THE INFLUENCE OF WAVE-DISPERSION CHARACTERISTICS ON THE DETERMINATIONOF MECHANICAL RELAXATION SPECTRA

A new rheometrical technique has been developed to determine the mecha nical relaxation spectra of viscoelastic materials by measurement of s hear wave group and phase velocities in a virtual gap rheometer. The t echnique's sensitivity and accuracy in providing relaxation spectra di rectly from measurements arises from exploitation of marked changes in wave dispersion characteristics in the vicinity of stress relaxation mechanisms. The new technique is compared with two conventional, wave- based rheometrical techniques (based on pulse-propagation and resonanc e measurements), which are adversely affected by wave dispersion. The superior accuracy and sensitivity of the new technique over convention al rheometry is demonstrated with reference to the Maxwell model and p ublished relaxation spectra. Experimental results obtained on a concen trated colloidal dispersion serve to demonstrate the measurement princ iples.