431119 Layering, Melting, and Recrystallization of a Close-Packed Micellar Crystal Under Steady and Large-Amplitude Oscillatory Shear Flows

Wednesday, November 11, 2015: 8:30 AM
Ballroom F (Salt Palace Convention Center)
Norman J. Wagner, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE, Carlos R. López-Barrón, Exxon Mobil Chemical Company, Baytown, TX and Lionel Porcar, Large Scale Structures Group, Institut Laue-Langevin, Grenoble, France

The rheology and three-dimensional microstructure of a concentrated viscoelastic solution of the triblock copolymer poly(ethylene oxide)106-poly(propylene oxide)68-poly(ethylene oxide)106 (Pluronic F127) in the protic ionic liquid ethylammonium nitrate are measured by small angle neutron scattering (SANS) under flow in three orthogonal planes. This solution's shear-thinning viscosity is due to the formation of two-dimensional hexagonal close-packed (HCP) sliding layer structure. Shear-melting of the crystalline structure is observed without disruption of the self-assembled micelles, resulting in a change in flow properties. Spatially resolved measurements in the 1–2 plane reveal that both shear-melting and sliding are not uniform across the Couette gap. Melting and recrystallization of the HCP layers occur cyclically during a single large amplitude oscillatory shear (LAOS) cycle, in agreement with the “stick-slip” flow mechanism proposed by Hamley et al. [Phys. Rev. E 58, 7620–7628 (1998)]. Analysis of 3D “structural” Lissajous curves show that the cyclic melting and sliding are direct functions of the strain rate amplitude and show perfect correlation with the cyclic stress response during LAOS. Both viscosity and structural order obey the Delaware–Rutgers rule. Combining rheology with in situ spatiotemporally resolved SANS is demonstrated to elucidate the structural origins of the nonlinear rheology of complex fluids. [JOR, 59, 793 (2015)].

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See more of this Session: Colloidal Hydrodynamics I: Soft and Active Systems
See more of this Group/Topical: Engineering Sciences and Fundamentals