What is Viscosity?

Viscosity describes how easily a fluid flows. Low-viscosity fluids like water pour quickly and rapidly take the shape of a container, while high-viscosity fluids like honey flow slowly and take much longer to settle. By comparing how long fluids take to move under the same force, such as gravity, we can determine which is more viscous. This time-based behavior can be captured by kinematic viscosity, which describes how fast a fluid flows under its own weight.

Imagine a very low-viscosity fluid, such as water, inside a syringe; you can easily push it out using your fingers. In contrast, if the syringe is filled with a high-viscosity fluid like honey, you would need a much sturdier syringe and likely some mechanical system to apply enough force to make the fluid flow.

In this scenario, a force (or pressure) is applied to drive the fluid through the syringe. If we impose a fixed flow rate (or injection rate) and measure the required force or stress, the resulting relationship between stress and flow rate gives an idea of the fluid’s dynamic viscosity.