Probing sample structure and properties to produce a rheological profile
In contrast to yield stress and thixotropy tests that apply a rotational force in one direction, amplitude sweep is a back-forth oscillatory assay (think clothes washer agitator) that uses an increasing amplitude (energy input) to probe sample properties such as rheological stability. The amplitude sweep is an important assay to properly determine the LVER (Linear Viscoelastic Region) to define rheological stability within either a % strain or stress value that is a critical input parameter for subsequent frequency based assays described in later sections.
Figure 1 shows amplitude as % strain or stress increasing until the upper limit of the Linear Viscoelastic Region (LVER) limit is reached, typically defined as a 5% G' (elastic modulus, solid nature) decrease beyond which the sample structure is increasingly destroyed. The value of the upper limit of the LVER tends to correlate with physical stability. Determining a % strain or stress within the LVER is a critical input for other oscillatory tests performed at the same temperature to ensure that the rheological integrity remains intact - otherwise confounding results may be obtained. It is important to keep in mind that the LVER can change with temperature.
Figure 2 shows amplitude sweep curves of hydrogel samples showing G' (elastic modulus = solid nature), response with increasing amplitude along with their LVER. Subtle product formulation and processing modifications may result in rheological and performance changes that may be identified by a rheometer, but not necessarily identified using a viscometer.