# The Impact of Single- and Multicomponent Liquid Drops on a Heated Wall: Child Droplets
## 2 Experimental Procedures  
To study the drop impact on a surface, we used the experimental setup involving a group of nozzles for producing the drops of fixed dimensions (\(R_{\text{d}}\)). During the experiments, a liquid immersible pump generated the drops having a certain velocity (\(U_{\text{d}}\)). After a drop was produced, it moved to the center of a substrate, i.e., solid surface. The surface was represented by plates made of copper and steel with different roughness grades, namely, grade 14--\(R_{\text{a}}\) = 0.01; 0.008 \(\upmu\)m; \(R_{\text{z}}\) = 0.05; 0.04; 0.032 \(\upmu\)m; grade 6--\(R_{\text{a}}\) = 2.5; 1.25 \(\upmu\)m; \(R_{\text{z}}\) = 10; 8; 6.3 \(\upmu\)m; grade 3--\(R_{\text{a}}\) = 20; 16; 10 \(\upmu\)m; \(R_{\text{z}}\) = 80; 63; 50; 40 \(\upmu\)m. In addition, we used a glass plate and a metal mesh with a mesh size of 1 \(\times\) 1 mm. The experiments involved drops of various compositions (Table 1). This will enable specialists to use the findings for improving such technologies as fuel spraying, thermal treatment of liquids, contact heat exchangers, and others. We chose the liquids with substantially different properties (Table 1) to cover more applications. Water is the main component of power fluids in heat-exchange technologies. Therefore, all liquids in the study contained this component. We tested water as a single-component liquid, and the slurry and two emulsions as multicomponent ones. The viscosity was measured usinga Brookfield DV3 TLV viscometer (AMETEK Brookfield, Middleboro, MA, USA) [11]. The surface tension was measured using the [PERSON] ring method [12]. In addition, the effect of surface hydrophobicity on the impact outcomes was addressed. To do that, the copper plate was covered with a paraffin or oil film. Also, we managed to account for the effect of surface temperature on the drop breakup. In this case, the copper plate was heated up to 50 \({}^{\circ}\)C, 100 \({}^{\circ}\)C, and 180 \({}^{\circ}\)C. A Testo 835-T2 pyrometer (Testo Industrial Services GmbH, Kirchzarten, Germany) with an accuracy of \(\pm\) 0.1 \({}^{\circ}\)C monitored the heating temperature. A Fastcam mini UX high-speed camera (Photron USA, Inc. San Diego, CA, USA) with a resolution of 1280 \(\times\) 1024 px and a sample rate of 4000 frames per second recorded drops impinging on a wall. By using the Photron Fastcam Viewer software 4.0.3.1 (Photron USA, Inc. San Diego, CA, USA), we managed to determine the velocity (\(U_{\text{d}}\)), size (\(R_{\text{d}}\)), and impinging angle (\(\alpha_{\text{d}}\)) (Table 2). Table 3 lists the errors in determining these parameters. To vary the drop velocity \(U_{\text{d}}\) before impinging on the surface, the altered the distance between the pump and substrate. In particular, a linear module moved the pump vertically above the substrate. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Liquid (Volume or Mass Concentrations of the Admixtures to Water)** & **Temperature** & **Density** & **Surface Tension** & **Dynamic Viscosity** \\
**of the Admixtures to Water)** & [\({}^{\circ}\)C] & **[kg/m\({}^{3}\)]** & **[N/m]** & **[P*s]** \\ \hline Water & 20 & 998 & 0.0726 & 0.0014 \\ Coal slurry (30 wt\%) & 20 & 1126 & 0.2472 & 0.0033 \\ Transformer oil emulsion (30 vol\%) & 20 & 963 & 0.0567 & 0.0069 \\ Castor oil emulsion (30 vol\%) & 20 & 987 & 0.0615 & 0.2857 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Properties of the liquids under study.