(124f) Measuring the Full Three-Dimensional, Nonequilibrium Microstructure of Complex Fluids Under Steady and Time-Dependent Shear, Including Laos, Using Spatial and Time-Resolved Small-Angle Neutron Scattering

Understanding the nonlinear shear rheology of complex fluids - including polymers, self-assembling solutions, colloids and nanoparticle suspensions - is advanced by directly measuring the microstructure(s) responsible for the stress under deformation. Rheo-optics, direct visualization, rheo-NMR, and light and x-ray scattering under flow are among the various methods applied to a wide variety of model and industrial systems to aid in the development of structure-property relationships and often, to rationalize seemingly anomalous rheological behavior. These methods are often system specific, however, and a quantitative, robust, and broadly applicable method capable of measuring the full three-dimensional microstructure remains an experimental challenge. Indeed, some of the more interesting nonlinear effects, such as shear-banding and shear-induced phase transformations, where the flow-field and microstructure are intimately coupled, require the ability to resolve the microstructure both with spatial and time resolution.

In this presentation we review a robust and broadly applicable method using neutron scattering to measure the microstructure in all three planes of flow, and with spatial and time-resolution. Neutron scattering has a distinct advantage as most materials are amenable to study by neutron scattering, unlike x-Ray and light scattering. Further, truly unique experiments are possible by selective isotopic substitution, which allows probing components in a mixture by contrast matching. We have addressed this challenge by developing a Couette geometry to access the 1-2 (velocity-velocity gradient plane of flow) by small angle and ultra-small angle neutron scattering.  Further, spatial resolution of order 100 microns is achieved by an aperture that scans across the flow.  Combined with a now commercial, rheo-SANS Couette geometry that enables radial (velocity-vorticity) and tangential (velocity gradient-vorticity) measurements, these instruments provide the first measurements of microstructure in all three projections of the flow and thus, reconstructing the full, three dimensional flowing microstructure. Further, time dependent deformations that are often used to test physically-based constitutive models, such as flow start-up and large amplitude oscillatory shear (LAOS), are probed by stroboscopically synchronizing the deformation field and the scattered neutron collection through time stamping and binning methods. Finally, neutron scattering is an absolute scattering method such that local chemical composition can also be measured under flow.  This enabled determining such effects as shear-induced concentration gradients. These instruments are now available for use by the community at the Institute Laue Langevin in Europe and at the NIST Center for Neutron Research in the U.S.

Recent results for the shear-crystallization of self-assembled block copolymer micelles in ionic liquids under LAOS, the shear banding of worm-like micelles under start-up flow, and the time-dependent microstructure of shear thickening colloidal dispersions and colloidal gels under LAOS are presented to demonstrate the breadth of the method and how these measurements provide fundamental microstructural data essential for understanding the rheology, as well as for critically testing rheological constitutive models of these complex fluids.