kinematic viscosity

Kinematic viscosity asks not just how sticky a fluid is, but how readily it flows under its own weight. It combines the fluid's internal resistance with how heavy it is: a light, thin liquid drains quickly, while a dense, thick one drains slowly. It is essentially the viscosity you 'see' when gravity is doing the pushing.

Numerically, kinematic viscosity is the dynamic (ordinary) viscosity divided by the density of the fluid. Its SI unit is the square metre per second, but the commonly used practical unit is the centistokes (cSt). Because the most familiar capillary instruments measure how long a fixed volume of liquid takes to drain under gravity, what they read out directly is kinematic viscosity.

The distinction matters whenever density differs noticeably between samples. Two oils could have the same dynamic viscosity but, if one is denser, the denser one will have a lower kinematic viscosity and drain faster through the same capillary. In pharmacopoeial methods using glass capillary viscometers, results are reported as kinematic viscosity and then converted to dynamic viscosity by multiplying by density when needed.