(573h) The Effect of Harmonic Number on Large Amplitude Oscillatory Shear (LAOS) Testing of Starch Suspensions

Starch functions as thickening, texture enhancing, stabilizing agent in numerous applications. Starch suspensions have unique property of shear thickening, which can be defined as abrupt increase in viscosity at elevated shear stress(Crawford et al., 2013). Due to perfect round shape of the starch granules, starch suspensions are known to be a non-Brownian particle system (Fall, Bertrand, Ovarlez, & Bonn, 2010). When applied amplitude and velocity of the strain exceeds the critical value for thickening, shear-induced formation of hydrodynamic clusters and transient fluctuations cause abrupt increase in viscosity (Fang et al., 2019). This phenomenon occurs when rigid particles at high concentrations are closely packed (Matignon et al., 2014). Shear thickening of starch suspension generates problems in production lines with small openings, paddles that have high shear rates and abrupt deformations.

Linear viscoelastic region rheological measurements probe the rheological characteristics of material under small strains, where material is not permanently deformed and response is in linear region. Although linear oscillatory sehar measurement is non-destructive method and suitable for relations of structure-property, it is inadequate for many engineering applications that involve large and accelerated deformations (P Deshpande & Deshpande, 2018). For characterization of shear thickening property of starch suspensions triggered by rapid and extended shears, large amplitude oscillatory shear (LAOS) measurement is necessary(Hyun et al., 2011). Findings of the LAOS measurements guide to design efficient processes and better quality products. When a material is exposed to strain in the form of sinusoidal wave, response wave appears as perfect sinusoidal. Increase in amplitude of the strain waves deviates material behavior from linear to nonlinear region. Nonlinearities of the material manifest as fluctuations in response stress waves, which interpretation of the waves require relatively complex mathematical tools (Ewoldt et al., 2013). Response stress curves of these measurements processed with Fourier Transform to encode nonlinear properties of sample. Fourier transform decompose complex time-domain signal into frequency-domain spectrum.

In this study starch suspensions prepared with different starch concentrations of 45, 47, 50 % wt. were subjected to strain range of 0.01-200% at frequency of 1 rad/s with DHR-3 rheometer. Starch suspensions are prepared by deionized water and 0.5% wt. pectin in order to inhibit suspension settling and measurements were done with concentric cylinder geometry. Stress responses of suspensions were evaluated by software provided by TA instrument, TRIOS. The raw responses of oscillation in non-linear region evaluated with Fourier transformation, their reconstructions, Lissajous-Bowditch curves and rheological parameters (G'_M, G'_L, η'_L, η'_M, e₃/e₁, v₃/v₁, S and T) were obtained for deeper understanding of non-linear behavior of starch suspension.

In all of the suspensions, there are three distinct regions; linear response region, shear thinning, early shear thickening region. At higher concentrations of starch, onset amplitude from linear to nonlinear region is increased. The raw stress response curves of starch suspensions at linear region are in the form of perfect sinusoidal. The generation of shear thickening behavior characterized by distorted response wave with uncommon beating patterns. For the evaluation of these waves, various harmonic numbers of 3, 5, 10, 50, 100, and 300 are included in the reconstruction of the waves. It is proved that increase in harmonic number diverge reconstructed wave into raw stress response curve. When the harmonic number decreased, reconstructed wave smoothens and characteristic waves of thickening are eliminated. Root mean square errors versus harmonic number included for the reconstruction are also calculated. The error, deviation of reconstructed wave from original wave, decreases as harmonic number for reconstruction is increased.

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