Rheometry test methods consist of two main categories: rotational and oscillatory. Most rheometer users begin with rotational tests, which apply a shear rate (or rotational velocity) and measure viscosity and shear stress. Although rotational tests are helpful for characterization and many applied tests, there is another half to the world of rheology – oscillatory measurements. This second half brings more insights and is growing in importance in the age of ever-changing formulations and raw materials.
The inputs and outputs of oscillatory tests can seem esoteric compared to the simplicity of rotational methods, but learning how to interpret oscillatory data is well worth the effort. Sample structure is the driving factor for the output moduli. Changes in particulate aggregation, polymer networks, and colloidal stability are shown immediately in the data instead of waiting for the effect to impact viscosity. Gelation, curing, sedimentation, evaporation, and gel fracture are just a few of the characteristics that can be measured with much more precision than their counterpart rotational methods.
Most modern rheometers can detect subtle changes in the solid to liquid balance of the sample as soon as the applied strain, temperature, or impact of time alters the sample structure. Unlike the majority of rotational methods, oscillatory tests do not rely on large shear forces to rip apart sample structure, but rather detect small changes. This is a big advantage for understanding how colloids and gels age over time, as well as for investigating the impact of low strain conditions, such as shelf-life, structural recovery, and hardening.
Oscillatory methods output shear moduli (G’, G”) that represent the state of the sample in terms of liquid-like characteristics (G”) and solid-like characteristics (G’). Regardless of the composition of the sample, the moduli display how the sample behaves as either a solid or a liquid and by what degree. Each point on the plot gives a snapshot showing if the sample is more solid-like (G’ > G”) or liquid-like (G” > G’) at that moment under the set conditions. Onset of changes to the physical properties can be identified simply by a change in slope. In this manner, G’ and G” are much more reliable than a viscosity, which may or may not display any change during subtle structural transitions.
To get started with oscillatory tests, begin with the classic Amplitude Sweep (aka Strain Sweep). Sinusoidal strain is gradually increased over time in order to discover the strain at which the sample structure starts to deform. Results can also be shown in terms of shear stress, which gives a threshold stress for the onset of deformation. The strain that is below the threshold for deformation can then be used as a constant value for carrying out frequency sweeps and structural recovery tests.
If you would like to learn more about oscillatory measurements, get in touch for a free 30 minute consultation.