Quantitative measurements of nanoscale thin frost layers using surface plasmon resonance imaging

This study reports the presence of a nanoscale thin frost layer. During the frosting process, the surface plasmon resonance (SPR) imaging method can be used to overcome conventional optical limits and quantify this layer. The research outlined here also provides quantitative thickness measurement of the thin frost layer via a proposed calibration method based on the measured SPR intensity. The SPR system established in this study consists of a 50 nm gold-coated BK7 cover glass, a prism, a light source, a polarizer, a lens and a filter for the collimated light of a 600 ± 5 nm wavelength, and a CCD camera. The SPR angle of the ice phase is 72°, which corresponds to the ice refractive index of 1.307. The gold-glass specimen is cooled from room temperature (23 ± 1 °C) to −4.0 ± 0.8 °C by using a thermoelectric cooler to maintain the relative humidity of 20 ± 3% (at the room temperature). As a result, it is found that the nanoscale thin frost layer between the frozen condensates exists on the surface. Also, the present study yields the spatial distribution of reflectance that is associated with the frost layer thickness, indicating that the local information about thin frost layer thickness can be obtained through this SPR imaging method. It is found that the SPR imaging method enables successful capture of the depthwise spatial variations of the thin frost layer, showing that the frost layer was grown over time as a result of the de-sublimation of water vapor.