Spin-probe-enabled sensing of fluids in confined geometries and interfaces

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When a solid surface confines the molecules of a fluid, new dynamicsemerges e.g., in the form of enhanced viscosity, local order, or collective motion ⎯ whose practical impact encompasses areas ranging from tribology and materials processing to membrane physics. Confinement also affects ion transport and/or the rate andequilibrium concentration in a chemical reaction, all of which makes the physical chemistry of small fluid volumes the subject of broad interest. Studying these effects, however, is notoriously difficult, mainly due to the lack of experimental methods with the required sensitivity and spatial or time resolution. This problem is particularlyacute in heterogeneous systems, common in catalysis or the biological sciences, because extracting the typically minute signatures of confinement from the macroscopic signals is often impractically difficult. Here we propose to explore a new route toinvestigating confined fluids through the use of near-surface point defects in solids as local, nanoscopic probes. We identify two complementary research fronts: The first one capitalizes on novel NV based magnetic resonance spectroscopy methods to investigatewater diffusion under variable confinement and surface hydrophobicity; also part of this effort is the development of novel sensing strategies adapted to heavy water, an area where we will combine experiments and path-integral molecular dynamics simulations. The second research thrust zeroes in on the use of external ferromagnetic-tip-induced gradients that we will leverage to non-invasively probe molecular diffusion and image surface-induced order in confined water. Here we shift the focus to the investigation ofmesoscale fluidic systems, where the use of paramagnetic defects remains virtually unexplored. To this end we introduce various experimental protocols, which we articulate with magnetic resonance techniques we helped pioneer to investigate nanoscopic volumes of model fluids under controlled conditions. In taking this route, wecapitalize on the superb versatility of magnetic resonance techniques, which we leverage here to simultaneously extract fluidic structure and dynamics in confined environments over a broad time scale. The intellectual merit of this proposal resides, therefore, in thefundamental value of the scientific problem we choose to tackle, the innovative measurement strategies we aim to explore, and the integrated approach we take — simultaneously combining nanofabrication, molecular dynamics modeling, and measurement.