Interactions between liquid flows and chemically modified microchannels

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The behavior of liquid flows in capillaries, including fluid-solute and fluid-wall interactions, is of utmost importance for the better understanding of technical and biomedical flow phenomena as found in microcapillary devices, electrophoretic biochips, blood and lymph circulation, protein adsorption, calcification, and dialysis. Our goal is to study aqueous, organic, and biological liquid flows in microchannels and their interaction with chemically modified walls under controlled conditions. The fluids used are homogeneous, e.g. pure solvents or electrolytes, or heterogeneous, e.g. aqueous solutions containing colloidal particles, proteins, or cells. The microchannels will be fabricated by deep X-ray lithography, which allows the free design of lateral flow geometries. The width of the channels can be altered from several microns to few hundred nanometers. Thereby, the systems can be studied from continuous flow conditions down to the scale, where molecular motion starts to affect the flow. The channel walls will be chemically modified by coating with ultrathin organic films, e.g., to control hydrophilicity of the walls or to graft polymer brushes of different density. Thereby, a variety of fluid-wall interactions can be assessed. For analysis of these systems we shall apply in-line holography and sum frequency generation spectroscopy (SFG). While in-line holography yields a detailed picture of the velocity distribution of the microsopic flow by tracking tracer particles, SFG as a nonlinear optical technique is highly sensitive to fluid-wall interactions, such as penetrating fluid molecules and order phenomena at the fluid-solid interface.