Investigating the Dynamics of Confined Colloidal Thin Films by a Novel Confocal Micron-Gap Rheometer

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National Science Foundation - Division of Chemical &Transport Systems ? Particulate & Multiphase Processes Program (1415)Proposal Number: 0730813 Principal Investigators: Zhu, Yingxi (Elaine) Affiliation: Notre Dame Proposal Title: Investigating the Dynamics of Confined Colloidal Thin Films by a Novel Confocal Micron-Gap Rheometer This project investigates the role of confinement on the structure and rheological properties of colloidal suspensions between two solid surfaces. While this project focuses on the fundamentals, the PI has in mind broad applications of using colloidal particles in numerous chemical and pharmaceutical products, chemical processing, and advanced functional materials for various micro/nano-scale devices and systems. Specifically, this proposal strives for a deeper understanding of confinement-induced glass transition and the film-thickness dependence of dynamic responses of colloidal thin films. The research team will elucidate the mechanical stability of a jammed glassy suspension by examining the critical yield stress and its scaling behavior. The team will further explore how to fluidize jammed colloidal structures by judicious actions of shear and surface texture. The immediate objectives of this research are: 1) by direct microscopic observation, to characterize the structure of buried colloidal thin films as the gap spacing between two solid surfaces decreases down to the dimension comparable to the particle diameter; 2) to determine the dynamical properties, i.e. mobility, relaxation processes and relaxation times of colloidal thin films in the quiescent state and also under the influence of applied shear excitation and relate them to the rheology of bulk suspensions; 3) to address the roles of more complex variables, such as compression rates, surfaces and topographical compositions on phase states and viscoelastic properties of interfacial colloidal layers. The proposed research will focus on a model hard-sphere system ? fluorescence labeled poly(methyl methacrylate) (PMMA) microspheres suspended in both density and index matched non-polar media and confined between two solid surfaces treated with: 1) smooth undyed PMMA thin layers, and 2) patterned templates with controlled topographical and chemical patchiness. Intellectual Merit. The fundamental understanding of glass transition and interfacial properties of confined colloidal thin films and of the roles of surface properties, compression rates and shear on slow dynamics of colloidal particles in complex media are great challenges. Going beyond material properties to address fundamental colloidal physics, a micron-gap rheometer is integrated with a laser scanning confocal microscope to simultaneously control surface separation, ?see? 3-dimensional structures of buried colloidal particles and their motion, and measure viscoelastic responses to external forcing fields. This novel experimental setup is unique to the PI's laboratory. It will offer a new approach, unavailable to rheometric and Brownian dynamics simulation studies, to reveal the slow (1sec) and long-range mechanism of confinement-induced glass transition of colloidal suspensions, molecular fluids and many other complex fluids in general. The technical significance of this work centers on a new approach and understanding of the physics of glass transition. The knowledge gained will be employed for rational materials design to control phase structures, interparticle interaction and friction on demand. This in turn may have a broad impact on synthesis of tough structural ceramics, lubricating layers and advanced functional materials, on fluidization in materials processing, and other technical areas by controlling the viscoelasticity of colloidal systems for painting, lubrication and etc. The techniques and knowledge gained will apply by rational extension to broader areas, such as efficient blood-cell filtration and energy saving during fluid transport in porous media.Broader Impact. A broad-based education/outreach program is integrated within this interdisciplinary research program. Already active in the local chapter of the Society of Women in Engineering (SWE), the PI is committed to the recruitment and retention of female students who continue to be under-represented in many engineering disciplines. The PI also actively participates in a coexchange program with Notre Dame's sister college, St. Mary's College, a leading private woman's Catholic university in educating woman students. Central to this proposal is curriculum development and research mentoring to strengthen the materials science and nanotechnology programs at Notre Dame. Finally, this program seeks to establish a strong coalition with the oil and automotive industries such as ExxonMobil and Ford to help students, scientists and engineers communicate and work well together.