Viscosity and thermal diffusivity in liquids and gases

[dRic2]

Hi PF, I was wondering about this for some time and I can get my head around it.

Viscosity $\mu$ is a "measure" of the momentum flux
Thermal diffusivity $\alpha$ is a "measure" of the heat flux (kinetic energy of molecules)

In gases both viscous stresses and heat flux take place due to collisions between molecules thus the value of viscosity and thermal diffusivity is similar. That doesn't apply to liquids. In particular for very viscous fluids
$\frac \mu \rho >> \alpha$ ($\rho$ is the density). Since both phenomena happen for the same reason (collisions) I can't explain myself why there is this huge difference.

Ps: for liquid metals $\frac \mu \rho << \alpha$

 

[Chestermiller]

The molecular derivation of these quantities and their comparisons (the Prandtl number) are explained in detail in Transport Phenomena.

 

[dRic2]

Yes, but there is a lot of statistical physics (kinetic theory of gases) which I know nothing about. I was looking for an easier explanation if possible... Otherwise I'll have to wait until I know something about statistical mechanics