Solid/aqueous interfaces play a crucial role in several important processes from micro/nanofluidics devices to biological systems where adsorption and interfacial hydrodynamics, in particular slip and wetting, are key problems. A common feature underlying these phenomena is water organization near the surface. In the case of hydrophobic surfaces, where the existence of a water gap is still controversial, it is now recognized that slippage occurs at the solid surface, but the discrepancy in the slip lengths reported for different surfaces is enormous. The case of electrolytes and charged interfaces is of utmost practical importance for surface driven transport phenomena such as electro-osmosis or electrophoresis which are sensitive to charge distribution at the solid/liquid interface. In a nanochannel, channel size, Debye length and slip length can all be on the same order of magnitude, leading to unusual phenomena. For example, continuum dynamics predicts that the channel will be filled with a unipolar solution of counter-ions suggesting that the type and concentration of ions can be controlled by the surface charge density of the channel wall. Ion specificity, referring to the broad range of phenomena particularly important in biology where ions of the same valency have a dramatically different effect, is expected to play an important role here, and is again related to the interfacial water structure. Enhanced slip lengths of a few 10s of nm have also been predicted from molecular dynamics simulations, leading to completely new flow mechanisms. However, these key issues, with high relevance for e.g. energy conversion, remain almost unexplored experimentally.



Project description: We will take advantage of recent technical progress on nanofabrication and advanced x-ray scattering techniques to determine complex structures at buried interfaces at the sub-nm to 10s of nm scale. Coupling these techniques to nanochannel devices we will measure the aqueous structuring or ion distribution near a solid surface and in nanochannels with unprecedented resolution. Both static and dynamic cases will be investigated. The surface will be either hydrophobic or hydrophilic. When necessary, slip lengths will be measured by atomic force microscopy. This project will advance the state of the art by directly establishing the relation between the hydrophobic gap and the slip length at aqueous interfaces or determining the distribution of different ions under flow in a channel narrower than the Debye length.