What is Viscosity?

You can think of viscosity as various types fluids taking different times to, for example, pour out of a cup. Low viscosity fluids, like water, will pour out of a glass in a fraction of a second and take the shape of the receiving container just as quickly. On the other hand, a high viscosity fluid like honey would take minutes to pour out and the interface between honey and air would take just as long to settle. Looking at the time moving the fluid takes, we can try to establish which of those two fluids is more viscous. This time-based measurement is what we refer to as kinematic viscosity: how fast does a fluid flow for a given force applied to it. In our example, the applied force is gravity.

Another way to think about viscosity measurements is using a piston or syringe. Imagine that you have a very low viscosity fluid, like water, inside a syringe. You can probably push the fluid out easily using your fingers. However, if you have honey inside the syringe, you better have a pretty sturdy syringe and would likely require some additional mechanical system to help you apply enough force to push the honey out of your syringe. In this case we have applied and measured necessary force to make the fluid flow. If we select a fixed amount of time we want to take for the injection, or the injection rate, and measure the force or stress, the resulting measurement is what we call dynamic viscosity.