CAREER: in situ Polymer Gelation in Confined Flows

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Polymer gels form in many natural and industrial flows: proteins gel in preparing foods like Jell-O or eggs; plastics can gel during 3D printing; blood proteins can crosslink to form clots in vessels. Flows become complex when polymer molecules crosslink, or gel, transforming the fluid into a soft solid. Crosslinking can result in intermittent flow or complete flow stoppage, resulting in clogging of small channels, nozzles, or vessels, and eventual failure of the system. The ability to predict flow regimes is key to assessing, controlling, and preventing intermittency and clogging. This award will advance our fundamental understanding of crosslinked polymer flows through small channels to enable such control. The outcome of this award could result in advances for a variety of applications including in bioprinting and 3D printing by enabling efficient printing of a wider variety of polymers. It will also help designing polymers to flow through small spaces, ranging from fluid fertilizers to biomedical and pharmaceutical products. Education and outreach activities are planned to engage and train high school, undergraduate, and graduate students in emerging topics in complex fluids. Curriculum development, engaging the general public, and supporting underrepresented populations in STEM are also planned.The goals of this award are twofold: (1) to understand the range of system parameters that lead to intermittency and clogging in crosslinking polymer flows through microchannels, and (2) to provide quantitative descriptions of intermittency and the rheological and material properties of intermittent flows. Video microscopy and in situ rheology combined with theoretical transport models will be employed for this purpose. Polymers can be crosslinked by chemical or physical methods, both of which are found in bioprinting. In chemical crosslinking, gelation proceeds quickly, as in alginate crosslinked by calcium. Gelation rate is controllable in physical crosslinking, as in gelatin crosslinked by lowering temperature. Both types of gelation will be investigated to determine the importance of reaction rate and mechanism. Dependence of flow regimes on chemical concentrations and flow rates will be studied. The outcome will provide parameters to predict and control flow regimes in a broad range of crosslinking polymer systems. The intermittent flow regime will be investigated by in situ rheological measurements to determine gel strength and viscoelasticity in flow, which will be compared to bulk rheological characterization of comparable systems. This will facilitate strategies to prevent and overcome intermittent flows in practical systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.