## 2 Experimental System and Measurement Methods  
### Experimental System  
As shown in Figure 1, the experimental system was set on a level optical platform to guarantee vertical impingement. It includes a droplet generating system, a one-dimensional positioner, an image acquisition system, and solid surfaces. Among them, the droplet generating system includes an insulated tank with thermostatic water bath circulation and a detachable nozzle with an inner diameter of 2 mm. The temperature in the tank is monitored in real-time by thermocouples and a data acquisition system. The insulated tank moves in the vertical direction to change the impinging height of the droplet in free fall and droplet impingement velocity. The adjustment range of height is 0-50 cm. In the experiment, the backlight photography method was adopted, and the data were collected by an image acquisition system, including a background light source (LED), photoelectric sensor, computer, and a high-speed camera (FASTCAM SA-Z, Photron, Tokyo, Japan) with a shooting rate of 20,000 fps and a resolution of 1024 \(\times\) 1024. In order to preserve the R1336 mzz(Z) in liquid and control the experimental variables as much as possible, the temperature of the water bath was controlled at 0 \({}^{\circ}\)C. The temperature of R1336 mzz(Z) was maintained at 5 \(\pm\) 1 \({}^{\circ}\)C accordingly, considering the heat dissipation of the tank and pipe. The properties of R1336 mzz(Z) at 5 \({}^{\circ}\)C are listed in Table 1. The experiment was conducted at room temperature (20 \(\pm\) 3 \({}^{\circ}\)C) and atmospheric pressure. The experimental procedures are as follows. First, the refrigerating cycle is turned on to reduce the R1336 mzz(Z) liquid temperature within the insulated tank to 5 \({}^{\circ}\)C. Afterward, a single droplet can be generated by gravity with no initial velocity once the nozzle valve is opened slowly. When the droplet freely falls to the area near the solid surface, it triggers the photoelectric sensor installed near the surface, and the signal from the photoelectric sensor triggers the high-speed camera to record the impacting images of the droplet. Each experiment was repeated twice to ensure repeatability. To explore the influence of surface characteristics on droplet impingement, four standard reference specimens with different roughnesses made of carbon steel 45, namely groove textured surface, were selected for the experiment. The parameter \(Ra\) well represents the surface roughness, which is 0.8 \(\upmu\)m, 1.6 \(\upmu\)m, 3.2 \(\upmu\)m, and 6.3 \(\upmu\)m. The selection of roughness and droplet size in the experiment referenced the work done by [PERSON] et al. [18]. Notably, nozzles were used to generate droplets. For the normal flat nozzle, droplets are more likely to attach to the plane surface before falling. This leads to a series of small droplets or liquid columns when the droplets fall, and the diameter of the generated droplets diverges greatly. In the process of experiments and observation, this paper changed the flat nozzle into a conical nozzle with an inner diameter of 2 mm, as shown in Figure 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Properties** & **R1336 mzz(Z)** \\ \hline Surface tension \(\sigma\) (mN m\({}^{-1}\)) [32] & 14.35 \\ Density \(\rho\) (kg m\({}^{-3}\)) [32] & 1428.07 \\ Viscosity \(\mu\) (ukg m\({}^{-1}\) s\({}^{-1}\)) [33] & 493.04 \\ Boiling point (\({}^{\circ}\)C) [31] & 33 \\ ODP [31] & 0 \\ GWP [31] & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Properties of R1336 mzz(Z) (5 \({}^{\circ}\)C,1 atm). Figure 1: Schematic of the experimental system. The phenomenon of accompanied droplets was obviously improved. In addition, the divergence of the droplet diameter was apparently reduced, as shown in the picture.