The non-Newtonian properties of polymers and other complex fluids stem from molecular- and meso-scale structural changes induced by flow. This proposal describes a research program centered on creation and application of novel x-ray scattering capabilities for studying mobile fluids in a planar extensional flow produced at the stagnation point of a cross-slot flow device. Prior efforts to apply x-ray and neutron scattering in this vein have been limited by design compromises in flow cell construction that result in kinematic inhomogeneity and spurious effects of parasitic shear gradients. Exploiting capabilities of the Advanced Photon Source (APS), the design concept proposed here provides a marked improvement in the definition of the applied flow field. While the methods to be developed here will have broad applicability to many classes of complex fluids, two targets are proposed for initial study: structural dynamics of viscoelastic wormlike micelle solutions, and the deformation of flexible polymers in dilute solution.
Intellectual Merit : Viscoelastic solutions of wormlike surfactant micelles are often viewed as models of both dilute and entangled polymers, but also show a propensity for a variety of flow transitions and instabilities driven by microscopic structural changes. The ability to quantify micellar orientation induced by a well-defined extensional flow will contribute valuable insights into many phenomena in these fluids, including structure formation associated with turbulent drag reduction, differences between shear-banding and non-banding micelle formulations, the origins of strain hardening in certain micellar systems, and the nature of flow-induced nematic phase transitions in concentrated micelle solutions. Measurement of dilute polymer solutions in extensional flow is a very challenging application for x-ray scattering, but has the potential to address a long-standing discrepancy between theoretical prediction (and more recent direct visualization in DNA) of large coil deformations, and prior measurements of coil size using light scattering.
Broader Impacts : Viscoelastic phenomena in polymer and surfactant solutions have profound consequences in many complex flows of practical significance, including drag reduction in turbulent pipeline flow and enhanced oil recovery processes that utilize polymer and/or surfactant solutions. Understanding complex fluid rheology in both shear and, especially, extensional flows is a prerequisite to understand the more complex flows that arise in these and other technologies. As a result, the direct measurements of microscopic fluid structure in planar extensional flow proposed here have tremendous potential to positively impact technologies important to society. In addition, this project will have broad impact in scientific infrastructure and human resource development. The PI has extensive experience in the development of novel instrumentation for x-ray scattering studies of polymers under flow, drawing upon his active participation in the DND-CAT research facility at APS. More importantly, he also has a track record of making this instrumentation available to scientific collaborators, as well as to the community of synchrotron users who access APS facilities through "general user" proposals. In a similar way, the instrument developed in this project will be made available to the broader research community, expanding the benefit of this project in both the research to be performed, as well as the training of graduate students and postdoctoral researchers.
