Abstract:
The talk will describe nonlinear rheology in extreme conditions that change fluid structure and flow mechanisms. One example is elastohydrodynamic lubrication where fluids flow at extreme pressures (>1GPa), strain rates (105-107 s-1) and shear stresses (>100MPa). The high pressures can lower friction and wear by elastically flattening the bounding solids. They also drive the fluid towards the glass transition and the rapidly increasing viscosity prevents lubricant from squeezing out of the contact. MD simulations of the viscosity of three model small molecule lubricants have been performed over a wide range of temperatures, pressures and strain rates that overlap experiments and reveal universal trends. At high temperatures and low pressures, where the Newtonian viscosity hN is small, the fluid shows simple shear thinning associated with molecular alignment. In this regime the shear thinning has a power law form. Increasing hN by about 2 orders of magnitude causes a transition to thermally activated hopping. Over more than 10 decades in strain rate the stress rises logarithmically with rate as implied by Eyring theory. We show that the nonequilibrium response can be used to determine changes in hN over more than 20 orders of magnitude that agree with existing experiments and extend them to test the nature and existence of the glass transition.
The second part of the talk will examine elongational flow of long entangled polymers. Nonlinear extensional flows are common in polymer processing but remain challenging theoretically because dramatic stretching of chains deforms the entanglement network far from equilibrium. Tube and slip-link models that describe flow of melts near equilibrium must be enriched to describe these states, but it has not been clear what physics is missing. Simulations of a coarse-grained model reproduce experimental trends in extensional viscosity with time, rate and molecular weight. These trends in viscosity are related to a crossover from the Newtonian limit to a high rate limit that scales differently with chain length. Contrary to some geometric models of entanglement, our results suggest the degree of chain confinement is independent of the orientation of the entanglement network during elongation and relaxation after flow stops, but the confining tube straightens and narrows during elongation. The results are used to test and constrain generalizations of tube and slip-link models to strongly nonlinear flows.
Bio:
Mark Robbins grew up near Boston and received his BA and MA degrees from Harvard University. He spent a year as a Churchill Fellow at Cambridge University and received his PhD from University of California, Berkeley, in 1983. Following a postdoctoral fellowship at Exxon's Corporate Research Science Laboratory in New Jersey, he joined the Department of Physics and Astronomy at Johns Hopkins in 1986. He was promoted to associate professor in 1988 and to professor in 1992.
Robbins received a Presidential Young Investigator Award in 1986, a Sloan Foundation Fellowship in 1987 and a Simons Fellowship in Physics in 2012. He became a fellow of the American Physical Society (APS) in 2000 and the American Association for the Advancement of Science (AAAS) in 2018. He served as chair of the APS Group on Statistical and Nonlinear Physics and of the advisory board of the Kavli Institute of Theoretical Physics (KITP) at the University of California, Santa Barbara, and is on the advisory board of the Boulder School for Condensed Matter and Materials Physics. He has organized symposia and workshops for the Materials Research Society, the Aspen Center for Theoretical Physics, and KITP, most recently “From the Atomic to the Tectonic: Friction Fracture and Earthquake Physics in 2005 and “Physical Principles of Multiscale Modeling, Analysis and Simulation in Soft Condensed Matter” in 2012. He chaired the 2010 Gordon Research Conference on Tribology.
Host: Professor Alison Dunn
