Consequently, the difference in the evaporation rates of water droplets from St\({}_{12.6}\) and St\({}_{15.3}\), as  
Figure 6: Specific evaporation rate versus evaporation time of water droplets from copper (**a**) and steel (**b**) surfaces heated to 80 \({}^{\circ}\)C at an ambient temperature of 23 \({}^{\circ}\)C.  
well as St\({}_{25.8}\) and St\({}_{46.2}\) (Figure 6b), can only be due to the influence of an increase in the surface area due to an increased roughness. According to the analysis of Figure 7, we have evaluated the effect of an increase in the surface area on an increase in the total evaporation rate of a water droplet (\(W_{\text{total}}\)) on St\({}_{12.6}\) and St\({}_{15.3}\), as well as St\({}_{25.8}\) and St\({}_{46.2}\) surfaces. It can be seen from Figure 7 that an increase in the surface area of 0.1% leads to an increase in the total evaporation rate of the droplet of 6.5% on the St\({}_{15.3}\) surface, and of 4% on the St\({}_{46.2}\) surface. This indicates that an increase in the surface area has a significant effect on the droplet evaporation rate, but this effect decreases with an increasing surface roughness. The results obtained should be used with caution when extrapolating too large surface area increments. It was found that the specific evaporation rate of a 10 \(\upmu\)L water droplet on steel surfaces is higher than that on copper surfaces (Figure 6). The result obtained is explained by the fact that the average mass heat capacity of steel (460 kJ/(kg\({}^{\circ}\)C)) is greater than that of copper (385 kJ/(kg\({}^{\circ}\)C)). The copper and steel surfaces had identical sizes; therefore, the volume of the samples was identical. Despite the fact that the mass of the copper sample was 14% greater than that of the steel sample, under identical conditions, where the surface heated to a temperature of 80 \({}^{\circ}\)C, the steel sample accumulated 4% more heat. Consequently, in the experiments performed, the specific heat flow from the steel samples to the droplet was higher than on copper samples. For these reasons, the heating of the droplet on the steel sample to the phase transition temperature was quicker, and the evaporation rate was higher. The effect of the higher thermal conductivity of copper compared to that of steel did not significantly affect the droplet evaporation rate due to the large size of the samples. A 10 \(\upmu\)L water droplet with a temperature of 23 \({}^{\circ}\)C dosed onto copper and steel samples heated to 80 \({}^{\circ}\)C did not significantly affect the cooling of the samples. According to the data recorded by thermocouples, it was established that the temperature on the sample surface and under the sample did not change after dosing the droplet. It should be noted that, with a decrease in the size of the samples, including a significant decrease in thickness, the phase transition of a water droplet placed on copper and steel surfaces can reduce their temperature. Under such conditions, in practice, when the surfaces of heat-loaded equipment sections are cooled with water droplets, due to the constant heat supply from the  
Figure 7: Total evaporation rate of water droplets versus Sdr parameter (increment of surface area due to increased roughness) for St\({}_{12.6}\), St\({}_{15.3}\), St\({}_{25.8}\), and St\({}_{46.2}\) surfaces. heating source and the high thermal conductivity of copper, the droplet evaporation rate on copper surfaces will be higher than on steel surfaces. Therefore, in practice, in order to increase the cooling intensity of heat-loaded equipment by spray cooling systems operating in the evaporation (single-phase) regime, it is necessary to use low-mass heat transfer surfaces with a small thickness. However, in practice, the use of heat transfer surfaces with a small thickness causes an increase in the risk of their thermal destruction due to overheating.