## 1 Introduction  
Spray cooling has the advantages of strong heat dissipation, low required flow rate, and high heat transfer coefficient [1; 2; 3]. It has been widely applied in the heat dissipation of photovoltaic cells [4], high-power electronic chips [5; 6], laser treatment for port wine stains [7; 8], and so on. It is relatively difficult to observe the micro-dynamic behavior and study the thermal and kinetic mechanisms during spray impingement because of the complex interactions between numerous droplets with cooling surfaces and between droplets themselves. Therefore, researchers pay more attention to the impact behavior of a single droplet. On the one hand, single droplet behavior, including impinging dynamics and heat and mass transfer, is one of the most important processes during spray impingement. On the other hand, single droplet impingement simplifies the process of investigating the fundamentals of spray cooling in a controllable system and offers the ability to investigate the influence of different parameters in isolation. Early in 1876, [PERSON] et al. [9] studied the morphology of water and mercury droplets impacting a solid surface or liquid with a certain depth, initiating the study of droplet impingement. Scholars have carried out numerous studies on morphology evolution and the heat and mass transfer process of droplet impingement. In the past 20 years, with the development of high-speed photography technology, the observation of droplet impact has made further progress. When droplets impact solid walls, the outcomes are spreading, splash, rebound, partial rebound, deposition, retreat (recoil), ejection, and fingering [10]. Identifying different outcomes and developing corresponding transition criteria are important [11]. These behaviors are dependent on the characteristics of the droplet, surface, and ambient conditions. Some researchers have investigated droplet properties characterized by diameters, viscosity, surface tension, density, velocity, and temperature. [PERSON] et al. [12] numerically investigated the impingement of heated water droplets at atmospheric pressure and suggested warm droplets spread more violently than cold droplets due to larger Re. [PERSON] et al. [13] investigated the effect of impact velocity and concluded that larger velocity resulted in larger spreading and amplitude of oscillation. [PERSON] et al. [14] noted that few studies investigated low-viscosity droplets, while most studies explored droplet impingement using water and glycerin. An empirical model for predicting the maximum spreading diameter was proposed by analyzing the impingement of gasoline, isooctane, and ethanol droplets. [PERSON] et al. [15] systematically investigated the effects of droplet diameter, impact velocity, viscosity, and surface tension on droplet impact dynamics using R113, deionized water, ethanol, and acetone. Large viscosity and surface tension were found to hinder droplet spreading, whereas the initial droplet diameter minimally affected the spreading factor. Moreover, [PERSON] et al. [16] found that reduction of the ambient pressure could suppress the corona splash by investigating methanol, ethanol, and 2-propanol impingements. As for low-boiling refrigerants commonly used in cryogen spray cooling, they show obviously different characteristics, such as lower viscosity, lower surface tension, and high volatility. However, the impingement dynamics of a single droplet with a low boiling point and high volatility are rarely reported. Surface conditions are characterized by temperature, roughness, wettability (i.e., hydrophilic or hydrophobic), and thermal conductivity. [PERSON] et al. [17] proposed that roughness was an important factor affecting the morphology of water droplet impingement on solid walls. Therefore, it is of great significance to study the influence of roughness on the dynamic behavior of droplet impingement. [PERSON] et al.