Slippage and Nanorheology of Simple and Complex Fluids in Confinement

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The properties of a liquid in contact with a solid interface are very important for applications involving e.g. the flow through porous media or in lab-on-a-chip devices. Solving the hydrodynamic equations for a liquid flowing over a solid surface, one usually assumes the relative velocity between fluid and solid to be zero. This is the so-called "no-slip boundary condition". Experiments with complex fluids and, very recently, with simple (Newtonian) liquids have shown that there can indeed be a non-zero velocity ("slippage") at the boundary. In the project we will study experimentally why and under which conditions this can take place. The sizes involved are expected to be of nm-scale. On the same scale we will therefore determine the velocity and the shape of a moving liquid front. Both are known to serve as extremely sensitive "nano-rheometers". It is expected from theory that slippage is enforced for smooth and low surface-energy substrates. In our experiments, we will vary these system properties and also properties of the liquid like viscosity and viscoelasticity. Since slippage can enhance the flow rate also on a macro-scale, slippage is an important issue in the design of microfluidic devices such as mixers etc. The vision thus is to be able to tailor surfaces in their ability to enhance slippage.