Kinematic Vs. Dynamic Viscosity

Here at SGS Polymer Solutions, we love to make the world of analytical testing and polymer science more understandable. From explaining our testing lab services to examining the “why?” of everyday science, we love what we do.

Today, we want to explain a complex idea: viscosity. At its basic level, viscosity is the measure of a material’s resistance to flow. Viscosity is one of the many metrics we can use for physical analysis of polymer materials.

A wide range of polymers we test are often the object of scrutiny across industries — from legal testing to complex analysis for research and development. Liquids, gels, and many polymers are often tested to ensure that they meet the standards and specifications as raw materials for the products they are destined to be included in. Because viscosity is a basic measurement of fluid dynamics, these measurements are important for the characterization of materials.

There are many types of viscosity. Today, we want to talk about two basic types: dynamic viscosity and kinematic viscosity. While the units used to measure each type of viscosity can be converted in order to understand the other, they differ in very fundamental ways.

Dynamic Viscosity

Dynamic viscosity is the measure of how a fluid resists flow when an external force is applied. You evaluate dynamic viscosity when you want to understand how the material reacts to deformation, which can be measured by Brookfield or rheometer testing.

A Brookfield test can evaluate if a product meets specific manufacturing or use specifications and is suitable depending on the type of substance being tested. We regularly test gels, lotions, and adhesives using this test. Brookfield testing requires that the substance flows readily in a certain fluid motion, but we can test less viscous substances by other techniques. Using as little as eight milliliters of a substance, we can measure the dynamic viscosity and report a value in centipoise.

Kinematic Viscosity

Kinematic viscosity differs from dynamic viscosity in that it only measures how gravity affects a substance’s resistance to flow. Kinematic viscosity assumes that no outside forces are acting upon the substance. This is reported using centistokes.

Because of the density-dependent relationship between kinematic and dynamic viscosity, two substances that have the same dynamic viscosity can have widely different kinematic viscosities. In order to convert the two, both the temperature and specific gravity of a substance are taken into account. Dividing by the density of a fluid converts centipoise to centistokes. This relationship allows us to determine both the kinematic and dynamic viscosities of a given sample using only single viscosity measurement if the fluid density is known.

So what test would we choose for a specific sample? That is informed by the type of fluid as well as the desired information. Fluids are divided into two basic types: Newtonian and non-Newtonian.

Newtonian fluids do not change viscosity with shear rate–that is, the rate at which one layer of fluid passes over an adjacent layer. Water and oil are examples of Newtonian fluids. While it isn’t always the case, we don’t normally test Newtonian fluids using Brookfield testing or rheometer testing. We generally test Newtonian fluids using a capillary viscometer tube, where changing the shear rate is not needed.

In the world of polymers, there are many non-Newtonian fluids. The viscosity of these substances does change with shear rate. Because we can readily change the shear rate of the testing, we test these using Brookfield and rheometer testing.

Want to learn more about the testing environments we use for testing viscosity? Get in touch today! Our scientists are always happy to learn more about your testing needs and design exact testing environments to meet the requirements of each individual client. We’re always up for a challenge when it comes to independent, analytical tests.

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