Prof. Ravi Prakash Jagadeeshan's Talk

Jan 12, 2015 - 16:00

Jan 12, 2015 - 17:00

Room 235 Chemical Engg.

Seminars

Prof. Ravi Prakash Jagadeeshan Monash University Melbourne Australia.

Abstract : Most models in polymer solution rheology assume that the steady state stress is uniquely determined by the rate of deformation. Yet in 1974 de Gennes suggested that the time history of deformation has a crucial bearing on the steady state value of stress. Since the hydrodynamic drag force experienced by a polymer molecule depends on its conformation de Gennes conjectured that in extension dominated flows depending on the deformation history it is possible for a dilute polymer solution to manifest multiple values of stress at a single strain rate and consequently exhibit hysteretic behaviour. The importance of de Gennes contention has paradigm changing implications for the modelling of polymer rheology since the accurate estimation of the relationship between stress and strain in a solution lies at the heart of being able to develop a realistic description of the flow of polymer solutions. It took 30 years before experimental evidence could be obtained in support of this argument. Individual DNA molecules in ultra-dilute solutions subjected to planar elongational flow were shown to be either coiled or highly stretched depending on the history of deformation. Since the stress in a polymer solution has its origin predominantly in the entropic resistance of individual polymer molecules to deformation from their equilibrium coiled state the experiments indirectly validate de Gennes hypothesis of different deformation histories leading to disparate states of stress. Subsequent Brownian dynamics simulations of dilute polymer solutions have confirmed the existence of hysteresis both in polymer chain conformations and in the steady state stress. Recent scaling arguments and experiments carried out at Monash University suggest that the concentration of polymers has a significant non-monotonic influence on the extent of coil-stretch hysteresis with a initial growth in hysteresis followed by extinction as the polymer concentration increases. Additionally it has been known since de Gennes early theory that increasing the fraction of shear in a mixed flow of planar shear and extension dramatically decreases the magnitude of coil-stretch hysteresis. In this talk I will discuss the development of a mesoscopic Brownian dynamics simulation algorithm that is capable of accurately describing polymer solutions undergoing planar mixed flows at finite polymer concentrations. The simulations permit the examination of the competing roles of polymer concentration and flow mixedness on the extent of coil-stretch hysteresis and provide a fascinating insight into the influence of non-linear phenomena on the molecular scale on macroscopic solution properties.Brief Biography: Ravi Prakash Jagadeeshan is currently a Professor in the Department of Chemical Engineering at Monash University and has been at Monash since January 2001. Before joining Monash Ravi was an Associate Professor at the Indian Institute of Technology Madras and did postdoctoral work on Sandpile dynamics with Prof. S. F. Edwards at Cavendish Laboratory in Cambridge and on Polymer solution rheology with Prof. H. C. Öttinger at ETH Zürich. He was a Humboldt Fellow in the Techno-Mathematik Department at the University of Kaiserlautern in 1999/2000. Ravi’s research interests revolve around understanding the interaction of flow and micro-structure in complex fluids. Ravi and his students use an array of modelling techniques in their research including nonequilibrium Brownian Dynamics closure approximations and finite element analysis.