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 an oscillation assay (think clothes washer agitator) that uses an increasing amplitude (energy input) to probe sample properties such as rheological stability. The amplitude sweep is very important to properly determine the LVER (Linear Viscoelastic Region) to define a % strain value that is a critical input parameter for subsequent frequency based assays described in later sections.
Figure 1 shows amplitude as % strain increasing until 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 proportionally correlate with physical stability. Determining a % strain well within the LVER is a critical input for other oscillation tests performed at the same temperature to ensure that the rheological integrity remains intact - otherwise confounding results may be obtained.
Figure 2 is a typical amplitude sweep plot showing LVER, G' (elastic modulus = solid nature), G" (viscous modulus = liquid nature), and phase angle response as amplitude increases. Formulation and processing modifications may result in product changes identified by a rheometer, but not necessarily identified using a viscometer.