Shear flows in far-from-equilibrium strongly coupled fluids

TL;DR

This paper investigates the behavior of shear flows in strongly coupled fluids far from equilibrium using holographic models, revealing universal late-time ratios of viscosity to entropy density that depend on shear rate.

Contribution

It demonstrates that dissipative transport coefficients like viscosity exhibit universal behavior even far from equilibrium in holographic models.

Findings

Viscosity-to-entropy ratio approaches a constant at late times.

This ratio depends on shear rate and can be smaller than hydrodynamic predictions.

Universal behavior persists even when the system is far from equilibrium.

Abstract

Despite the viscosity of a fluid ranges over several orders of magnitudes and is extremely sensitive to microscopic structure and molecular interactions, it has been conjectured that its (opportunely normalized) minimum displays a universal value which is experimentally approached in strongly coupled fluids such as the quark-gluon plasma. At the same time, recent findings suggest that hydrodynamics could serve as a universal attractor even when the deformation gradients are large and that dissipative transport coefficients, such as viscosity, could still display a universal behavior far-from-equilibrium. Motivated by these observations, we consider the real-time dissipative dynamics of several holographic models under large shear deformations. In all the cases considered, we observe that at late time both the viscosity-entropy density ratio and the dimensionless ratio between energy density and entropy density approach a constant value. Whenever the shear rate in units of the energy density is small at late time, these values coincide with the expectations from near equilibrium hydrodynamics. Surprisingly, even when this is not the case, and the system at late time is far from equilibrium, the viscosity-to-entropy ratio approaches a constant which decreases monotonically with the dimensionless shear rate and can be parametrically smaller than the hydrodynamic result.