Frequency sweep generates a rheological "fingerprint”.  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.

 

Figures 1 and 2 show 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 visoelastic liquid (G" dominant); whereas at higher frequencies it is a viscoelastic solid (G' dominant).  Figure 2 further illustrates this concept.  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.

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Figure 3 illustrates the concept of complex modulus (G*) that quantifies the stiffness of a sample across a range of frequencies.