Figure 1 Experimental arrangement. The sensing application of the SPR system can be realized by modulating either the wavelength or incident angle [11]. The controlling of light injection angle requires a fine adjustment of the physical configuration precisely; therefore, we choose to implement such a wetness sensing through controlling and analyzing the reflection spectrum under SPR, i.e., wavelength modulation surface plasmon resonance. Since under different incident angles, SPRs occur in different wavelengths, we fix the incident

angle to be 69.3° which simplifies the system as well as provides high enough sensitivity. Results and discussion We first focus on the case where RG-7388 purchase part of the top surface area of a rectangular prism is immersed in water (see Figure  2a). The reflection spectra

under different immersion percentages are measured and plotted in Figure  2b, which actually exhibits the spectral response of SPRs contributed from both water-Au and air-Au interfaces. However, according to our calculation, under an identical injection angle, SPR excited from air-Au interface occurs BAY 63-2521 at a much shorter wavelength that is beyond the scope of our spectrometer; thus, the dips observed in Figure  2b are mainly from the Au-water interface. From this measurement, the adjustment of immersion ratio leads to a substantial change of the reflectivity (especially at the SPR dip at Dichloromethane dehalogenase around 693 nm), however, without shifting the resonant wavelength noticeably. This further confirms that the SPR is primarily from a given metal-dielectric interface (i.e., water-Au interface); the variation of the surface areal coverage modifies the portion of incident light to couple into the SPR, therefore resulting in a significant change of the dip reflectivity. From the varying dip reflectivity, the coverage of water or air can be estimated. The corresponding calibration

curve for the reflectivity of SPR peak is shown in Figure  2c. The SPR reflectivity follows a linear decrease with the gradually increased immersion area. A linear fitting indicates that the adjusted R squared is about 0.9959. The error term comes mainly from uncertainty of our immersed area Vactosertib calibration and measurement noise and can be further reduced with an optimized experiment setup. Figure 2 Schematic and results of the measurement system with top surface partially immersed in water. (a) Schematic of top view of the measurement system. (b) SPR spectra under various immersion percentages. (c) Dependence of the reflectivity at 693 nm against the immersed area: (dots) experimental data and (line) linear fit. Figure  3a,b,c,d illustrates the measured surface patterns, where the size and distribution information of water droplets can be achieved, with wet steam continuously spraying on the hydrophobic coating layer.