# Dynamic and Kinematic Viscosity

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### Dynamic Viscosity

The dynamic viscosity is the internal friction coefficient, which is a measure of the viscosity of a fluid (flowing substance). Different substances have different viscosities. The viscosity of common liquids decreases with increasing temperature, and the viscosity of common gases increases with increasing temperature. In general, a substance with a higher viscosity is less likely to flow; a substance with a lower viscosity is easier to flow. The unit of dynamic viscosity is N·s/㎡ or Pa.s.

### Relationship Between Dynamic Viscosity and Kinematic Viscosity

The kinematic viscosity (v) is the ratio of the dynamic viscosity (η) of the fluid to the density (ρ) of the fluid at the same temperature. The unit is (m^2)/s, and the equation is: Equation: ν=η/ρ.

### Fluid Viscosity

When there is relative movement between two adjacent layers of fluid, there will be resistance to deformation on the contact surface of the two layers of fluid. Different from solids, this resistance is not related to the deformation size of the fluid, but proportional to the deformation speed of the fluid. The resistance to deformation of the fluid is called viscosity.

### Viscosity of Some Common Substances

The viscosity of different fluids varies greatly. At a pressure of 101.325kPa and a temperature of 20℃, the dynamic viscosity and kinematic viscosity of common fluids are as follows:

• Water: μ=1.01×10^(-3) Pa.s, v=1.01×10^(-6) ㎡/s;
• Glycerine: μ=1.499Pa.s, v=1.19×10^(-3) ㎡/s;
• Air: μ=17.9×10^(-6) Pa.s, v=14.8×10^(-6) ㎡/s;