UNS: The Effect of Flow-Induced Concentration Inhomogeneities on the Flow of Polymer Solutions

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1510333(Leal)It has been found that during the processing of complex fluids, such as polymer and surfactant solutions, liquid crystals, etc., large molecules in the solution can migrate to more concentrated areas. The goal of the proposed research is to understand this phenomenon, known as shear-induced banding, for the case of polymers. Discovering the fundamental reasons for this phenomenon has significant technological implications in processing and manufacturing of complex fluids that are common in the petrochemical, consumer products and the food industry. When a macromolecular suspension (e.g., surfactants, polymers, and possibly proteins) is sheared, there is what is known as stress-induced migration of the macromolecules or micelles toward regions of higher concentration of the solution, via the so-called Helfand Fredrickson (HF) mechanism. In this work, the co-PIs propose to explore whether this mechanism can produce macroscopic concentration gradients in polymer solutions - a case where this phenomenon is somewhat controversial. The co-PIs, through prior theoretical work, have suggested that the coupling between the polymer stress and concentration in entangled solutions produces a linear instability for simple shear flow, and this instability leads to shear banding. In this proposal, they propose a coordinated experimental and computational approach to investigate this problem, and then to generalize the results to other flows: (a) pressure driven flow in a tube which exhibits a "spurt" instability, and (b) flow in 2- and 4-roll milling flows, i.e., testing their models under different stress patterns and even extensional flows. The proposed approach will involve both careful experiments (using rheo-NMR and neutron scattering) and computations. Outreach and dissemination of research is planned, involving graduate students in research. In addition, results will be disseminated to the industry.