What Is Kinematic Viscosity?

Kinematic viscosity is a property of liquids and gases that represents how easily a given substance can flow. In practical terms, it is closely related to how thick the substance is. Both absolute and kinematic viscosity change according to temperature.

To obtain kinematic viscosity, the absolute viscosity of a substance is divided by its density. Kinematic viscosity is represented by the Greek letter nu, which resembles a "v;" absolute viscosity is represented by mu, which resembles a "u;" and density is represented by rho, which resembles a "p." The equation, therefore, is v = u/p.

Absolute viscosity, also called dynamic viscosity, measures a substance's resistance to flow. It is determined experimentally by sandwiching a liquid or gas between two plates and applying a known amount of pressure to move the top plate — the dynamic viscosity depends on the pressure, the amount of time it was applied, and the distance the plate moved in that time. Dynamic or absolute viscosity is based on the International System of Units (SI) units of pascal-seconds (Pa*s), which means that if a pressure of 1 Pa is applied for 1 second, the plate will move the same distance as the distance between the two plates. Centipoise (cP) is also a common unit for dynamic viscosity — 1 cP is the viscosity of water around room temperature. The imperial units, pound-seconds per square foot (lb*s/ft^2), are very seldom used.

Density measures the mass of a substance relative to its volume, which means that it has units of mass per volume. The units are kg/m^3 in SI units or slugs/ft^3 in imperial units. Density can be understood by comparing it to weight — a piece of a denser material will weigh more than the same-size piece of a less dense material.

Since kinematic viscosity is dynamic viscosity divided by density, it has units of square meters per second (m^2/s) in the SI system or square feet per second (ft^2/s) in the imperial system. As for absolute viscosity, the imperial units are almost never used. Centistokes (cSt) are commonly used units — one cSt is the kinematic viscosity of water at about room temperature. One cSt equals 10^-6 m^2/s.

Heat affects material properties, so both types of viscosity change at higher temperatures. When a liquid is heated, it flows more easily and thus the viscosity decreases. Kinematic viscosity is somewhat less affected than absolute viscosity, as heat also reduces the density because molecules move farther apart as a substance is heated. The viscosity of gases increases at higher temperatures — as a gas expands, it exerts more pressure on the plate, making it harder to move.