On the critical exponent of η/s and a new exponent-less measure of fluidity

Abstract: We discuss on the critical exponent of $\eta/s$ for a fluid, and propose a new exponent-less measure of fluidity based on a mode-mode coupling theory. This exhibits a remarkable universality for fluids obeying a liquid-gas phase transition both in hydrodynamic as well as in nonhydrodynamic region. We show that this result is independent of the choice of the fluid dynamics, {\em viz.}, relativistic or nonrelativistic. Quark-Gluon Plasma, being a hot relativistic and a nearly perfect fluid produced in relativistic heavy-ion collisions, is expected to obey the same universality constrained by both the viscous and the thermal flow modes in it. We also show that if the elliptic flow data in RHIC puts a constraint on $\eta/s$ then the new fluidity measure for Quark-Gluon Plasma in turn also restricts the other transport coefficient, {\it viz.}, the thermal conductivity.