"Getting all shook up"
Probing viscoelastic properties and molecular interactions
Frequency sweep generates a rheological "fingerprint” or spectrum. It is used to probe viscoelastic properties such as stiffness (complex modulus, G*), solid nature (elastic or storage modulus, G'), liquid nature (viscous or loss modulus, G"), solid or liquid tendency (phase angle), complex viscosity, and tan delta (G"/G') across a frequency range. This assay is useful to probe polymer and biomolecule viscoelastic behavior and arrangement in various matrices. It is important to ensure that this assay is performed within the LVER (Linear Viscoelastic Region) that is determined with an amplitude sweep.
Figure 1 shows the classic viscoelastic response vs frequency. Specifically, Figure 1 shows the frequency sweep output of Silly Putty placed between an oscillating upper plate and stationary lower plate. At low frequency it is a viscoelastic liquid (G" dominant); whereas, at higher frequencies it is a viscoelastic solid (G' dominant). As the pictures show, at rest (slow event=very low frequency), Silly Putty slowly flows as a viscoelastic liquid, yet when rolled into a ball and dropped on a surface (fast event=high frequency) it bounces as a viscoelastic solid.
Figure 2 describes other frequency sweep measurables that include complex modulus (G*), complex viscosity (n*), and tan(delta).
Figure 3 shows application of frequency sweep to compare complex modulus (G*) for 6 arthritis formulations to model effect of increasing joint movement and efficacy. Increasing G* correlates to increasing stiffness.