*3.1. Fluorescent Images and Thickness of Adherent Paint Droplets*

Figure 5 shows fluorescent and pseudo-color images of an adherent paint droplet, measured from time-lapse photography taken under the conditions of *μ* = 0.037 Pa·s and *P*in = 0.3 MPa. It is shown that the area where the paint adheres becomes green. The red solid line represents the measurement position of the cross-sectional fluorescence intensity. Figure 6 shows the time change in the cross-sectional thickness of the droplets when adhering to the acrylic plate. To derive the film thickness, the aforementioned calibration data were used. The coating thickness, *h,* was calculated from the average fluorescence intensity using the calibration data. The horizontal axis is distance, *X,* and the vertical axis is coating thickness, *h*. *T*<sup>D</sup> = 0 s was defined as one frame before droplet impact, and the droplet behavior was measured at *T*<sup>D</sup> = 1/90, 3/90, and 6/90 s. It was confirmed that the center part of the droplet is thinner than the outer parts at *T*<sup>D</sup> = 1/90s. After adhesion, the center part becomes thick at *T*<sup>D</sup> = 3/90 s. This behavior is caused by the impact when the droplet adheres. Here, the high, spike-like values seen in the cases of *T*<sup>D</sup> = 3/90 s and 6/90 s are the parts of the bubbles existing in the adherent droplet. The presence of air bubbles resulted in a high value due to the increase in local thickness, as shown in the fluorescence images. Finally, the droplet thickness converged to about 50 μm at *T*<sup>D</sup> = 6/90 s. In addition, there was almost no difference in position between *T*<sup>D</sup> = 1/90 and *T*<sup>D</sup> = 6/90 s. Therefore, it was confirmed that the paint droplet adhering to the acrylic plate did not spread on the painted surface.

**Figure 5.** Fluorescent and pseudo-color images of the paint droplet.

**Figure 6.** Coating thickness on a red line in Figure 5.

Figure 7 shows fluorescent and pseudo-color images of the droplet on the formed coating surface. Other conditions are the same as in Figure 5. Figure 8 represents the time change in the cross-sectional thickness of the droplet. It was confirmed that both ends of the droplet show a high value when the droplet adheres to the coating at *T*<sup>D</sup> = 1/90 s. However, the center part of the droplet did not become thick after adhesion. The spike-shaped part seen in the *T*<sup>D</sup> = 3/90 s data indicated the presence of air bubbles. The coating thickness before (*T*<sup>D</sup> = 0/90 s) and after (*T*<sup>D</sup> = 6/90 s) the droplet adhesion was compared. The coating thickness did not change in the range where the droplet adhered (70 < *X* < 170). However, the coating thickness increased in the range where the droplet did not adhere (1 < *X* < 70, 170 < *X* < 205). This result shows that the adherent droplet spread around and integrated with the coating. Using the fluorescence method, it is possible to analyze the change in the coating thickness in a short time (within minutes) by examining the fluorescence intensity distribution in the cross-section.

**Figure 7.** Fluorescent and pseudo-color images of the paint droplet.

**Figure 8.** Coating thickness on a red line in Figure 7.
