Nano- & Microfluidics:

The use of wetting gradients is an interesting tool to drive liquid flow along surfaces, micro or nano-channels. Separation processes of liquid mixtures or polymer solutions are discussed mainly phenomenologically until now and a basic understanding of the influence of boundaries on those processes, which is essential for an improved design of new microfluidic devices, is needed. The goal of the proposal is to gain a fundamental understanding of separation processes on gradient surfaces in confined geometries on the nano- and microscale. The key idea of the project is a combination of newly developed versatile experimental tools with a powerful theoretical approach in order to investigate phenomena which are not at all understood until now. Central part is the investigation is the demixing of multi-component fluids during the flow. Different types of multi-component fluids starting from simple liquids and ending with polymer blend solutions will be focussed on. A gradual change of surface properties is achieved by generating a gradient of the geometrical structure ("roughness”) or by a variation of the chemical composition of the surface. Wetting gradients will be prepared by binary polymer brushes made of two largely different polymers introducing a gradient of hydrophilicity/hydrophobicity or surface charge via variations in the grafting density. The flux and hydrodynamics of the fluid will be investigated as well as the spatial variation of its composition due to changing boundary conditions. The comparison of experimental results with computer simulation data will result in a high level of understanding.