# Sample, Scalable Route to Produce Transparent Superhydrophobic/Hydrophilic Film Surfaces
## 3 Results and Discussion
### The Effect of Spraying Parameters  
Distance between the nozzle and substrate  
The critical particle velocity, which depends on particle characteristics, such as density and size, is the most crucial element in the spray process that influences coating development. The particles continue accelerating as they leave the nozzle throughout the cold-spray procedure. In this regard, substrate-nozzle distance is crucial and needs to be optimized. Shock waves arise on the substrate surface when the nozzle-substate distance exceeds the  
Figure 6: The value of average transmittance of the coating varies with the content of f-SiO\({}_{2}\) with the corresponding WCA images and values. critical value, which results in a decrease in velocity. The particle's velocity will drop if the distance exceeds the critical value. Figure 7 shows a schematic illustration of the nozzle substrate distance. Figure 8 displays the WCA and transmittance of the as-sprayed coatings at various spraying distances illustrated Figure 7 and with varying f-SiO\({}_{2}\) contents. As the spraying distance increases, the coating's WCA for a given f-SiO\({}_{2}\) percentage stays constant. However, the transmittance changes significantly as you get closer to the nozzle and surface. It is known that the farther the nozzle is from the top surface of the substrate, the higher the porosity of the sprayed coatings, so the WCA decreases. At a higher distance, the deformation of a particle is determined by its strength and density, which will influence the particle's kinetic energy at the same velocity [36]. Therefore, it is difficult to form a dense coating due to its low density [37; 38]. However, for our coatings, the results showed that only the f-SiO\({}_{2}\) content influences the surface reactivity. This can explain why the coating with the same f-SiO\({}_{2}\) content was less transparent but with approximately the same WCA and a relatively high deposition efficiency. Figure 8: Value of average transmittance of the coating varies with the content of f-SiO\({}_{2}\) the with the corresponding WCA images at two different spraying distances—30 mm and 50 mm. Figure 7: Schematic illustration of the substrate–nozzle distance. Shaping air pressure  
The used AccuMist nozzle system creates a soft, sharply focused beam of tiny spray drops by combining low-pressure air with an ultrasonic atomizing nozzle. A uniform flow of compressed air is built around the nozzle stem by introducing compressed air into the diffusion chamber of an air shroud, which surrounds the nozzle and is typically compressed at 3 psi (see Figure 9). The low-pressure air stream immediately incorporates the ultrasonically created spray at the stem's tip. The air shroud's focusing mechanism can be adjusted to provide total control over spray width. The spray container has an hourglass-like form and is very narrow. The results shown in Figure 10 demonstrate that for the exact content of f-SiO\({}_{2}\), the transmittance and WCA are higher when using higher-shaping air. The liquid droplets tend to follow, molding air to diverge radially and producing higher overspray under high rotating speeds and low-shaping air-flow conditions. The smaller droplets that follow the air with a strong forward momentum will produce a more concentrated spray and better conveyance at high rotating speeds and high-shaping air flow [39; 40]. Figure 10: Value of average transmittance of the coating varies with the content of f-SiO\({}_{2}\), the corresponding WCA images, and the values of samples sprayed with shaping air of 0 psi and 3 psi. Figure 9: Schematic illustration of the resulting flow (**left**) without shaping air and (**right**) with shaping air. In the spray process, the area spacing substantially impacts the coating profile. The area spacing is defined as the distance of two neighbor scanning passes (Figure 11). In our experiment, we tested the effect of different area spacing on the WCA and the transmittance of the resulting coatings.