On the ratio of the shear viscosity to the density of entropy of the rare gases and H2,N2,CH4, and CF4

- The shear viscosity to density of entropy ratio is minimal at the critical point.
- The minimal ratio obeys the conjecture proposed by Kovtun, Son and Starinets.
- The minimal ratio depends on mass and collision diameter of atoms and molecules.

The ratio κ of the shear viscosity η to the volume density of entropy s,κ=η/s, of the rare gases He, Ne, Ar, Kr, and Xe and the molecules H2,N2,CH4, and CF4 is studied in the critical region. It is shown that the minimum value κm of η/s is observed in the vicinity of the critical point. In addition the universal inequality κ⩾ℏ/(4πk) proposed by Kovtun et al. (2005) holds for these gases. For the rare gases and 6Li κm depends linearly on m1/2σ2, where m is the mass and σ defined via the potential energy function as U(σ)=0. κm of the molecules H2,N2,CH4, and CF4 falls on the same straight line provided that the vibrational and rotational contributions to the entropy density s are removed.