Gelatinized starch dispersions under small constant shear stress applications: Strain instabilities, chaotic behaviour and fractality

Corn and wheat gelatinized starch dispersions (GSD, 5.0% w/w) were prepared (90 °C, 20 min). The apparent viscosity-shear rate (0.0001–1 s−1) profile of the both GSD showed a shear thickening behaviour region (∼0.0002–0.003 s−1) followed by a shear thinning behaviour (∼0.003–1 s−1). The response to the application of small constant shear stresses applied with a biconical disc in the bulk of the GSD was studied. In the region of shear thickening behaviour the GSD showed that the shear strain exhibited irregular fluctuations (i.e., rheological instabilities) about a steady-state value. The detrended fluctuation analysis (DFA) was used for characterising self-correlations and fractality of the measured time series. The computations showed that the irregular shear strain fluctuations contained long-term correlations that can be related to micro-structural features of the GSD. Besides, the strength of the long-term self-correlations increased with the applied shear stress, suggesting that the rheological instabilities can be attributed to elastic turbulence as induced by the gel micro-components. Light microscopy indicated that the rheological instabilities can be related to spatial heterogeneities of starch granules and leaked amylose acting as the inter-particle viscoelastic material.