# The Effect of Adsorbed Volatile Organic Compounds on an Ultrathin Water Film Measurement
## 2 Materials and Methods
### Adsorbed VOC Characterization Using SPRi  
SPRi was used for real-time tracking of adsorbed VOCs on a surface. Our in-house-developed automated SPRi instrument was used for the imaging. Figure 1a shows a schematic of this instrument. P-polarized monochromatic collimated light required for the SPRi forms by passing a divergent light from a LED illumination system through the optical array (focusing lens, biconvex lens, apertures, 550 nm bandpass filter with a full width-half maximum of 10 nm, and p-polarizer). A stationary mirror and a rotating mirror direct the light to launch on a prism. Reflected light from the prism is guided toward a charge-coupled device (CCD) camera using a second pair of rotating and stationary mirrors. The recorded light contains information on the thickness and refractive index of the medium on top of the gold-coated glass. The use of a motorized rotating and linear stage allows for sweeping the incident angle to obtain the SPRi reflectance curve of a sample. The reflectance curve of the sample reveals the surface plasmon resonance (SPR) angle (the angle corresponding to the minimum reflectance), which works as a fingerprint for characterizing the materials on top of the gold substrate. The characterizations of adsorbed VOCs on samples 2 and 4 were done separately. For each sample, first, the corresponding SPR angle of the sample was found by sweeping the incident angle from 40\({}^{\circ}\) to 60\({}^{\circ}\). Then, the incident angle was adjusted to 1.5\({}^{\circ}\) below the SPR angle (46.3\({}^{\circ}\)), i.e., 44.8\({}^{\circ}\), and the  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Preparation** & **Sample 1** & **Sample 2** & **Sample 3** & **Sample 4** \\ \hline Cleaning procedure & - & SCP + Plasma & SCP + Plasma & SCP \\ \hline Storing method & Open container & Open container & Sealed container & Open container \\ \hline \hline \end{tabular}
\end{table}
Table 1: Details of the cleaning procedure and storing method of the samples.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Analysis**} & \multicolumn{4}{c}{**Exposure Time (h)**} \\ \cline{2-5}  & **Sample 1** & **Sample 2** & **Sample 3** & **Sample 4** \\ \hline XPS & - & 0, 4, 17, 43, 180, 893 & 0, 4, 17, 43, 180, 893 & 0 \\ \hline AFM & - & 2, 17, 41, 185 & 2, 17, 41, 185 & 2, 17, 41, 185 \\ \hline Contact Angle & 0, 4, 18, 46, 180, & 0, 4, 18, 46, 180, & 0, 4, 18, 46, 180, & 0, 4, 18, 46,time-lapse images (one frame per 1 min) of each sample were recorded for 110 h at room conditions (relative humidity of 33% and temperature of 28 \({}^{\circ}\)C), Figure 1b. To ensure the repeatability of the result, SPRi was repeated three times for sample 2 and sample 4. The schematic of the SPRi of dropwise condensation is illustrated in Figure 1c. First, the reflectance curve for a sample cleaned with SCP was obtained using SPRi with angle-scanning. Then, the incident angle was fixed at 44.8\({}^{\circ}\) and the setup was prepared for the SPRi of dropwise condensation at 3000 FPS. For inducing condensation on the sample, we placed an indium tin oxide (ITO)-coated glass slide as a heater at a distance of 5 mm above the gold surface. A water column (between the heater and the gold-coated glass) with a size of 15 \(\pm\) 5 \(\upmu\)m was formed using a syringe. By heating the water column, part of the water column evaporated and condensed on the surface. The temperature and relative humidity above the study area were 45 \({}^{\circ}\)C and 60 \(\pm\) 4%, respectively, while the condensing surface temperature was 35 \({}^{\circ}\)C. The details of the dropwise condensation experiment, image processing, statistical noise analysis, uncertainty of SPRi, and a monolayer water film measurement are discussed elsewhere [5; 23; 24].