Rheometers are necessary laboratory instruments for manufacturers and researchers of fluids and soft solids. With quality assessment, product development, and process control reliant upon the physical properties of these materials, a rheometer should be selected to stand up to the demands and last for a decade. Identifying key features and checking resolutions of various models will help you obtain an instrument that can meet your needs while also minimizing cost.
Measuring systems, also known as “fixtures”, “geometries”, and “tools” for the rheometer, are the pieces that hold the sample in place during testing. Different measuring systems have different surface areas in contact with the sample and different torque output per rotational speed, which gives each one its own limitations. When selecting a rheometer, it is important to consider future measuring system needs so that if you need to test different kinds of samples you can purchase modular components to augment your testing instead of having to buy a whole a new rheometer.
Most labs start off with a single measuring system and then purchase others if needed in the future. With limited budgets, it is important to assess if the one measuring system can perform for the variety of sample types and test conditions. For example, a parallel plate is often a good choice for running samples that range between a paint-like consistency and a thick lotion. The adjustable gap size helps accommodate samples with micron-scale particulates as well as samples that need higher shear rates. However, some samples may be ejected from the gap at high shear rates.
Another common measuring system used by rheometry beginners is the cup and bob (aka “concentric cylinder”) system. Any samples that can be poured into the cup - without exerting a high normal force on the cylinder – can be tested. There is no sample trimming involved, and it is rare that a sample would ever come out of the cup. However, the lowest viscosity samples (such as dilute water-based solutions) typically require higher torque sensitivity for accurate readings at low shear rates. Furthermore, the high sample volume required and more involved cleaning of the cup and bob surfaces can be a detriment.
Beyond the common measuring systems, there are many more that are required to obtain data for specific sample circumstances. Small plate diameters are useful for measuring thick gels and sticky pastes that produce high torque, whereas large diameter plates are well suited for measuring low viscosity fluids in small sample volumes. Likewise, stirrers are used in cups for samples that slip at the cylindrical surface. Going past conventional rheometer samples, films and fibers require a tension-inducing measuring system to obtain an accurate measurement. Similarly, oscillatory torsion can only be applied to hard solid samples when clamped in the appropriate sample holder.
In addition to the type of measuring system, rheometer users should also be aware of environmental factors influencing the sample in the measuring system. Temperature equilibration times can vary drastically between cup systems and plates, as well as the sample surfaces exposed and prone to evaporation. When designing rheometry tests, make sure the measuring system required for the sample is capable of being used at the temperature and rheometer settings. Oven heating systems are usually limited to plate or cone systems, while tack tests are limited to plates.
Measuring system variety and selection are just one aspect of rheometers that needs to be considered when making a purchase. Next week’s article will examine the need for rotational and oscillatory testing capability.
Have questions about rheometer measuring systems? Click here to ask a rheologist.