The property of fluids that leads to flow resistance. There is fluid friction between layers of fluid that move past each other (shear stress).

Oil between two plates. The upper plate is pulled to the right, the layers of oil move at different speeds (highest at the top, not at all at the bottom).

The motion (momentum) imparted to the upper plate leaves this plate and flows down through the layers of oil to the lower plate. The quantity of momentum flowing through the oil every second and per square meter (j_p, unit: Pa) depends upon the gradient of speed in the y direction (dv/dy). The factor depends upon how viscous the oil is, and is called the viscosity (eta or µ).

eta, or µ.

Shear stress or momentum current density j_p = eta · dv/dy

Kinematic viscosity nu = Viscosity / Density.

[eta] = Pa·s

dynamic viscosity, momentum conductivity

Hagen and Poiseuille, Reynolds number, turbulent flow laws, laminar flow laws, conductive transports, electric conductivity, thermal conductivity

The inverse of the momentum conductivity drops to zero for water when it freezes (ice is superconducting). Values of viscosity in Table 3 (Chapter 1).

Viskosität