An entropy scaling demarcation of gas- and liquid-like fluid behaviors

Ian Bell (1) , Guillaume Galliero (2) , Stephanie Delage Santacreu (3) , Lorenzo Costigliola (4)

1 NIST - National Institute of Standards and Technology [Boulder]
2 LFCR - Laboratoire des Fluides Complexes et leurs Réservoirs
3 LMAP - Laboratoire de Mathématiques et de leurs Applications [Pau]
4 Department of Science and Environment [Roskilde]

In this work, we propose a generic and simple definition of a line separating gas-like and liquid-like fluid behaviors from the standpoint of shear viscosity. This definition is valid even for fluids such as the hard sphere and the inverse power law that exhibit a unique fluid phase. We argue that this line is defined by the location of the minimum of the macroscopically scaled viscosity when plotted as a function of the excess entropy, which differs from the popular Widom lines. For hard sphere, Lennard-Jones, and inverse-power-law fluids, such a line is located at an excess entropy approximately equal to −2/3 times Boltzmann’s constant and corresponds to points in the thermodynamic phase diagram for which the kinetic contribution to viscosity is approximately half of the total viscosity. For flexible Lennard-Jones chains, the excess entropy at the minimum is a linear function of the chain length. This definition opens a straightforward route to classify the dynamical behavior of fluids from a single thermodynamic quantity obtainable from high-accuracy thermodynamic models.

Domaines: Chimie-Physique [physics.chem-ph]

Ian Bell, Guillaume Galliero, Stephanie Delage Santacreu, Lorenzo Costigliola. An entropy scaling demarcation of gas- and liquid-like fluid behaviors. Journal of Chemical Physics, 2020, 152 (19), pp.191102. ⟨10.1063/1.5143854⟩. ⟨hal-02749763⟩