### Abstract

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.

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.

Original language | Danish |
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Article number | 191102 |

Journal | Journal of Chemical Physics |

Volume | 152 |

Issue number | 191102 |

Number of pages | 7 |

ISSN | 0021-9606 |

DOIs | |

Publication status | Published - 20 May 2020 |

## Cite this

Bell, I., Galliero, G., Delage-Santacreu, S., & Costigliola, L. (2020). An entropy scaling demarcation of gas- and liquid-like fluid behaviors.

*Journal of Chemical Physics*,*152*(191102), [191102]. https://doi.org/10.1063/1.5143854