Nonlinear Pressure Drop Oscillations during Gelation in a Kenics Static Mixer

Static mixers are useful to fabricate hydrogels for various applications. However, although spatial heterogeneity of formed hydrogels is frequently observed probably due to temporal oscillations during gelation and extrusion in static mixers, this nonlinear oscillation has not yet been studied. Here, we studied the mechanism of nonlinear pressure drop oscillations inside a Kenics static mixer for two hydrogels: alginate/calcium and poly(vinyl alcohol)/borate. The pressure drop behavior varied depending on the composition of the precursor polymer and operating conditions. Pressure oscillations were observed in both hydrogels. A flow visualization suggested that the deposition of hydrogels on the wall and their subsequent detachment by shear stress can induce pressure oscillations. In addition, we constructed a mathematical model of hydrogel deposition and detachment, which may explain the experimental results. A parameter space expressed by four dimensionless numbers was proposed. We categorized the pressure drop behaviors into four modes: plateau, increase, oscillation, and divergence.