones model of a fluid in the supercritical region. Emanating from the liquid-vapor critical point, there is a locus of isobaric specific heat maxima, called the Widom line, which is often regarded as an extension of the liquid-vapor coexistence line. Our simulation results show that the noise power spectrum of the density fluctuations on the Widom line of the liquid-vapor transition exhibits three distinct $1/f^{\gamma}$ behaviors with exponents $\gamma$= 0, 1.2 and 2, depending on the frequency $f$. We find that the intermediate frequency region with an exponent $\gamma \sim$ 1 appears as the temperature approaches the Widom temperature from above or below. On the other hand, we do not find three distinct regions of $1/f^{\gamma}$ in the power spectrum of the potential energy fluctuations on the Widom line. Furthermore, we find that the power spectra of both the density and potential energy fluctuations at low frequency have a maximum on the Widom line, suggesting that the noise power can provide an alternative signature of the Widom line.