The 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.
Understanding the flow of liquid in narrow channels is important in a range of natural and chemical phenomenon, including purification of minerals, chromatography, microfluidics, and flow of blood through narrow vessels. Gas flows between particles and containing particles are important for chemical engineering processes such as pneumatic transport, catalytic converters, fluidized beds and cyclone separations, and also for spray coatings and air filtration. An understanding of natural phenomena such as the coalescence of water droplets to form rain also depends on knowledge of the gas flow around particles. In this project we determined the boundary condition for flow in narrow channels containing air or liquid using Atomic Force Microscopy squeeze film experiments. Knowledge of the boundary condition is necessary for modeling fluid flows in the processes described above. These boundary conditions are usually parametized in terms of the slip-length, which is the extra radius of a pipe required in a model where the fluid adopts the velocity of the solid, in order to fit with measured data where the fluid velocity at the solid might be different to the solid velocity. Our principal findings were that: Finite slip lengths occur in water between hydrophobic surfaces. This suggests that hydrophobic coatings on the inside of narrow channels can be used to reduce the energy required to pump water through narrow channels. Finite slip lengths occur in weakly bonded alkane liquids. This, together with the result for water in hydrophobic channels shows that the bonding between the solid and liquid is important in determining the slip length. Polymer lubrication forces can be reduced by the dynamic migration of polymer molecules into regions of high shear force. Very large slip-lengths occur for molecularly smooth surfaces in air. The slip length in air can be altered by hundreds of nanometers by addition of a thin organic film. The contact angle method can be used to measure enantioselectivity in adsorption. This project also provided training for a Post-doctoral researcher, one Ph. D., one masters student, and three undergraduate student. Students have been trained in the use of Atomic Force Microscopy, ellipsometry, contact angle measurement, x-ray photoelectron spectroscopy, designing experiments and writing scientific papers. All of the undergraduate students went on to study for Ph.D. degrees at other Universities. The post-doc and Masters student have gone on to use their training in the computer chip manufacturing industry.
