Dynamic viscosity is known by the Greek symbol η, eta.
It is sometimes also referred to as shear viscosity. Dynamic viscosity is obtained by reformulating Newton's Law.
The SI unit is pascal-second [Pa.s] or millipascal-second [mPa.s]:
- 1 Pa.s = 1000 mPa.s
- The SI unit is named after Blaise Pascal.
Other commonly used units are poise [P] or centipoise [cP]:
- 1 P = 100 cP
- This unit is named after Jean Poiseuille
Relation between units: 1 cP = 1 mPa.s
Dynamic viscosity is rather used in life science and research applications.
Kinematic viscosity ν, ny, describes a substance's flow behavior under the influence of Earth's gravity. It is dynamic viscosity divided by density ρ, rho, which is defined as mass per volume. The quantity mass carries the gravitational influence.
Kinematic viscosity is widely established due to historical reasons: Gravity as the driving force does not require any elaborate technical equipment. It is simply available everywhere on earth.
The SI unit is square-meters per second [m2/s] or square-millimeters per second [mm2/s]:
- 1 m2/s = 1 000 000 mm2/s
The SI units can be derived from the equation for the kinematic viscosity:
Other commonly used units are stokes [St] or centistokes [cSt]:
- 1 St = 100 cSt
- This unit is named after George G. Stokes.
Relation between units: 1 cSt = 1 mm2/s
Kinematic viscosity is mostly used to specify petrochemical fluids such as fuels or lube oils.
Relative viscosity is an important parameter when testing polymers in solutions.
For most polymers there is a definite relationship between molar mass and viscosity. You can measure the viscosity to determine the molar mass. The higher the molar mass is, the more viscous the polymer solution is. Molar mass is one of the most important quality parameters of polymers.
Though most polymer solutions are actually non-Newtonian fluids, they behave like Newtonian fluids as long as the applied shear rate is low enough.
The relative viscosity is calculated by dividing the viscosity of the polymer solution η by the viscosity of the pure solvent ηs. The unit is .
The relative viscosity ηr can be used to calculate other parameters used for polymer quality control, such as
- logarithmic viscosity number (or inherent viscosity)
- specific viscosity (or viscosity relative increment)
- reduced viscosity (or viscosity number VN or Staudinger Function)
- intrinsic viscosity (or limiting viscosity number LVN or Staudinger Index)
- molar mass1)
1) Molar mass is almost always expressed in [g/mol]. It is defined as the mass of a given substance divided by its amount of substance.
The definition of the dynamic or shear viscosity η is based on the two-plates-model and defined for ideally viscous fluids, i.e. the viscosity is constant independent of the shear rate applied to the sample.
If the viscosity is influenced by the shear rate, it is important to specify that the values are different from the constant ones of an ideally viscous fluid. The values obtained are 'apparent viscosity' or 'apparent shear viscosity' values. They represent one point of the viscosity function only. A viscosity function displays η over shear rate. The shear conditions need to be specified for apparent viscosity values.
Read more about shear-dependent materials in World of Rheology.
Example of apparent viscosity: