Peter D. Olmsted

Georgetown University

Physicist

Fellow, Elected 2021

Olmsted has studied flow and rheology in many contexts, embracing theory, simulation, and often in close collaboration with experiment. His most important contributions to rheology have been in the area of shear banding, in which flow can introduce apparent ‘phase transitions’ between different microstructural states. He developed the theory for calculating shear banding based on spatial gradient (‘diffusive’) terms, and demonstrated this in models for liquid crystals, wormlike micelles, and entangled polymers by calculating full non-equilibrium ‘phase diagrams’. He introduced and explained the differences between gradient and vorticity banding, and delineated the signatures for shear banding measurable in specific flow geometries (e.g. pipe, cylindrical Couette), as well as identifying kinetic signatures during startup and the approach to steady state. He and Fielding introduced a simple model for rheochaos, calculated kinetic signatures for concentration-coupled shear banding, and calculated non-equilibrium ‘phase diagrams’ for wormlike micellar solutions. His group showed how Doi-Edwards (DE) based tube models can account for numerous rheological features of shear banding, including transient banding, ‘fracture’ upon startup, non-monotonic reentanglement kinetics, banding-like behavior under large amplitude oscillatory shear, and edge fracture induced by shear banding. This work helped usher in a new paradigm for comparing rheological models with experiments: it is now recognized that visualization of the flow field, and ideally microstructure, is the only way to unambiguously discriminate among candidate constitutive models. Moreover, the spatial stress gradient of most rheological devices [Couette, plate-plate, cone-plate] plays a crucial role in understanding the detailed observations in shear banding fluids.

Other topics have included participating in the development of microrheology, which is now a standard method for studying delicate and small samples; the identification of how ‘soft’ liquid crystalline elastomers that can deform with no stress or change in free energy based on symmetry arguments; and a microscopic theory of flow-induced crystallization of polymers. Most recently he the first molecularly-informed calculation of polymer deformation during the fused filament fabrication method of additive manufacturing. He has collaborated with numerous experimental groups in the areas of shear banding (micelles, lamellar surfactant solutions, liquid crystals), flow-induced crystallization, microrheology, and polymer scission in contraction flows.