Levich Institute Seminar Announcement, 09/09/2008

Tuesday, 09/09/2008 2:00 PM 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"

ABSTRACT

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:

• Professor, 1983-Present, Department of Mathematical Sciences, University of Delaware, Newark, DE 19716, (Assoc.Prof. 1983-1989), Secondary appointment as Professor of Chemical Engineering 2005- present.
• 2003- Present, Associate Dean, College of Engineering, University of Delaware.
• Fall 2004 Visiting Researcher, IMA and Dept. Math, Univ. Minnesota, Minneapolis.
• 2000-2001, Associate Dean for Planning & Research,College of Arts and Science, University of Delaware.
• 1992-2000, Chair, Department of Mathematical Sciences, University of Delaware (Interim Chair 1991-1992).
• 1987-1988, Visiting Associate Professor, Mathematics Department, University of Maryland, College Park, (NSF-VPW).
• 1973-1983, Mathematics Department, University of California at Los Angeles, California, Associate Professor with tenure, 1980-1983; Assistant Professor, 1975-1980; Adjunct Assistant Professor, 1973-1975.

CURRENT RESEARCH INTERESTS:

Complex fluids - highly entangled fluids and thin films.

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