# The Impact of Single- and Multicomponent Liquid Drops on a Heated Wall: Child Droplets
## 3 Results and Discussion  
### Patterns, Observations, and Outcomes of Drop Impact on a Wall  
We established that a critical velocity, at which the first drop breaks up after impact, equaled 2.5 m/s (We \(\approx 206\)) for the copper substrate and 2.4 m/s (We \(\approx 380\)) for the aluminum one. If We \(<100\), the second drop did not break up because it coalesced with the first one or rather with the liquid film on the surface. A wave structure on the liquid film surface did not appear in this case (Figure 2a). With an increase in the Weber number up to 100-150 in the case of the second drop impact on the liquid film of the first drop, the waves emerged but there was no splashing. A further increase in the Weber number caused the resulting film to splash (Figure 2b). Figure 1: Diagram of the drop impingement and the main recorded and calculated parameters. \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Systematic Error [\%]** & **Random Error [\%]** \\ \hline \(R_{\rm d}\) & 1.6 & 2.1 \\ \(U_{\rm d}\) & 2.1 & 3.4 \\ \(\alpha_{\rm d}\) & 2.3 & 3.1 \\ \hline \end{tabular}
\end{table} 
Table 3: Systematic and random errors in determining the parameters. The tests with the copper substrate showed a noticeable increase in the number of child droplets, from about 20 to 30, when changing the impinging angle from about 30\({}^{\circ}\) to 60\({}^{\circ}\) (Figure 2c). If the angle is reduced to 30\({}^{\circ}\), the number of the child droplets will go down to about 8-10 (Figure 2d). Having studied the effect of the thermal factor on the impact outcomes, we found out that when the copper substrate was heated up to 50 \({}^{\circ}\)C, the drops first spread over the surface. Then, larger child droplets are produced at the solid-liquid interface, and smaller ones (\(r_{\rm d}\approx 0.01\) mm) emerged at the liquid-gas interface (Figure 2e). The number of child droplets was vast and could reach several dozen. As for rebound, the parent drops were strongly deformed. Only 3-4 fragments with \(r_{\rm d}\approx 0.05\) mm were detached in this case. With an increase in the wall temperature up to 100 \({}^{\circ}\)C, the number of child droplets rose (Figure 2f). A further temperature growth up to 180 \({}^{\circ}\)C resulted in mostly large fragments (\(r_{\rm d}\approx 0.1\) mm) detaching from the parent droplet. Their number approximated 25-40. In general, the drops rebounded from a surface without scattering to the sides, i.e. the spreading angle was considerably limited. In this case, almost no splashing of small child droplets was observed (Figure 2g). When studying the effect of roughness on outcomes of the impact of the first drop on a surface, we used an aluminum substrate with miscellaneous grades of finish. At the Weber number of We \(\approx 380\), breakup was the most frequent outcome of the impact of the first drop on a rough surface (Figure 2h). In the case of a smooth surface in the identical impact conditions, the first drop spread over the surface, and each successive one interacted with the resulting liquid film. Upon impinging on a wall covered by transformer oil, all successively falling drops were observed to coalesce (Figure 2i). The emulsion film formed on the surface caused all the falling droplets to merge into a monolayer. This stems from the fact that the transformer oil enhances the surface tension and interfacial interaction in the liquid. During the impact on the surface covered by paraffin, only very small particles and several larger fragments emerged (Figure 2j). The main part of the drop broke up at the surface and spread over it as separate fragments. This phenomenon could be explained by the high hydrophobicity of paraffin. Figure 2: _Cont._  
During the impact of emulsion drops containing 30 vol.% of transformer oil and 70 vol.% of water, we recorded the phenomena similar to those observed during the impact of water drops on a wall covered by transformer oil (Figure 2k).