# Surface Tracking of MgO/Epoxy Nanocomposites: Effect of Surface Hydrophobicity
## 3 Results and Discussions
However, it should be marked that this conclusion was derived from a single factor comparison, while the tracking resistance was related to a variety of factors. Further research is necessary for understanding the process of surface tracking. Different morphologies of water droplets were found in different ablated regions, after the tracking process. Pictures of the water droplets, taking a sample with a doped 1 wt% nano-particles, as an example, in Region B and Region C, are shown in Figure 5. In Region B, the droplets showed an irregular strip shape, with significant difference in the front and side morphology. In Region C, however, the shape of the droplet was a regular circular shape, i.e., the front and side profiles of the droplet were almost the same. This phenomenon arose from the different morphologies in those two regions. During the tracking process, the dendritic ablation region (Region C) was relatively uniform, while the ablation area between electrodes (Region B) was a long 'tank'. The 'tank' distribution of Region B finally resulted in irregular shapes of the water droplets. Figure 4: Comparison of surface contact angle and CTI of the nanocomposites. Figure 5: Pictures of water droplets on the surface of the epoxy sample. Front view (**a**) and side view (**b**) of the droplet, after the tracking process in Region B; front view (**c**), and side view (**d**) of the droplet after tracking process in Region C.  
To further study the effect of SCA on the nanocomposites, the samples were subjected to a tracking test with a number of experimental droplets--5, 10, 15, 20, 25, 35, 45, and 50--controlled separately. Then the SCA measurement of each sample was carried out to investigate the hydrophobicity of the epoxy nanocomposites, during the tracking process. When the surface of the specimen was carbonized, the water droplets will be directly applied to the surface of the specimen, resulting in an inability to measure the SCA. Therefore, the surface contamination was wiped before the measurement. Considering the difference of Region B and Region C in the tracking process, dependences of the SCA on the number of droplets that were remaining, in both Region B and Region C, are presented in Figure 6. As can be seen from Figure 6b, the front contact angle of the droplet was greater than the side contact angle in Region B, during the tracking process, while the two contact angles were almost equal in Region C, in Figure 6c. This result was consistent with the observations in Figure 5. The common phenomenon of the two ablation regions was that the contact angle decreased, gradually, with the development of the tracking process, which indicated that the hydrophobicity of the sample gradually decreased. The decreasing of hydrophobicity resulted in the increase of the water film area, as well as the contaminant area of the specimen surface. It is worth noting that fifteen droplets might be a crucial number for the tracking process. Ablation area slowly increased in Figure 6a, while SCA decreased rapidly in Figure 6b,c, by the time the fifteenth droplet was administered. When the number of droplets administered was over fifteen, the ablation area increased rapidly, while the SCA gradually tended to be constant. This could be because losing the hydrophobicity led to a more severe ablation and s faster approach to the failure stage, as shown in Figure 6a. Surface resistivity of the nanocomposite samples is plotted in Figure 7. It decreased rapidly and then slowly increased, with increasing nano-MgO particles. Lowered surface resistivity was detected in the nanocomposite samples, on the whole, which is beneficial for reducing the accumulation of surface charges and distortion of the electric field. In addition, great effects of surface tracking on surface resistivity was also observed. The resistivity was about one magnitude lower, after the surface tracking process. This is predictable, as a result of formation of conductive pathways, during the surface tracking measurement.