# Dropwise Condensation Mechanism on Smooth Hydrophilic Surfaces
## 3 Results and Discussion
### Visualization of USDC
Figure 5: Selected time-lapse images at (**a**) 5.92 s, (**b**) 6.12 s, (**c**) 9.18 s, and (**d**) 20.14 s during the first stage, Onset (stage I) of USDC provide inconclusive evidence on whether a thin film larger than a monolayer exists on the surface; the images are recorded using SPRi at 680 nm. Figure 6: Formation of multilayer water film between droplets during Stage II of USDC, (**a**) recorded image using SPRi at 680 nm, and (**b**) processed image with thickness map; dark blue represents the droplet of water. Stage III of USDC is the growth of the thin film between droplets beyond the detection range (>8 nm) of the current SPRi system. Figure 7a depicts the correlation between the reflectance and the thickness of water film for SPRi with intensity modulation (IM, experimental angle of 43.15 degrees) at a 680 nm wavelength. As this figure shows, the reflectance associated with just air (no film on the surface) is 0 and the reflectance associated with a liquid droplet on the surface is 0.87. The reflectance values of 0 and 0.87 are practically the minimum and maximum reflectance that we can measure using our current SPRi system. In terms of thin film measurements, the water film with a thickness of 8 nm produces a signal that is equivalent to the reflectance of a water droplet, i.e., reflectance of 0.87. Therefore, the range in which we can measure a thin film is 0 to 8 nm. If the thickness of the thin film goes beyond 8 nm, it shows itself in our image with a reflectance higher than a water droplet, i.e., the color of a thin-film larger than 8 nm is slightly brighter than the water droplets in the recorded images. As noted, it is not possible to accurately measure film larger than 8 nm with the current SPRi setup. In this case, we can only comment as to whether a film with a thickness larger than 8 nm exists on the surface. The thickness of this film could be anywhere from 8 to 250 nm. Figure 7b,c show the raw and corresponding processed images from Stage III of USDC, respectively. In the raw image, the area between the droplets has a brighter color, as compared to the color of the droplets. In the processed image, droplets are represented in dark blue, areas with a thickness larger than 8 nm are identified in yellow color, and areas with a thickness less than 8 nm are shown in a bright blue color. Online Resource 3 is the video from Stage III of USDC. Figure 7: **(a)** Theoretical correlation between the reflectance and thickness of the water film, as visualized using SPRi at an experimental angle of 43.15 degrees and a wavelength of 680 nm, **(b)** a raw image of a thin film between droplets during USDC (bright and dark gray represent thin films and droplets, respectively), and **(c)** the corresponding processed image of thin films grown between droplets during USDC (dark blue, light blue, and yellow colors represent droplets, film with thickness smaller than 8 nm, and film with thickness larger than 8 nm, respectively). Figure 8 shows time-lapse images of coalescence during Stage III of USDC. It can be observed that after the coalescence, the swept area on the surface is not dry. In the example case shown in Figure 8, the thickness of the swept area just after the coalescence increases beyond 8 nm, i.e., the reflectance of this area is higher than 0.87, the reflectance of a droplet (and 8 nm thick film). Then, reflectance drops to a value lower than the reflectance of a droplet, i.e., the thickness of film becomes smaller than 8 nm. This fluctuation in film thickness may be an indicator of the thin film instability or lateral transition of water molecules in the thin film. Figure 8d shows the new droplets emerging on the surface after 42 ms from the start of coalescence. Online Resource 4 provides the video of the droplet coalescence illustrated in Figure 8. Our USDC results confirm the findings of [PERSON] et al. [12] regarding the presence of a thin-film larger than a monolayer between droplets.