Films were prepared using oxidized banana starch and glycerol. Oxidized banana starch was synthesized at three oxidation levels (0.5, 1.0 and 1.5% of active chlorine). Thermal gelatinization and casting were used for film preparation. Tensile strength, elongation, X‐ray diffraction pattern and water vapor permeability were tested. The tensile strength of the films was higher when the oxidation level of the banana starch was higher. When films were stored for 30 and 60 days, there were no statistical differences in tensile strength. However, differences occurred when those films were compared with the films stored for 90 days. The film with added sunflower oil presented the lowest value of elongation at 30 and 60 days of storage. When elongation values were assessed at 90 days of storage no statistical differences (α=0.05) were found among the films prepared from native starch with added sunflower oil, oxidized banana starch with 1.0% and oxidized banana starch with 1.5% oxidation level. All films had a B‐type X‐ray pattern. When the oxidation level in the starch increased, the peaks of crystallinity of the films increased as well. An additive effect was shown due to the higher interactions among polymer chains and among polymer and glycerol molecules as determined by the mechanical properties of the films analyzed. The corrected water vapor permeability (WVPc) of the films stored for 30 and 60 days did not show statistical differences (α=0.05). The films prepared with oxidized banana starch had the highest WVPc and those values increased when the oxidation level in the starch was higher. Oxidized banana starch can be used for preparation of films with adequate physicochemical characteristics.

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The shear-thickening behavior of an equimolar semidilute aqueous solution of 40 mM/L cetylpyridinium chloride and sodium salicylate was studied in this work by using a combined method of rheometry and particle image velocimetry (PIV). Experiments were conducted at 27.5 degrees C with Couette, vane-bob, and capillary rheometers in order to explore a wide shear stress range as well as the effect of boundary conditions and time of flow on the creation and destruction of shear-induced structures (SIS). The use of the combined method of capillary rheometry with PIV allowed the detection of fast spatial and temporal variations in the flow kinematics, which are related to the shear-thickening behavior and the dynamics of the SIS but are not distinguished by pure rheometrical measurements. A rich-in-details flow curve was found for this solution, which includes five different regimes. Namely, at very low shear rates a Newtonian behavior was found, followed by a shear thinning one in the second regime. In the third, shear banding was observed, which served as a precursor of the SIS and shear-thickening. The fourth and fifth regimes in the flow curve were separated by a spurtlike behavior, and they clearly evidenced the existence of shear-thickening accompanied by stick-slip oscillations at the wall of the rheometer, which subsequently produced variations in the shear rate under shear stress controlled flow. Such a stick-slip phenomenon prevailed up to the highest shear stresses used in this work and was reflected in asymmetric velocity profiles with spatial and temporal variations linked to the dynamics of creation and breakage of the SIS. The presence of apparent slip at the wall of the rheometer provides an energy release mechanism which leads to breakage of the SIS, followed by their further reformation during the stick part of the cycles. In addition, PIV measurements allowed the detection of apparent slip at the wall, as well as mechanical failures in the bulk of the fluid, which suggests an extra contribution of the shear stress field to the SIS dynamics. Increasing the residence time of the fluid in the flow system enhanced the shear-thickening behavior. Finally, the flow kinematics is described in detail and the true flow curve is obtained, which only partially fits into the scheme of existing theoretical models for shear-thickening solutions.

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