## 3 Results and Discussion
However, the LIPSS in that case was of poor quality and showed some nano/micro sized bead-like structures on the ripples. For the angle-dependent (iridescent) samples prepared at 500 mm/s, the SEM images show the appearance of sharp ripples (Figure 6D). For comparison we show here the SEM image of untreated surface as well (Figure 6E). The last column of Figure 6 shows the results of the wettability measurements of the aluminum surfaces demonstrating different colors. While laser processing of the samples initially resulted in their wettability shift towards hydrophilic behavior, dramatic transformation into superhydrophobic response was observed after those samples were kept in vacuum for 24 h. In order to reveal the effect of vacuum treatment explicitly, two sets of samples of each color were prepared at the same conditions. The wettability response of the first set of samples was measured right after the laser processing. All colored samples exhibited contact angle values less than 15\({}^{\circ}\) and were correspondingly classified as hydrophilic. The second set of processed samples, was kept in vacuum at \(2.5\times 10^{-4}\) mbar for 24 h prior to the contact angle measurements. It was found that, after vacuum storage, different colors provide different variations of wettability. We observed that the golden color surface had the highest contact angle close to 156\({}^{\circ}\), thus exhibiting superhydrophobic behavior. The contact angles of black, brown, and multicolor samples were 149, 147, and 145 respectively. When we placed 5 \(\mu\)L water droplets, the droplet was repelled back by the hierarchical rough micro-nanostructures and the low surface free energy of the structured material. The presence of nano/micro-sized bead-shaped particles on top of the ripples of golden color samples resulted in a decrease of the surface free energy and caused a high contact angle. This set of experiments showed that the laser-structured surfaces that were stored in vacuum for 24 h were extremely water repellant and hence superhydrophobic. Moreover, to test the sustainability of these superhydrophobic surfaces, we repeated the contact angle measurements after keeping the samples in air for many weeks; and yet they showed the same superhydrophobic behavior. Earlier, vacuum storage of nanosecond and picosecond laser-structured aluminum and titanium allowed the superhydrophobicity response of their processed surfaces to be increased due to enhanced adsorption of carbonaceous species on the laser textured surfaces [57; 58]. These organic compoundswere assumed to emanate from the walls of the chambers and other remaining hydrocarbons in the vacuum chamber maintained at the base pressure \(\sim\)10\({}^{-4}\) mbar [58]. While hydrogen-bonded H\({}_{2}\)O molecules on the hydroxylated layer were believed to hinder further adsorption of the hydrocarbons in ambient atmosphere, the extremely low water vapor pressure in the vacuum chamber allows faster chemisorption of hydrocarbons. This gave rise to speedy realization of the superhydrophobic characteristics on the structured surface, as observed in our studies. Figure 6: Contact angle measurements obtained by drop shape analyzer for different colors produced on aluminum along with the SEM images. The images correspond to (**A**) black, (**B**) brown, (**C**) golden, (**D**) multicolor and (**E**) untreated aluminum. Most treated surfaces demonstrated superhydrophobic characteristics as manifested in the high value of the measured contact angle. Black, brown, golden, and multicolor (iridescent colors) were obtained at the speeds of 10, 70, 200, and 500 mm/s, respectively. The scale bars for the SEM pictures are 200 and 20 \(\upmu\)m for the first and second columns, respectively. The processing times for disks of 20 mm diameter for black, brown, golden and multicolor were 520 s, 75 s, 27 s, and 11 s respectively. Figure 7 shows the Atomic Force Microscope and Confocal Microscope topographic profiles for the surfaces of different colors of the aluminum sample.