Wetting transparency of graphene across length scales: Macroscopic transparency but nanoscopic mirror-like behavior

Graphene supported on a substrate in contact with water underpins a wide range of processes and technologies, yet its wettability remains controversial. Understanding how substrate charges and graphene’s properties influence water organization is crucial. Here, we combine heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy with molecular dynamics simulations to investigate CaF2-supported graphene interfaces in contact with water. We find that interfacial water orientation is primarily governed by the CaF2 substrate’s pH-dependent local electrostatics, confirming graphene’s macroscopic wetting transparency. However, at the nanoscale, graphene’s polarizability gives rise to a local reorientation of interfacial water above substrate charges, partially counteracting the electrostatic field originating from the substrate, revealing subtle structural ordering that is masked in spatially averaged measurements. These insights elucidate the molecular origins of graphene’s wetting behavior and suggest new avenues to tailor interfacial phenomena in graphene-based nanofluidic, sensing, and energy applications.