Properties of the quantum fluid at RHIC

Collective flow in collisions between large nuclei at RHIC is a dominant process in single-particle transverse momentum distributions and in azimuthal correlations. Thus, viscosity must be large enough to prevent strong turbulent flow but not too strong as to dissipate all collective motion into random thermal excitation. It is shown that the transition from hadrons to quarks and gluons has behaviour similar to classical fluids near their phase transitions in the ratio of shear viscosity to entropy density.