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

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.