# Dropwise Condensation Mechanism on Smooth Hydrophilic Surfaces
## 3 Results and Discussion
### Visualization of USDC  
[PERSON] et al. [12] did not use any drainage mechanism on their condensing surface. Similarly, we have not considered any drainage mechanism on the gold surface in our study. Therefore, it is intuitive to expect the dropwise condensation to degrade over time in our experiments. We break down the USDC into three stages: (i) onset of condensation, (ii) growth of the multilayer film up to 8 nm, and (iii) thin-film growth more than 8 nm. These stages are separated by the presence and/or size of the thin film between droplets. Note that these stages are defined for the sake of our analysis' convenience because 8 nm is the maximum thickness that is measurable in our SPR imaging. Figure 5 illustrates the recorded images at the first stage, onset of condensation from the USDC experiments. The corresponding video of the onset of USDC is available as Online Resource 2. When steam is introduced to the surface, the vapor molecules in the ambient air strike the surface and form a film-like layer on the surface (see Figure 5a,b). This layer propagates from the inlet of steam, i.e., in the example case of Figure 5 from the right side to the left side of the image. Then, the reflectance of this film-like layer increases (up to the reflectance of a droplet, which is 0.5941 and 0.8710 for SPRi at 550 nm and 680 nm, respectively, see Figure 5c). Eventually, distinct droplets emerge on the surface (see Figure 5d). It can be seen in the example case of Figure 5a that some droplets also emerge randomly on the surface. The visualization results may make it look like a thin film forms on the surface and that eventually this film ruptures. However, due to low lateral resolution (the lateral resolution is 4 \(\upmu\)m and 10 \(\upmu\)m for SPRi at 550 nm and 680 nm wavelengths, respectively) of the experiments, the visualization can be misleading. In other words, as the size of each pixel is bigger than the size of initial droplets that form on  
Figure 4: (**a**) Water bridge on the gold surface, (**b**) relocation of the water bridge, leaving behind an area that is darker in color; the gold surface in ambient condition (location 2) is covered with a monolayer of adsorbed water molecules, (**c**–**e**) time-lapse images of dropwise condensation on the surface, and (**f**) the reflectance of location 2 remained relatively constant up to 45 s, while the reflectance of location 1 increased. the surface, the colonies of sub-micron size droplets at the onset of condensation can also create an image similar to a thin film. Therefore, the current visualizations cannot provide sufficient evidence regarding the mechanism of droplet formation at the onset of USDC. In a future study, we will equip the SPRi with a high-aperture objective lens to study the onset of dropwise condensation of steam in more detail. After Stage I, droplets grow and coalesce to become larger droplets. As no drainage mechanism was designed on the condensing surface, it is intuitive to expect that the long-term dropwise condensation of steam is not sustainable on the gold surface. At stage II of USDC, the results show the reflectance of the area between droplets increases. This increase in the reflectance indicates the formation of a multilayer water molecule film between droplets. Figure 6a,b show a raw image and the corresponding processed image from Stage II of USDC, respectively. The dark blue in Figure 6b represents that water droplets and thickness of thin-films between droplets varies between 3-6 nm. As the USDC continues, the thickness of thin films between droplets increases. As our study of the mechanism of dropwise condensation of steam is at its early stages, we have not conducted any quantitative analysis on the effect of thin films between droplets on the heat transfer rates. In our future work, we will include the measurement of heat transfer on the surface for USDC.