What is the difference between kinematic and dynamic viscosity?

The difference between kinematic and dynamic viscosity lies in their definitions, units, and applications:

Dynamic Viscosity (Absolute Viscosity, μ):

Definition: Measures a fluid's internal resistance to flow when an external force (shear stress) is applied. It reflects the fluid's "thickness" or internal friction.

Units: Pascals-seconds (Pa·s) or Poise (P), with 1 Pa·s = 10 P.

Application: Used in engineering contexts involving force calculations, such as lubrication, pipe flow resistance, or designing pumps.

Kinematic Viscosity (ν):

Definition: Represents the fluid's resistance to flow under gravity, incorporating both dynamic viscosity and density. It relates viscous forces to inertial forces.

Units: Square meters per second (m²/s) or Stokes (St), with 1 m²/s = 10,000 St.

Application: Relevant in scenarios where gravity or buoyancy matters, such as fluid flow measurements (e.g., capillary viscometers), weather (airflow), or fuel flow in engines.

Key Distinction:

Dynamic viscosity focuses on the fluid's internal resistance to shear.

Kinematic viscosity accounts for how that resistance interacts with the fluid's density, influencing behavior under gravitational or inertial forces.

Temperature Dependence:

For liquids: Dynamic viscosity decreases with temperature, while kinematic viscosity may increase due to reduced density.

For gases: Dynamic viscosity increases with temperature, and kinematic viscosity rises even more as density drops.