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 resolution of various models will help you obtain an instrument that can meet your needs while also minimizing cost.
Oscillatory Testing Capability
There are two main types of rheometry tests – rotational and oscillatory – and it is important to recognize if your rheometer can perform both types, and when to use each one. Rotational testing is the more common methodology due to the application of an angular velocity or shear rate to examine viscosity and shear stress. Conversely, oscillatory testing, developed in the 80s and 90s, provides key insights to sample structure through measuring the two components of shear moduli (G’ and G”) while also producing much more detail on low-shear behavior. Knowing which test type to run and the limitations of your rheometer will help you design optimized rheometry methods for your application.
A vast majority of rheometers both old and new are capable of running rotational tests. At the most basic level, applied angular velocities can be set through a simple motor that spins the measuring system at specific speeds with appropriate consideration of the geometry of the measuring system (set shear rate). Ball-bearing motors are an economical way to conduct rotational tests at moderate to high shear rates, and modern torque transducers in newer rheometers help maintain accurate readings. However, to collect data from low-viscosity samples such as water, a more refined motor may be required due to the low torque sensitivity. Air bearing motors provide a high torque sensitivity that can more easily measure low viscosity under low shear. In addition to the refinement offered by air bearing instruments to rotational testing, oscillatory measurement capability is a key feature that should not be minimized.
Oscillatory shear strain enables a wide variety of mechanical probing for soft solids and concentrated colloids. Data from the sample’s response to low strain can be interpreted for aging studies, evaporation tests, and crosslinking kinetics. Furthermore, analyses of the outputted shear moduli (G’ and G”) reveal the deformation limits and the structural changes from increased strain. In this way, viscoelastic properties are directly quantified through the delicate and sensitive measurements of oscillation.
To set an oscillatory test, sinusoidal shear strain must be achieved through oscillating the measuring system with a highly controlled angular frequency. To perform oscillatory tests, much more specifications are required of a rheometer than for rotational testing. A high torque sensitivity and precise rotor control are imperative for applying an accurate shear strain. When selecting a rheometer for purchase, consider the torque range and inquire about oscillatory test capability. You can get a good impression of the rheometer’s capability by looking at example data from samples with a similar consistency taken over decades of small shear strain. Additionally, consider the rheometer’s options for data averaging and optimized oscillatory input to get an idea of how much the instrument will automate achieving sinusoidal strain without user input.
Whether purchasing a rheometer for everyday needs, such as basic quality control or advanced work in research, take into account future testing requests. Refined motor control and a vast torque range are features that enable cutting-edge measurements that are essential to industrial development.
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