Techniques in oscillatory shear rheology - Indian Institute of ...

Techniques in oscillatory shear rheology 17Q = 1 I 3γ02 , (23)I 1This coefficient has been claimed to be helpful in distinguishing molecular architectureof polymers based on LAOS.Through analysis of shapes in Lissajous plots and Fourier spectra, such materialfunctions are defined. It should be noted, however, that these material functions willprove useful provided they give insight into material behaviour and help in identifyingthe microscopic mechanisms involved. Therefore, physical interpretations ofthese material functions are still being proposed and applied to different complexfluids to examine their utility.2.4 Experimental issuesIn general, oscillatory rheological measurements are difficult at very low and highfrequencies. At very low frequencies, with lower deformation and strain rates, signalto noise ratio is low due the sensitivity of sensors (or least count). At higherfrequencies, inertia effects are significant, while the rheometric analysis assumesabsence of these. Mechanical and electrical disturbances during the operation of therheometer can also lead to errors, which may be significant during analysis of higherharmonics in LAOS [26].The presence of higher harmonics during oscillatory shear is not always due toterminal nonlinear response of the materials. One has to be careful about contributionsfrom sample inertia [5]. It has been shown that during startup of oscillatoryflow (during the time before the terminal steady state is reached), higher harmon-(a)(b)Fig. 9 Proposed elastic moduli and their geometric interpretation for (a) linear and (b) nonlinearviscoelastic behaviour [7]