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

Techniques in oscillatory shear rheology 21Particle Image Velocimetry [11]. Initially, the reaction mixture consists of monomeronly, and therefore behaves as Newtonian fluid. However, as crosslinking reactionsprogress, viscoelastic behaviour is developed (as shown in Fig. 2(c)). The periodicvariation of velocities at different points was further examined by obtaining Fouriertransforms of the velocities’ time series.Fourier transforms of x and y-components of velocities at a point are shown inFig. 13. These Fourier transforms were collected at different times after introductionof the initiator. These results show that point velocities fluctuate at higher harmonicsduring the time of reaction. It is interesting to note that such higher harmonicswere observed neither with Newtonian glycerol/water mixtures nor with viscoelasticpolyacrylamide/water mixtures.Nonlinear response arising out of structural changes has been highlighted in thecontext of LAOS by showing appearance of higher harmonics during crystallizationin a polymer melt [6]. It will be interesting to examine if such behaviour is observedduring LAOS experiments during gelation.Fig. 11 Scaling of I 3 /I 1 with strain amplitude for different (a) linear and branched polymer meltsand (b) their blends [12]Fig. 12 Periodically drivencavity to observe bulk oscillatoryflow