Phase Transitions and the Perfectness of Fluids

TL;DR

This paper investigates how the ratio of shear viscosity to entropy density (eta/s) behaves during phase transitions in scalar field theories, revealing patterns consistent with empirical data and suggesting universality across different physical systems.

Contribution

It provides mean-field calculations of eta/s in scalar theories with phase transitions, aligning with experimental data and discussing implications for universal fluid properties.

Findings

eta/s behaviors match empirical data for water, helium, nitrogen, etc.

The results suggest eta/s patterns are universal across different systems.

Clarifies issues regarding counterexamples to the eta/s bound conjecture.

Abstract

We calculate the ratio eta/s, the shear viscosity (eta) to entropy density (s), which characterizes how perfect a fluid is, in weakly coupled real scalar field theories with different types of phase transitions. The mean-field results of the eta/s behaviors agree with the empirical observations in atomic and molecular systems such as water, He, N, and all the matters with data available in the NIST database. These behaviors are expected to be the same in N component scalar theories with an O(N) symmetry. We speculate these eta/s behaviors are general properties of fluid shared by QCD and cold atoms. Finally, we clarify some issues regarding counterexamples of the conjectured universal eta/s bound found in Refs.[16,17].