# The Impact of Single- and Multicomponent Liquid Drops on a Heated Wall: Child Droplets
## 3 Results and Discussion
### Effect of a Group of the Governing Parameters
#### 3.2.4 Surface Roughness and Hydrophilicity  
Figures 7 and 8 demonstrate that surface roughness and hydrophilicity may considerably affect not only the threshold drop velocity and size but also the number and size of child droplets. In contrast to the previous governing factor (Figure 6), Figures 7 and 8 show that the \(S_{1}\)/\(S_{0}\) ratio and the number of child droplets could increase by 2-6 times.  
The experimental results in Figure 7c gave grounds for other conclusions. The functions \(N(r_{\rm d})\) demonstrated that the surface roughness noticeably affected the number of child droplets when parent drops were small. When parent drops were comparable in size with the roughness cell, the number and size of child droplets were the same as for the impact on a smooth surface. If the cross dimension of the drop was smaller than size of the roughness cell, then the inner of the roughness cell did not affect the drop behavior. When impinging on a roughness cell, the drop broke up in the same way as during the impact on a smooth surface. In other cases, roughness did affect the characteristics of  
Figure 8: (**a**) Regime map of the drop impact on the wall (circle—deposition of the first drop and coalescence with all others, square—breakup) as a function of We(Oh): 1,2—copper surface; 3,4—surface covered by paraffin; 5,6—surface covered by oil; (**b**) \(S_{1}\)/\(S_{0}\)(We); (**c**) size distribution of child droplets at We = 200 and at different wetting properties of a surface: 1—copper surface; 2—surface covered by paraffin; 3—surface covered by oil.  
Figure 7: (**a**) Regime map of the drop impact on the wall (circle—deposition of the first drop and coalescence with all others, square—breakup) as a function of We(Oh): 1, 2—\(R_{\rm a}\) = 0.01; 0.008 μm; \(R_{\rm z}\) = 0.05; 0.04; 0.032 μm; 3, 4—\(R_{\rm a}\) = 2.5; 1.25 μm; \(R_{\rm z}\) = 10; 8; 6.3 μm; 5, 6—\(R_{\rm a}\) = 20; 16;10 μm; \(R_{\rm z}\) = 80; 63; 50; 40 μm; (**b**) \(S_{1}\)/\(S_{0}\)(We); (**c**) size distribution of child droplets at different surface roughness: 1—\(R_{\rm a}\) = 0.01; 0.008 μm; \(R_{\rm z}\) = 0.05; 0.04; 0.032 μm; 2—\(R_{\rm a}\) = 2.5; 1.25 μm; \(R_{\rm z}\) = 10; 8; 6.3 μm; 3—\(R_{\rm a}\) = 20; 16;10 μm; \(R_{\rm z}\) = 80; 63; 50; 40 μm; 4—smooth surface.  
child droplets. If the parent drop partially falls into a roughness cell and partially on the edge of a roughness element, the drop interacts with the surface in the same way as with a metal mesh.  
According to Figure 8, the higher the hydrophobicity of the wall surface, the more intensely the parent drop broke up. For the hydrophilic surfaces, even heated up to high temperatures, rebound or deposition occurred. As for hydrophobicity, it could be provided in different ways. [PERSON] et al. [21] studied how the chemical treatment of a metal surface affects the behavior of impinging drops. During the impact on a hydrophobic etched surface, a drop splashed and a corona was formed. [PERSON] et al. [22] demonstrated a 10 % increase in the maximum spreading diameter of a drop over a super-hydrophilic surface heated up to 300 \({}^{\circ}\)C, i.e., from 30% to 40% relative to the initial diameter of a drop, as compared with the case of the drop impact on an unheated surface. During the experiments with a large group of drops impinging on such surfaces, we established some patterns. In particular, it is more efficient to enlarge the contact area of drops with such surfaces in order to increase \(S_{1}/S_{0}\). However, to enlarge a number of small child droplets, We should be increased.