Non-Newtonian Fluids in Squeeze Films

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CBET-0828163DuckerThe flow of fluid in narrow channels is receiving increased attention because of the growing importance of both microfluidics and nanoparticle science and engineering. Reduction to smaller length scales raises interesting questions, including: what is the boundary condition at the solid-liquid interface? How can the flow of fluid be enhanced in narrow channels? How is the flow of complex fluids affected by confinement into a thin film? Here we propose to address these questions by using colloidal probe microscopy to study the flow of fluids in the squeeze film between a sphere and a plate. This method has recently been successful in confirming the no-slip boundary condition at the solid-liquid interface for simple liquids, and is now ready for the study of more complex fluids. The outcomes of this work have implications in two fields. First, the forces acting on small particles as they approach plates are important for in their own right because of the widespread use of particles and the presence of particle contamination on membranes, semiconductor wafers etc. Second, these measurements open a window on fundamental questions such as the fluid boundary conditions and the importance of confinement on fluid flow. The proposal is to use force microscopy to measure the force, velocity and displacement of a colloidal particle as it approaches the plate. The particle will be immersed in a fluid. The measured parameters will be compared to theoretical values to validate theories. The first order theory will be Brenner's lubrication result for simple liquids under creeping flow. Comparison to this theory allows us to determine the effective slip-length and the effective viscosity. Measurements will be made on a variety of non-Newtonian fluids, including polymer melts, surfactant solutions, nanoparticle dispersions, and rarefied gases. Some development of theory will be necessary to interpret these measurements. Attempts will be made to enhance flow through the adsorption of low viscosity fluids or films at the interface. The proposal is also to improve the colloidal force measurement by increasing the range of frequencies and shear rates that are accessible. Greater shear rates will be accessed by incorporating a high velocity drive, and greater frequencies will be accessed by development of an oscillatory drive. This oscillatory drive will enable the simultaneous measurement of elastic and dissipative responses over a small range of separations. The advantage of colloidal probe microscopy is the very high resolution in displacement and force that is achieved. For example, colloidal probe microscopy can determine the slip-length with only a few nanometers of uncertainty. The educational component of this proposal will be to train a post-doctoral researcher, a graduate student and undergraduate students.