# The Effect of Adsorbed Volatile Organic Compounds on an Ultrathin Water Film Measurement
## 3 Results and Discussion  
### Proper Cleaning Procedure Prior to the Dropwise Condensation Experiment  
Surface defects [7; 8; 9] and adsorbed VOCs [10; 11; 12] on a surface are key factors that contribute to surface heterogeneity. Surface heterogeneity can cause abnormality in the mechanism of dropwise  
Figure 1: (**a**) Schematic of the automated surface plasmon resonance imaging (SPRi) instrument (a p-polarized monochromatic (550 nm) collimated light was used for SPRi), (**b**) schematic of volatile organic compound (VOC) adsorption experiment using SPRi, and (**c**) experimental conditions for SPRi of dropwise condensation (brighter color represents droplets and darker color represents the gold substrate). condensation on a smooth surface. As our final goal was to study the effect of surface treatment on the measurement of ultrathin water film during the dropwise condensation process on a smooth surface, we studied the effect of surface cleaning methods on the surface properties (surface uniformity, surface wettability, and surface chemistry). Figure 2a,b show the surface morphologies of sample 2 with exposure times of 2 h and 185 h, respectively. The root-mean-square of the surface roughness for samples 2 and 3 was reduced slightly (within 183 h exposure time) from \(1.10\pm 0.02\) nm to \(0.95\pm 0.10\) nm and from \(1.14\pm 0.02\) nm to \(0.83\pm 0.06\) nm, respectively. Meanwhile, the root-mean-square of surface roughness for sample 4 changed from \(1.57\pm 0.01\) nm to \(1.70\pm 0.1\) nm. The AFM analysis showed that the surface had a uniform, smooth coating. However, the error of measurement (based on three measurements at three different locations of each sample) increased for all three samples over time. This showed that the adsorption of contamination on the surface can slightly affected the homogeneity of the surface. As the associated error was smaller than 0.10 nm over the course of 183 h, we can consider these samples to have had a uniform surface structure. Figure 2b shows a defect on the surface of the gold-coated glass; a defect can cause heterogeneous nucleation during the dropwise condensation process [9]. It can be seen in the example case in Figure 2c that atomic-scale defects and surface edges on the gold-coated glass caused heterogeneous nucleation on the surface (bright spots are micro-size droplets; the darker color represents the gold substrate). This image was recorded at the onset of dropwise condensation using SPRi [23]. As the surface defects were limited on the gold-coated glass surface, and these defects were prescreened prior to the dropwise condensation experiments, the presumption of homogenous nucleation due to the smooth condensing surface is reasonable. Figure 2: AFM images of the sample 2 surface (gold-coated glass, plasma-cleaned and stored in an open container at room conditions) after exposure times of (**a**) 2 h and (**b**) 185 h. (**c**) SPRi of dropwise condensation (the darker color represents the gold substrate and the brighter color shows the droplets) shows surface defects can cause heterogeneous nucleation. (**d**) Contact angle measurements of the samples. Equilibrium contact angles of DI water drops on the gold surface are shown in Figure 2d. The averaged contact angle for sample 1 remained relatively constant at 79.4\({}^{\circ}\) over the 1100 h period. The contact angle variations over the exposure time of 1100 h were more significant for plasma-cleaned samples -sample 2 (contact angle increased exponentially from 27\({}^{\circ}\) to 70\({}^{\circ}\)) and sample 3 (contact angle increased exponentially from 23\({}^{\circ}\) to 68\({}^{\circ}\)). For sample 4, which was cleaned with the SCP, the averaged static contact angle was slightly increased (from 70\({}^{\circ}\) to 76\({}^{\circ}\)) in 1100 h. The results showed that the cleaned samples had smaller contact angles, as compared to the sample with no surface cleaning (sample 1).