# Experimental Study on the Surface Properties of Nanoalumina-Filled Epoxy Resin Nanocomposites
## 3 Results and Discussion
### Surface Roughness  
The surface roughness of the EPNCs was studied through AFM. The arithmetic average roughness \(R_{a}\) and the root mean square roughness \(R_{q}\) were computed from multiple AFM topographical images for areas of \(10\times 10\)\(\mu\)m\({}^{2}\) at different locations of the EPNC surface. The obtained results were averaged and the results are presented in Table 1. From this table, the trends observed for both amplitude parameters (\(R_{a}\) and \(R_{q}\)) are very similar. For this reason, in Figure 4 only the results related with \(R_{a}\) are presented graphically. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample** & _w/lev\%_ & \(R_{a}\)**nm** & **St.Dev. nm** & **Coef. var.\%** & \(R_{q}\)**nm** & **St.Dev. nm** & **Coef. var. \%** \\ \hline DDM\_0 & 0 & 197.6 & 26.8 & 13.6 & 267.7 & 59.0 & 22.0 \\ \hline DDM\_1 & 1 & 202.9 & 5.4 & 2.7 & 243.7 & 5.2 & 2.1 \\ \hline DDM\_3 & 3 & 68.7 & 12.5 & 18.2 & 95.1 & 16.8 & 17.7 \\ \hline DDM\_5 & 5 & 233.6 & 16.3 & 7.0 & 289.8 & 40.4 & 13.9 \\ \hline DDSA+MNA\_0 & 0 & 331.6 & 56.7 & 17.1 & 487.1 & 120.8 & 24.8 \\ \hline DDSA+MNA\_1 & 1 & 256.2 & 55.7 & 21.7 & 324.3 & 68.8 & 21.2 \\ \hline DDSA+MNA\_3 & 3 & 17.4 & 2.7 & 15.5 & 21.6 & 3.6 & 16.7 \\ \hline DDSA+MNA\_5 & 5 & 136.0 & 16.9 & 12.4 & 186.5 & 22.4 & 12.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Roughness of EPNC surfaces. From Table 1 and Figure 4, it is possible to observe that for both hardeners (DDM and DDSA+MNA), a general decrease of the surface roughness (\(R_{a}\)) is observed until 3 wt. (%) is reached, following by an increase for 5 wt. (%). In the case of DDM hardener, the higher value for the surface roughness is obtained for 5 wt. %, while for the case of DDSA+MNA hardener the higher value is obtained for the CS. From Table 1, it is observed that EPNCs with 3 wt. (%) show decreases in \(R_{a}\) (\(R_{q}\)) of around 65% and 64% (95% and 96%) when compared with the CS with DDM and DDSA+MNA, respectively. According to Figure 4, the EPNC surface that shows the highest average surface roughness is the CS; for both DDM and DDSA+MNA, the is the highest average surface roughness values are for super-hydrophobic and hydrophilic EPNCs, respectively. For DDM, the highest average surface roughness was for the surface corresponding to the EPNC with 5 wt. (%) NPs, which was hydrophilic; and for EPNC this corresponded to the 1 wt. (%) NP for the case of DDSA+MNA, which was also hydrophilic. For the DDM hardener, the results are in agreement with [PERSON] and [PERSON] [41], who stated that for smooth hydrophilic surfaces, the wettability is improved by roughening them, while for smooth hydrophobic surfaces the opposite is observed. In addition, CA will increase by roughening the surfaces. However, in this study and for EPNCs with DDSA+MNA, the opposite effect is observed. In fact, considering the hydrophobic surfaces for EPNCs with 3 and 5 wt. (%) NPs, the contact angle decreases with the roughness. However, [PERSON] and [PERSON] [41] studied films, which could explain why the behavior is different. According to the author's knowledge, there are no studies concerning the study of CA and roughness for nanoalumina-filled epoxy resin nanocomposites. Figures 5 and 6 present representative 2D and 3D AFM images for EPNCs cured with DDM and DDSA+MNA, respectively. From Figures 5 and 6, it is possible to observe some variability of the surface roughness of the studied EPNCs on the selected regions. However, the influence in the roughness results is not very high, since the coefficients of variation presented in Table 1 are generally below 20%. Figure 4: Arithmetic average roughness of EPNCs using DDM and DDSA+MNA as hardeners. Figure 7 illustrates the comparison between the roughness (\(R_{a}\)) and the CA (\(\theta^{*}\)). From the figure, it is possible to conclude that in the case of EPNCs with DDM hardener, the wettability is improved for the more hydrophilic surfaces with the increase in roughness.