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Bulk viscosity and n-component fluids

Saga Säppi*

  • *Contact author: sappi@ieec.cat
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Phys. Rev. D 114, 016009 – Published 9 July, 2026

DOI: https://doi.org/10.1103/3hqb-q7h6

Abstract

Understanding the hydrodynamics of out-of-equilibrium dense viscous fluids is of key importance to accurate descriptions of physical systems such as compact stars, particularly their mergers. We consider a near-equilibrium relativistic fluid with n independent and small chemical potentials restoring the system back toward equilibrium. By diagonalizing the evolution equation for the out-of-equilibrium chemical potentials, we construct an explicit evolution equation for the bulk scalar of the system in second-order hydrodynamics in terms of equilibrium quantities. We find expressions for the 2n transport quantities and show that, in the rest frame of the fluid, the system admits a Green’s function corresponding to an n-component fluid.

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Phys. Rev. D 114, 016009– Published 9 July, 2026
  • Received 16 September 2025
  • Accepted 16 June 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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