Scientists, engineers, and technicians all want to have accurate and reproducible rheology data. The following tips explain how to prevent messy data points in order to refine the result.
Data Point Collection
It is understandable that researchers want to perform rheometry tests as quickly as possible, but using a fast rate of data collection can produce disastrous results. Each data point averages the sample response, and the longer time given per data point ensures a more accurate output. Unless performing a shear start-up test to capture the very initial moments of shear rate, data collection should allow at least 1 s/point for low-viscosity fluids and 5 s/point for colloids. These are the recommended quickest data collection rates, and users are advised to allow more time for data collection when testing a sample for the first time.
Select the Appropriate Measuring System
A one-measuring-system-fits-all approach to rheometry does not give accurate data for
every sample. Water-like viscosity samples should be tested with a double-gap cylinder while the thickest gels and pastes are best used with a small diameter plate. Larger surface areas of the measuring systems give greater sensitivity, but are prone to anomalous data for thicker samples. Gap size should also be considered, as micron-scale particulates or structures can distort readings taken with micron-scale gaps. Using multiple measuring systems will help you get the best data from the rheometer and is well worth the investment.
Noisy data towards the start of the test can be a sign of a lack of temperature equilibration in the sample. Samples that warm up or cool down after the data collection has started alter the results, at times subtly or even drastically. Regardless of the test temperature, the sample needs to be allowed enough time to reach the temperature set in the rheometer. For testing around room temperature, allow at least five minutes for temperature equilibration prior to starting the test. Longer temperature waiting times are needed for more extreme temperatures. The extra time for temperature equilibration is worth it – otherwise, the data can show inaccurate trends induced by the temperature rather than by the test itself.
Subtle evaporation can have a large impact on rheology data. Even results from water-based samples can be influenced by evaporation, typically through film formation along the surfaces exposed to air. Use a solvent trap cover sealed with a water-well to ensure that the sample chamber is quarantined from the ambient air. If no solvent trap is available, a simple cover can be used to at least mitigate evaporation. Furthermore, samples ran in a cone or plate may be suitably guarded from evaporation by pipetting silicone oil around the exposed edge after sample loading. If this method is used, ensure that the oil does not mix with the sample and that the impact of the oil is less than the impact of evaporation on the data.
Modern rheometers are sensitive to vibrations that may not be detectable to humans. The hum of nearby equipment or the vibration from construction across the street can manifest itself in messy data. Vibration can be detected in tests showing smooth data interrupted by outlier points that are followed by more smooth data. Granite or marble slabs are commonly used to prevent vibration, and a rheometer should not be located in an area with high foot traffic or spaces where carts and forklifts are operated.
For more information on how to improve rheometry measurements, contact us for a free 30 minute consultation.
Click here for a free copy of our Lab Checklist for How to Get Better Rheology Data!