Kinematic viscosity measures how easily a substance flows and is related to its thickness. It is obtained by dividing absolute viscosity by density. Absolute viscosity measures a substance’s resistance to flow and is determined experimentally. Density measures mass per volume. Both types of viscosity change with temperature.
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 viscosities change with 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 looks like a “u”; and density is represented by rho, which looks like a “p”. The equation, therefore, is v = u/p.
Absolute viscosity, also called dynamic viscosity, measures the resistance of a substance to flow. It is determined experimentally by placing a liquid or gas between two plates and applying a known amount of pressure to move the top plate: dynamic viscosity depends on the pressure, the amount of time it was applied, and the distance the plate traveled in that time . Dynamic or absolute viscosity is based on 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. Imperial units, pound-second per square foot (lb*s/ft^2), are very rarely used.
Density measures the mass of a substance relative to its volume, meaning 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 sized piece of a less dense material.
Since kinematic viscosity is dynamic viscosity divided by density, it has units of meters squared per second (m^2/s) in the SI system or feet squared per second (ft^2/s) in the imperial system. As for absolute viscosity, imperial units are almost never used. Centistokes (cSt) are commonly used units: a cSt is the kinematic viscosity of water at 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 therefore the viscosity decreases. Kinematic viscosity is somewhat less affected than absolute viscosity, as heat also reduces density because molecules move further apart when a substance is heated. The viscosity of gases increases at hotter temperatures: as a gas expands, it puts more pressure on the plate, making movement more difficult.
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