What is Viscosity?
If the definition of the word “viscosity” seems as though it should be simple, then just Google it. In the world of fluid mechanics and in academic circles where mixing processes are analyzed in great depth, terms such as kinematic viscosity, dynamic viscosity, impeller viscosity and process viscosity are used to describe material properties very specific to their occurrence in a given batch. We’ll leave the specifics of each definition and its equation derivation to academia.
A very simple way to think about viscosity is that it is a measure of how much a liquid resists flow. Most people think of “thickness” as being associated with a liquid’s viscosity as those that are thicker tend to flow more slowly or very little at all.
For example, think about the speed in which water and honey come out of their respective containers. Water falls out of the container faster than honey because it has a low viscosity, a low viscosity entails that the molecular bonds in water itself are weak and makes the fluid fall faster. Meanwhile, honey has a high viscosity because the molecular bonds are very strong and thickens the fluid, which is why honey moves slowly out of a container.
However, for the average customer the practical thing to understand is that as the viscosity increases the needed impeller size and mixer type changes.
These liquids need more surface area from an impeller to push them. For a center-mounted mixing setup the impeller size can be as high as 75% of the vessel diameter and be driven using a rigidly mounted gear-reduced mixer.
A quick rule of thumb is that as liquid viscosity moves from “momentum mixing” where the liquid remains in motion when agitation stops to a situation where the liquid wants to stop quickly we generally recommend a large diameter impeller turning slowly with a bit of extra horsepower capacity and variable speed control. This combination performs well as “process insurance”.