# Direct Fabrication of Ultrahydrophobic Laser-Induced Graphene for Strain Sensors
## 3 Results and Discussion
### Superhydrophobic Property
For all the samples, the pristine samples had a carbon content of below 8%. By nature, carbonaceous materials are hydrophobic. The hydrophobic surface chemistry in combination with a hierarchical surface texture renders the surface ultrahydrophobic. Further, to analyze the surface functional groups responsible for the superhydrophobic nature, FTIR spectroscopic analysis was performed. Therefore, the SH-LIG is substrate-independent as demonstrated in Figure 6.  
Interestingly, the fabricated SH-LIG was substrate-independent. Figure 6 shows the water droplets with high contact angles on SH-LIG on various substrates. In all the cases, the contact angle was greater than 160\({}^{\circ}\) and the roll-off was below 5\({}^{\circ}\). FTIR spectroscopy was performed on the fabricated samples to analyze the surface chemical composition of the SH-LIG surface and understand its origin. To compare the effect of laser parameters on the resulting chemical composition of the SH-LIG, another laser-processed sample that used high average laser power (16 W) was prepared. Figure 7 represents the FTIR spectrum of both the samples mentioned above. The FTIR spectrum collected from the SH-LIG shows an absorbance peak at 1290 cm\({}^{-1}\) which is attributed to the presence of C-O bonds in alcohol or C-O-C functional group [51]. The absorption at \(\sim\)1630 cm\({}^{-1}\) corresponds to the C=C stretching in the aromatic rings [52]. Additionally, the absorption at 1690 cm\({}^{-1}\) is associated with the typical C=O stretch of the carbonyl group [53]. Further, the absorption at \(\sim\)2880 and 2960 cm\({}^{-1}\) corresponds to the stretching of CH\({}_{2}\) and CH\({}_{3}\) groups, respectively [54]. In comparison, the FTIR spectra of the sample modified at high average power do not show the presence of any significant peak at corresponding wavenumbers. This could be a result of the strong photothermal effect at high temperatures leading to strong oxidation and thus superhydrophilicity of the fabricated LIG [55]. An important aspect of superhydrophobic surface functionality lies in the stability of the property, especially in extreme environments. Therefore, SH-LIG was fabricated on both glass and composite materials to test their stability during immersion under water for 25 h. In the case of the composite material, an identical line circuit pattern with silver paste contacts, similar to the one used in the three-point bend test, was prepared to test the resistance change with immersion time. However, an 8 mm \(\times\) 8 mm area of SH-LIG was generated on a glass substrate to measure the variation in water contact angle with immersion time. As shown in Figure 8, the dynamic contact angle measurements demon  
Figure 7: Comparison of FTIR spectra of superhydrophobic LIG and superhydrophilic laser modifications. strate the stability of the SH-LIG even after continuous immersion underwater for 25 h. An important aspect of the dynamic contact angle measurement is the contact angle hysteresis (CAH), which is the difference between advancing and receding contact angles. In this study, even after 25 h of immersion, the CAH is -6.8\({}^{\circ}\), thus maintaining a stable superhydrophobicity. This demonstrates the high stability of the superhydrophobic property, which is a combined effect of the hydrophobic carbonaceous LIG and the hierarchical and fibrous surface morphology efficiently trapping air and repelling water penetration [56; 57; 58; 59]. From Figure 8, the stability in superhydrophobicity is further confirmed by the circuit resistance measurements performed on a SH-LIG conduction line circuit of 2.5 cm length, at various time intervals on the LIG fabricated on the composite material which was immersed underwater. The resistance values remain almost the same after 25 h due to the stability of its superhydrophobic property. Figure 9 shows the effect of total internal reflection by the trapped thin air layer, also termed plastron, on the SH-LIG, causing a shiny appearance of the superhydrophobic region on water immersion.