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I am trying to get an intuition for the following argument: In heavy ion collisions the central collision area can be described by almost ideal hydrodynamics at very early times after the impact. This leads to an asymmetric distribution of the final state hadrons in the angle $\phi$ around the beam axis rather than a uniform distribution. This was measured at RHIC. The fact that ideal hydrodynamics is a good description in particular means that the shear viscosity $\eta$ is small.

So far so good. Now, the crucial step is that this means that the underlying field theory is strongly coupled. The argument I read is, that strong coupling prevents a quasiparticle description. Quasiparticles would transport momentum and therefore damp shear flows. The absence of quasiparticles leads consequently leads to low shear viscosity.

My intuition would by somewhat opposite. Strong coupling suggests that the constituents of matter "stick together" strongly and therefore the viscosity is high. This intuition is obviously wrong.

Is the idea that some new degrees of freedom arise in the strong coupling limit whose effective coupling is small? Or is there a different explanation why strong coupling leads to low shear viscosity?

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  • $\begingroup$ I also wanted to know this for quite some time now $\endgroup$ – Hydro Guy Oct 6 '14 at 20:03
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In a weakly coupled fluid the viscosity scales as $\eta\sim 1/g^a$, where $a$ is a positive power and $g$ is the coupling. In QCD, up to logs, $a=4$. This means that in perturbation theory $\eta$ is parametrically large, and strong coupling is required to describe low viscosity fluids.

There are many counter-intuitive aspects of viscosity. For a recent discussion, see for example Sect. 1.2 (the final few paragraphs) of arXiv:1403.0653

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    $\begingroup$ Thanks, the references seems very interesting and comprehensive. However, I could not find an intuitive explanation of the low shear viscosity -> strong coupling argument. It only states on page 9 that the naive intuition is well motivated but wrong. Only the viscosity of very vicuous fluids is determined by force chains and therefore by the coupling. For other fluids they do not give another explanation except for the mean-free-path argument I mentioned in the question. $\endgroup$ – physicus Oct 7 '14 at 19:51
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    $\begingroup$ The argument is weak coupling --> large viscosity. Therefore, if low viscosity is observed then the coupling must be large. The statement strong coupling --> low viscosity is not always true. $\endgroup$ – Thomas Oct 8 '14 at 14:22

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