Andy Pye caught up with Malvern Panalytical at the 28th International Forum for Materials Testing, which was held in Ulm, Germany, as it always is, on the premises of ZwickRoell. Malvern Panalytical is a leading provider of scientific instrumentation, including rheometry.

Standard Operating Procedures (SOP): Viscometerdrawellanalytical.com

The resistance of a substance to flow is referred to as the material's viscosity, and its mathematical definition is the shear stress divided by the shear rate.

When dealing with Newtonian fluids, such as water, the viscosity of the fluid is generally unaffected by the shear rate that is applied to it.

In contrast, non-Newtonian fluids either shear thin, meaning that their viscosity decreases when subjected to higher shear rates, or shear thicken, which means that their viscosity increases when subjected to an increased shear rate. Shear thickening is a phenomenon that occurs less frequently.

Viscosity is a performance-defining parameter for a large number of industrial products, of which only a small percentage are Newtonian. Knowing the properties of a substance under the conditions that are typically used is necessary in order to have an accurate understanding of how it will act when it is being used or processed. It should come as no surprise that this presents a significantly greater challenge for non-Newtonian materials than it does for Newtonian ones.

There are a lot of materials and formulations that are used frequently, and a lot of them have complex rheological properties. The viscosity and viscoelasticity of these materials and formulations can change depending on the external conditions that are applied, such as the stress, the strain, the timescale, and the temperature.

In many cases, it is necessary for the viscosity to be high when the shear rate is low in order to prevent sedimentation or slumping, but it must become less thick when the shear rate increases in order to make application or processing easier. Consider the most recent time you went to a restaurant that served you food with a bottle of ketchup already sitting on the table. What did you do if the ketchup was stubborn and wouldn't pour? You gave the bottle a good shake, which made the ketchup easier to pour out of the bottle. Ketchup is a non-Newtonian fluid, which is why the experiment was successful.

Therefore, a single viscosity measurement is not enough to adequately describe the viscosity of such materials; rather, the viscosity should be measured over a range of shear rates or stresses.

Viscometer or rheometer?

Simple flow measurements on Newtonian materials are the bread and butter of a viscometer's design. A viscometer can be made portable so that it can be used for testing in the field or in remote locations.

Rheometers are more versatile and have a much wider dynamic range of control and measurement parameters than viscometers, despite the fact that rheometers are more expensive than viscometers. Using a rheometer, one is able to more accurately characterize the flow, deformation, and even the tacky quality of a material (for both Newtonian and non-Newtonian materials). Benchtop viscometers made by one manufacturer cost between $2,000 and $4,000, while rheometers made by the same company have a price range of $4,500 to $25,000.

Viscometers typically have a measurement range of about 0.1 to 1000 s-1, whereas rheometers have a measurement range that extends from 0.000001 to 10,000 s-1. A greater measurement range makes it possible to obtain relevant data by subjecting the sample to conditions that are representative of those that may be encountered during the manufacturing process or while the product is in use.

However, viscometers and rheometers are complementary: it is not uncommon to find viscometers being used for quality control testing on products that have been developed using a rheometer within the same organization. The line that divides the two options is becoming increasingly blurry. The majority of digital viscosity meter come equipped with the capability to measure a non-Newtonian material and show how its viscosity changes as a function of spindle rotational speed. Some digital viscosity meter also have the ability to measure multiple speed ranges.

In recent years, there has been significant development in the software that drives both of these. These advancements include systems that help the user choose the appropriate test for investigating particular behaviors, that guide through measurement, that interpret the data that is gathered, and that create permanent test records. Some instruments come equipped with an inbuilt capability to perform mathematical modeling on a wide variety of data sets and analyze the results.

When trying to gain insight into the ways in which molecular structure influences processing characteristics, rotational rheometers are the method of choice. The accuracy of their measurements is at least +/-10%, despite the fact that they are relatively simple to use. There are many different configurations, but each one has some kind of component that rotates at a consistent rate inside the liquid. In one common configuration, there are two coaxial cylinders with the liquid to be measured contained between them. In another configuration, known as the cone-and-plate design, the liquid is placed on a horizontal plate, and a shallow cone is placed into the space created by the liquid.

Knowing a material's rheological properties is essential because the vast majority of polymeric materials are worked on in their molten state, either by extrusion or injection molding. It was specifically developed for the purpose of simulating the processing conditions of molten polymers at high temperatures, which is why high-pressure capillary extrusion is now a well-established method for doing so. It allows for the flow properties to be measured under conditions that are typical of processing and extends the shear rate range that can be achieved in the laboratory beyond that which is available in a rotational instrument.