# Dropwise Condensation Mechanism on Smooth Hydrophilic Surfaces
## 3 Results and Discussion  
### Onset of SDC  
As SPR imaging is a powerful technique for the detection of thin films, we designed our experiment to understand whether a thin film can be detected at the onset of dropwise condensation. Figure 3a shows the SPRi (at 680 nm) results of SDC at the onset of condensation. Online Resource 1 shows the corresponding video of SDC at 680 nm. The light gray and dark gray represent water and gold surfaces, respectively. As Figure 3a shows at t = 18 s, the first condensates emerge in the vicinity of the water bridge. The reflectance variation is measured by tracking the averaged reflectance of pixels in the red dotted box over time; the measurement area is chosen in the vicinity of the water bridge, as the initial droplets emerge on the part of the surface closer to the water bridge. As mentioned, the formation of a water film with the thickness of a molecule layer corresponds to a 4% change in the measured reflectance. However, this result indicates that the reflectance variations before the emergence of the first droplets are infinitesimal (see Figure 3b). This means no film, even one as small as a molecule-thick layer, forms on the surface prior to the emergence of droplets at the onset of SDC, i.e., the initial droplets form by heterogeneous nucleation on the surface. In our other work [14], we showed that the area between droplets is covered with a monolayer film during SDC. Our results confirm the finding of [PERSON] and [PERSON] [6] in support of nucleation theory as the mechanism of dropwise condensation.  
Figure 2: Theoretical SPR plots showing the effect of 1 nm water film on the reflectance at two wavelengths, (a) 550 nm and (b) 680 nm.  
In a different set of experiments, the water bridge on the surface was slightly relocated on the surface by moving the ITO coated glass that is on the top of the water bridge (see Figure 1a). After ~16 s, SDC was started by heating the water bridge using ITO-coated glass. Figure 4 shows selected time-lapse images of this experiment recorded using SPRi at 550 nm. The relocation of a water bridge on the surface left behind an area with a lower reflectance, compared to the rest of the surface. The white-dashed line shows the boundary of the area that was previously covered by the water bridge. The area on the right side of the dashed line can be identified with its darker gray color from the rest of the surface. Locations 1 and 2 are sample areas that are chosen from these two distinct parts of the surface for image processing purposes. Our analysis shows that the reflectance of location 2 is 1% higher than the reflectance of location 1, i.e., for ambient conditions, the gold surface is found to be covered with a molecule-thick layer of adsorbed water. The presence of adsorbed water film on the metal surface has been reported in the literature [18,19]. Figure 4f shows the temporal changes of reflectance at locations 1 and 2. It can be observed the reflectance at location 1 increases when the relative humidity increases (from 35% to 60%) on the surface due to dropwise condensation. The increase in the reflectance of location 1 indicates that water molecules are adsorbing on the surface. It is worth noting that the adsorbed molecules on the surface mostly consist of water molecules and not volatile organic compounds (VOCs). In our previous work [15], we showed that the rate of adsorption of VOCs on the gold surface is much slower than the rate of adsorption of the molecularly adsorbed water layer on the surface.  
Figure 3: (**a**) Selected time-lapse images recorded (using SPRi at wavelength of 680 nm and temporal resolution of 250 FPS) from the onset of SDC, (**b**) the averaged reflectance of the area in the red box shows no thin film forms on the surface prior to the droplet formation.