Instabilities and shear banding in polymeric fluids
Georgetown University
Abstract
Entangled polymer liquids have enormous non-Newtonian effects due to the elasticity of the long molecules. This gives rise to spectacular phenomena such as rod-climbing, shear thinning, and numerous elastic instabilities. One contentious prediction is ‘shear banding,’ in which a sheared solution divides into ‘bands’ of different shear rates, with well-aligned and poorly-aligned polymers respectively in the low and high shear rate bands. There have been predictions of this phenomenon since the 1960s, and more detailed theories and experiments suggest that banding might not occur in chemically inert polymers (such as polystyrene), but is possible in ‘living’ polymers. Experiments show clear shear banding in living polymers (such as wormlike micelle solutions), but the evidence is poor for chemically inert polymers, except for possibly polymer solutions. I will (1) show computer simulations of entangled polymers that show evidence for shear banding; (2) present a newly realized two-dimensional instability that may render the high shear rate band of banding wormlike micelles (and polymers) to be in an elastic turbulent state; and (3) speculate on why banding is not seen experimentally for simple polymer melts such as polystyrene.
About the speaker
Dr. Peter Olmsted received an A.B. in Physics from Cornell (1984); and a Ph.D. in Physics from the University of Illinois at Urbana-Champaign (1991), where he studied the theory of liquid crystal hydrodynamics and non-equilibrium phase transitions. Following post-docs at Exxon, Cambridge, and the University of Michigan, where he worked on the theory of a variety of different soft materials (microrheology, membrane dynamics, polymer phase separation, biophysics, liquid crystals, and liquid crystalline elastomers), he moved to Leeds in 1996 for a University Research Fellowship. He became a Professor in 2005 and led the Soft Matter Physics group in Leeds from 2008–2013. While in the UK he was in several European Union networks on Soft Matter and led the EU Integrated Training Network DYNACOP (Dynamics of Architecturally Complex Fluids) from 2008–2012. In 2014 he moved to Georgetown University in Washington, D.C., as Joseph Semmes Ives Chair in Physics, where he joined the Institute for Soft Matter Synthesis and Metrology (ISM2), where he was Director from 2015–2021. He was the Secretary/Treasurer of the APS Topical Group on Soft Matter (DSOFT) from 2014 to 2018. Olmsted is a Fellow of the Institute of Physics (UK), the American Physical Society (Division of Polymer Physics), and the Society of Rheology, and he was awarded the British Society of Rheology Annual Award in 2008. His research achievements include models for polymer crystallization at rest and under flow, theories for shear banding in complex fluids such as polymer and surfactants, an experimental–computational collaboration that revealed how mechanical force unfolds proteins, and recent work on polymer dynamics and disentanglement during additive manufacturing.
