# Direct Fabrication of Ultrahydrophobic Laser-Induced Graphene for Strain Sensors
## 3 Results and Discussion
### Superhydrophobic Property  
In addition to the good strain sensing performance of the fabricated LIG, the laser parameter for the strain sensor was primarily selected to result in the ultrahydrophobic property of the fabricated LIG sensor, here termed SH-LIG. Figure 5a shows a 6 \(\upmu\)L water droplet on the fabricated superhydrophobic surface. The droplet maintains a high contact angle (165\({}^{\circ}\)) and low roll-off (5\({}^{\circ}\)) from the surface, an indication of an extreme superhydrophobic surface. Additionally, as demonstrated in Figure 5b and the video in the supplementary information, the impinging water droplets are totally repelled from the surface as a result of minimal surface interaction, causing the water droplets to rebound and exit the surface. It is worth mentioning that the contact angle values obtained in the single-step laser processing in this study stand better than previous studies on superhydrophobic LIG fabrication with water contact angles closer to 155\({}^{\circ}\). Figure 4: (**a**) Relative resistance change vs applied flexural strain, (**b**) Three-point bend test setup during measurement, (**c**) Strain-gauge connected to the multimeter, (**d**) Close view of the LIG sensor circuit. Figure 5: (**a**) 6 \(\upmu\)L water droplet above SH-LIG surface on a glass substrate, (**b**) Water jet bouncing off the SH-LIG. The laser ablation of the polyimide tape generates the LIG, which is directly deposited over the substrate itself. The unmodified polyimide tape can then be peeled off, rendering the pristine substrate with the specific laser patterned SH-LIG. The SH-LIG can be generated on different substrates as shown in Figure 6. Topographical surface analysis was performed over the SH-LIG on different substrates to determine the surface roughness, which is an important parameter to define superhydrophobic surfaces. Table 1 shows the measured average surface roughness (Sa) of the SH-LIG region over the substrates. Noticeably, the average surface roughness of SH-LIG modification varies drastically between the different substrates. This effect can be attributed to the difference in the material properties of the substrates. Although the average surface roughness varies noticeably, all the samples exhibited excellent superhydrophobic property with static contact angles above 160\({}^{\circ}\) and roll-off angles below 5\({}^{\circ}\) for all the samples. Therefore, in terms of surface morphology, eventhough the microtexture varies on different samples, the excellent superhydrophobicity can be considered a result of the hierarchical micro/nano surface feature of LIG. The laser-modified micro-scale roughness along with the porous and fibrous nano-scale complex graphene oxide network provides an excellent surface feature to facilitate ultrahydrophobicity. Apart from the topographical analysis, the surface elemental compositional analysis was performed using energy dispersive X-ray spectroscopy. EDS analysis was performed on the glass, aluminum, and silicon substrates. Table 2 shows the quantitative information of the SH-LIG on different substrates. The corresponding EDS spectra are shown in Figure S1. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Substrate** & **Roughness [Sa (\(\mu\)m)]** \\ \hline Al 6061 & 6.22 \\ \hline Silicon & 1.81 \\ \hline Glass & 4.58 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Profilometer analysis of average surface roughness of SH-LIG on various substrates. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & & **Atomic \%** & \\ \hline
**Element** & **Al 6061** & **Silicon** & **Glass** \\ \hline Al & 40.5 & - & - \\ \hline Si & - & 69.2 & 18.7 \\ \hline C & 28.5 & 23.3 & 45.9 \\ \hline O & 29.1 & 7.4 & 35.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: EDS composition information of different SH-LIG substrates. Figure 6: SH-LIG fabricated on different substrates in a one-step laser process. The EDS analysis confirms the presence of carbon as a major elemental component in all the samples.