## 3 Results and Discussion
### SEM Characterization of Hair Surface Morphology Before and After Plasma-Activated Water Treatments  
The SEM images presented in Figure 2 detail the morphological characteristics of the pristine hair samples summarized in Table 1, highlighting structural differences among the hair types, particularly surface texture, cuticle arrangement, and strand integrity. Figure 2a displays the wavy hair sample (Sample 1, Type 2), showing a relatively smooth surface with a mild undulating pattern along the hair shaft. The cuticle layers are mostly intact, with slight lifting observed, consistent with the described texture. This moderate surface structure contributes to characteristic shine and volume. Figure 2b,c depict straight hair samples (Samples 2 and 3, Type 1), characterized by uniform, smooth surfaces without significant undulations. The flat, compact cuticle scales minimize strand friction, contributing to the sleek appearance typical of straight hair. SEM studies confirm that straight hair tends to exhibit uniform cuticle alignment with minimal porosity, which enhances shine and mechanical resistance [21]. Minor surface irregularities, potentially due to environmental exposure or treatments, do not affect overall structural consistency. Figure 2d shows the coily/kinky hair sample (Sample 4, Type 4) with tightly packed coils and a notably textured surface. Pronounced cuticle layers and areas of slight lifting, typical of Type 4 hair, increase surface area and exposure, making this hair type susceptible to dryness and breakage. These findings are consistent with prior reports showing that Type 4 hair has higher curvature and cuticle lifting, which contributes to increased fragility and water loss [22]. The SEM images in Figure 3 reveal marked alterations in hair surface morphology after exposure to nitrate-rich plasma-activated water (PAW-N), and when compared to the corresponding pristine samples in Figure 2, these changes underscore the impact of PAW-N's acidic pH (2.15), elevated nitrate concentration (~500 mg/L), and high ionic strength (6244 \(\upmu\)S/cm). In Figure 2(a), the wavy hair (Sample 1, Type 2), originally presenting a relatively smooth surface with mildly undulating cuticle layers (Figure 1(a)), displays moderate cuticle lifting, roughening, and localized erosion. This indicates disruption of hydrogen bonding and lipid layer degradation, potentially increasing porosity and water loss. Figure 2(b) shows straight hair (Sample 2, Type 1), which originally exhibited a compact, aligned cuticle (Figure 1(b)), but now presents severe fragmentation and partial detachment of cuticle layers--evidence of oxidative degradation induced by PAW-N's reactive nitrogen species and ionic environment. In contrast, Figure 2(c), corresponding to another straight hair sample (Sample 3, Type 1), shows less pronounced damage compared to Sample 2, with partial cuticle lifting and moderate surface roughness relative to its initially smooth profile (Figure 1(c)). This difference may reflect intrinsic variation in strand thickness or cuticle compactness, as thicker strands often present more cortical mass relative to surface area, reducing the impact of surface oxidation, while compact cuticles can act as a physical barrier  
Figure 2: Scanning Electron Microscopy (SEM) images of untreated (control) hair samples corresponding to Table 1, captured at 2000\(\times\) magnification: (**a**) Sample 1—wavy hair (Type 2), exhibiting a slight curl pattern without fully developed curls; (**b**) Sample 2—straight hair (Type 1), with a smooth surface and minimal natural wave; (**c**) Sample 3—straight hair (Type 1), showing a very smooth and aligned cuticle structure; (**d**) Sample 4—caily/kinky hair (Type 4), characterized by tightly coiled, dense strands and naturally lifted cuticle edges. that limits the diffusion of reactive species into deeper layers of the hair fiber.