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.
In addition to the bearing that rotates the measuring system, some rheometers also have force sensors to detect vertical forces along the measuring shaft. The perpendicular force is called a “normal force” because it is “normal” to the surface of the sample. Normal force detection and control are important to rheometry because they impact how the sample is held in place during the measurement. However, not all samples and test types require normal force to produce an accurate result.
Normal force control is often set by the rheometer to hold in place samples that have drastic shrinkage or expansion during the measurement, such as liquids that become gels that are tested in a cone or plate system. The downward normal force applied by the measuring system helps retain the sample during rotation by preventing discharge from the cone or plate. A variety of gels that form during a rheology experiment benefit from having either zero or one Newton of force applied in this manner.
Additionally, it is helpful to measure the normal force of the sample pushing up against the measuring system during sample loading to prevent unnecessary compression. Delicate gels, foams, and pastes need to remain securely in place during the measurement but can be easily destroyed if too much force is applied by the plate or cone. Observing the normal force during loading can serve as a guide to when to stop lowering the measuring system if normal force levels increase drastically.
On the contrary, normal force is not as helpful when evaluating samples with a cup measuring system. Due to the sample confinement within the cup, the application of low normal force or high normal force does not prevent the sample from being discharged during testing. Regardless of normal force detection, thicker fluids and soft solids should be unloaded from the rheometer by detaching the measuring system before moving the motor upward. This will prevent the normal force from the sample wearing down the motor. Likewise, many samples can be loaded onto a lower plate and wedged with an upper plate or cone without an applied normal force. Generally, these include paints, coatings, colloids, and adhesives.
Rheometers that lack a normal force sensor can be a great way to save money while still receiving a decent torque range and motor control. Samples prone to slip out of the gap during measurement may be deterred through use of roughened measuring systems, either sandblasted or profiled, which use friction to help hold samples in place. As long as the instrument has gap control, gaps can be set to help apply a significant normal force by eyeing the sample without a normal force sensor. This does require more attention during sample loading, but this method can be used to secure thicker samples.
In addition to normal force control there are other key aspects of a rheometer that need to be considered as well. Next week we will take a look at the role of measuring systems on rheometer measurement capability.
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