Levich Institute Seminar Announcement, 09/09/2008
Steinman Hall, Room #312
(Chemical Engineering Conference Room)
Professor L. Pamela Cook
University of Delaware
Department of Mathematical Sciencies
"Highly Entangled Wormlike Micellar Solutions - Model and Predictions in LAOS and Extensional Flows"
Surfactant molecules (micelles) can self-assemble in solution into long flexible structures known as wormlike micelles. These structures entangle, forming a dense network and thus exhibit viscoelastic effects, similar to entangled polymer melts ∓ solutions. However, in contrast to polymeric networks, wormlike micelles break and reform leading to a new relaxation mechanism. Steady shearing flows of these solutions exhibit spatial inhomogeneities such as 'shear-bands' during deformation which have been well-studied both experimentally and theoretically. In this talk the dynamical response of two recently formulated constitutive models (denoted respectively VCM, PEC+M) under Large Amplitude Oscillatory Shear (LAOS) and strong extensional deformations will be presented. In LAOS, kinematic inhomogeneities develop across the gap for a wide range of strains and frequencies. Lissajous figures in conjunction with a Chebyshev polynomial decomposition of the viscous and elastic stress contributions are used to probe the behavior of the model. The complex dynamics of shear-banding in this unsteady large amplitude deformation can be conveniently represented using a Pipkin diagram. In filament stretching experiments with these wormlike micellar solutions it has been observed that the elongating fluid filaments can experience a sudden rupture near the midplane at high strain rates [Rothstein]. This newly observed failure mechanism is not related to the visco-capillary thinning observed in viscous Newtonian fluids, but is conjectured to be due to a viscoelastic cohesive failure event. The dynamics of the new VCM model in transient uniaxial elongation are examined to aid in further understanding this elongational rupturing mechanism.
BRIEF ACADEMIC/EMPLOYMENT BACKGROUND:
CURRENT RESEARCH INTERESTS:
Complex fluids - highly entangled fluids and thin films.