Unpredictable polymeric flow dynamics with reaction between HPAM and Al³⁺ by comparison between pre- and...

A chemical reaction in the flow field affects the physical property of fluids, resulting in changing flow dynamics. The effect of the reaction on the flow dynamics is normally evaluated by comparison between the physical properties of the fluid between pre- and post-reaction. However, our group found a polymeric reacting flow in which a comparison between pre- and post-reaction fluid properties is insufficient to evaluate the influences of the reaction on the flow dynamics (T. Ueki et al., J. Phys. Chem. B, 2019, 123, 4587-4593). In the study, we examined the stirred flow by an impeller in a beaker with a chemical reaction between partially hydrolyzed polyacrylamide (HPAM) solution and Fe(NO₃)₃ solution. We found that a so-called Weissenberg effect, where the stirred solution climbs up the stirring shaft because of an increase in the viscoelasticity of the mixed solution, transiently appeared in only the halfway of the reaction, although the reaction slightly decreased the solution's viscosity at the final stage under a certain pH and we proposed the mechanism for the temporal increase in the viscoelasticity. In the present study, we conducted similar experiments as our previous study using HPAM solution at various pH and two concentrations of Al(NO₃)₃ solutions. The unpredictable flow dynamics by comparison between pre- and post-reaction fluid properties, i.e., the appearance and disappearance of the Weissenberg effect, has been observed under a certain pH and Al(NO₃)₃ concentration. We investigated the mechanism by means of the simultaneous measurement of pH and the stirring torque and the identification of the dominant species of aluminum aqua complexes by the Henderson-Hasselbalch equation. As a result, we proposed two mechanisms of dissapearance of the Weissenberg effect: one is the same for the Fe(NO₃)₃ case but the other is unique and novel for the Al(NO₃)₃ case. This study can contribute to the comprehensive understanding of the conditions for the occurrence of the unpredictable dynamics of polymeric reacting flow by comparison between the pre- and post-reaction fluid properties.