"A Thermomechanical Approach to Developing Parabolic Focal Conic Defects in Concentrated Surfactant Lamellae Through Large Amplitude Oscillatory Shear" 

Matthew Kaboolian, MSE PhD Candidate 

Advisor: Professor Kendra Erk

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ABSTRACT

Concentrated surfactants have recently been a point of significant research as consumer care companies look to reduce the amount of water in their products. As concentrations increase liquid crystalline phases emerge that impact processing. One such liquid crystalline phase is the lamellar stack of surfactant bilayers. Morphologically, lamellar surfactants are diverse, existing as planer bilayers, highly curved parabolic focal conic defects, pFCDs, or rolled spherical onion shaped multi-lamellar vesicles, MLVs. These defects originate from stresses activating certain deformation modes, which create complex three-dimensional geometric structures. Work in collaboration with Procter and Gamble has revealed that these focal conic defects, FCDs, that make up the apparent cross-polarized microstructure, are often poorly characterized, particularly under shear, and especially under oscillatory shear. Understanding the evolution of these FCDs is critical to understanding the mechanical history of surfactant formulations and selectively altering their properties. An area of open exploration is using large amplitude oscillatory shear and temperature as a thermomechanical processing tool. The related theories of recovery rheology will be applied to concentrated, confined, and aligned lamellar surfactants in an effort to create pFCDs. This has been inspired by similar techniques which have been used to convert thicker lamellar surfactants into MLVs. The application of recovery rheology can allow for characterization of the rheologically and structurally complex lamellar phase, particularly within the context of FCDs and their interplay with initiation and cessation of flow. A deeper understanding of the dynamics and flow transitions of lamellar surfactants, and all surfactant phases offer the opportunity to develop a framework for understanding morphological and mechanical changes in a material agnostic way. While also answering important industrial questions about formulations regarding the onset and arrest of flow.