|Katie Mary Weigandt
Developing structure-property relationships in complex fluids is essential to enable the design of new materials with specific properties. This is especially true when investigating the mechanical properties of non-Newtonian fluids. Non-Newtonian fluids will exhibit a non-linear relationship between stress and strain over some domain in the stress-strain curve. These non-linearities can often be attributed to a structural transition in the material. Examples include shear thinning observed concurrant with alignment of wormlike micelles, shear thickening observed during clustering in colloidal suspensions or irreversible changes such as strain hardening in fibrin gels when fibers align and subsequently stretch, and structural rearrangement of fractal particle networks (eg bohemite, metal oxide nanoparticles, carbon black) under shear. Neutron scattering is especially well suited to enable the direct measurement of complex fluid and soft material strucure (from nanometers to microns) undergoing deformation in specially designed shear cells and rheometer tools. NIST has been at the forefront of shear SANS development over the last decade and our scientists have contributed to development of a widely adopted commercial rheoSANS package, as well as speciallized shear and extensional flow devices. My team has recently focused on the development of shear cells for simultaneous measurement of rheology and scattering at shear rates up to and in excess of one million s^-1.[2,3] These new tools are particularly advantageous for understanding a wide array of industrially relevant processes and materials including spray coating, lubrication and injection through thin needles, but also opens up broad new realms for developing structure-function relationships in soft materials well beyond the existing status quo.
This opportunity seeks applicants to either continue developing the sample environments necessary to enable direct measurement of structure-function relationships in soft materials and complex fluids, or to use the existing measurement tools to explore and characterize soft materials including: micellar solutions, polymer solutions, colloidal suspensions, protein solutions, gels, mixed materials, etc. The successful applicant will work with the nSoft consortium (a NIST-industry consortium focused on using neutrons to study soft matter), attend consortium meetings, and present both at the consoritum meetings and more broadly to the academic community. The research can be both industially relevant and/or fundemental materials science oriented.
Additional opportunity exists for the study of non-homogeneous hierarchical structures using the, still under development, far field interferometer (tomographic SANS). This method will enable the study of local structure in ~50 µm voxels within a larger heterogeneous sample. For materials that undergo irreversible change, but remain stable after deformation (for instance stretched composite solids), tomographic SANS can be used to show where the structural failure may occur with additional applied strain, or how various process conditions lead to different heterogeneous microstructures in soft material systems. Opportunites to participate in the development of this new measurement technique include instrumentation development projects, data science, and fundemental materials characterization projects.
1. A. Bharati, S.D Hudson and K.M Weigandt. “Poiseuille and Extensional FlowSAS for Developing Structure-Rheology Relationships in Soft Matter Systems”. Current Opinion in Colloid and Interface Science (2019)
2. RP Murphy, ZW Riedel, MA Nakatani, PF Salipante, JS Weston, SD Hudson, KM Weigandt. “Capillary RheoSANS: measuring the rheology and nanostructure of complex fluids at high shear rates”. Soft Matter 16 (27): 6285-6293 (2020)
3. J.S. Weston, D.P. Seeman, D.L. Blair, P.F. Salipante, S.D. Hudson, K.M. Weigandt, “Simultaneous slit rheometry and in situ neutron scattering”, Rheologica Acta, 57 (3): 241-250 (2018)
Rheology; Shear; Neutron Scattering; Complex Fluids; Soft Matter; Non-Newtonian; Tomographic SANS; SANS; RheoSANS;