Probing interfacial water via color-center-enabled spin magnetometry

The behavior of interfaces between immiscible fluids is central to various natural and engineered processes, but their experimental characterization is often challenging due to sensitivity constraints or lack of spatial and/or temporal resolution. Here, we make use of shallow nitrogen-vacancy (NV) centers to investigate the dynamics of water confined between the host diamond crystal and a fluorinated oil droplet on its surface. Leveraging NVs as local spin probes, we implement 1H- and 19F-selective nuclear magnetic resonance protocols to separately probe interfacial water and oil molecules. We find distinctive dynamics for either molecular system, with the former diffusing much faster than the latter. Further, from a comparison with a reference, oil-free diamond surface, we unveil a slow, week-long process leading to the gradual removal of water from the adsorbed layer. Molecular dynamics simulations suggest oil molecules progressively gain direct contact with the diamond surface due to greater binding affinity, effectively increasing water confinement and hence impacting its mobility. Our results shed light on the dynamics of adsorbed water while informing new approaches in the investigation of fluidic hetero-interfaces.