# Influences of Brass Surface Morphology on Leidenfrost Effect during Liquid Nitrogen Cooling
## 5 Conclusions  
In this study, the Leidenfrost effect was studied by cooling brass samples in liquid nitrogen from room temperature. The surface of the brass samples was polished by using different sandpapers in prior; thus, different surface morphologies with texture were prepared with roughness \(Sq\) and fractal dimension \(D_{\text{RSE}}\), which were highly negative-correlated. An apparatus for boiling experiment was setup to record the cooling curves, which was converted to the boiling curves with the lumped parameter method, and the SEM and WLI were used to observe the surface morphologies and obtain the height matrix to calculate the \(Sq\) and \(D_{\text{RSE}}\) of the surface, respectively. The experimental parameters of the Leidenfrost effect (\(\Delta T_{\text{LP}}\) and \(\Delta T_{\text{CHF}}\)) were obtained. To further illustrate the scale effect of the \(Sq\) on the \(\Delta T_{\text{LP}}\), multi-scaled analysis was performed. The findings of this study could be instructive for the community of HTS wires, whose protecting layer is usually brass or copper, because the \(\Delta T_{\text{LP}}\) increasing could help to suppress the quench propagation during the HTS device operation. The conclusions obtained are as follows:  
(1) It was found that the surface morphology of brass could influence \(\Delta T_{\text{LP}}\), but neither \(Sq\) nor \(D_{\text{RSE}}\) had a clear correlation with \(\Delta T_{\text{LP}}\). With \(Sq\) increasing from 0.192 \(\upmu\)m to 1.961 \(\upmu\)m, i.e., \(D_{\text{RSE}}\) decreasing from 2.664 to 2.287, \(\Delta T_{\text{LP}}\) could increase from 102.74 K to 107.58 K with a significant fluctuation. Besides, for \(\Delta T_{\text{CHF}}\) ranging within the scope of 94.1-104.3 K, the \(Sq\) or \(D_{\text{RSE}}\) of brass samples had no clear influence. (2) The measurement scale was from 3.9 \(\upmu\)m to 491.4 \(\upmu\)m, and the influence of \(Sq\) with the measurement scales was much more obvious than the \(D_{\text{RSE}}\). The multi-scaled property of \(Sq\) showed an influence on the Leidenfrost effect and showed that the utilization of smaller measurement scale (\(L\)) could reveal a closer relationship between \(Sq\) and \(\Delta T_{\text{LP}}\), which might be attributed to the scale of microcavities where vapor bubbles can nucleate, and could be promising to obtain more understanding beyond the current one of film boiling and the Leidenfrost effect. Besides, a higher correlationly between \(\Delta T_{\text{LP}}\) and roughness smaller \(L\) was found. The RSE algorithm used in this study could be applied to the characterization of more engineering surfaces, and the experimental apparatus and results could provide reference for other low-temperature experiments. Data curation, Z.L. and F.F. ; Funding acquisition, P.F. and F.F. ; Investigation, Z.L., J.C. and F.F. ; Project administration, D.Y., P.F. and F.F. ; Visualization, Z.L., J.C. and F.F. ; Writing--original draft, Z.L. ; Writing--review and editing, D.Y., J.C., P.F. and F.F. All authors have read and agreed to the published version of the manuscript. **Funding:** This study was supported by National Natural Science Foundation of China under Grant No. 51875311, Guangdong Basic and Applied Basic Research Foundation under Grant No. 2020A1515011199, and Shenzhen Foundational Research Project under Grant No. WDZC20200817152115001. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The data presented in this study are available on request from the corresponding author. **Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.