Water surface with nanometer thickness probed by oblique-incidence reflectivity difference

The water surface thickness has been expected to reach the nanometer level for the bulk aqueous phase. To verify this prediction, in this work, the oblique-incidence reflectivity difference (OIRD) measurement was applied to characterize the water film thickness on a dynamically fluctuating water surface. By extracting wave parameters such as period, wavelength, and attenuation coefficient from the recorded optical signals, a physical model describing the surface wave dynamics was established and then integrated with the high-temporal-resolution OIRD data to reconstruct the temporal and spatial variation of the water film thickness during wave propagation. Our experiments reveal that the interfacial water film is only ∼1 nm thick and exhibits spatial heterogeneity that varies with position during fluctuations. As the oscillation amplitude gradually attenuates under the surface tension, the variation range of the water film thickness decreases accordingly and finally approaches a constant. This study combines OIRD with interfacial fluid dynamics, which provides a reference for further interdisciplinary research.