Figure 4. SEM images of the laser textured samples prepared in various gas environment and in the vacuum. With blue arrow pointed direction of the scanning by laser beam. Direction of polarization marked with white double side arrows in case of linear polarization and with white circle for circular one.
During these experiments, laser induced periodic surface structures (LIPSS) in the form of ripples with the period close to laser wavelength and with the direction perpendicular to laser polarization were formed (Fig. 3, 300 mm/s row, left and central panels). With increasing number of overlapped pulses (i.e. decreasing scanning speed) the microgrooves were directed mainly parallel to polarization direction and with period of a few laser wavelengths appear (Fig. 3, 50 mm/s row, left and central panels). In the case of circular polarization, the ripples were replaced with micropikes and with increasing number of pulses bigger structures in form hills can be observed (Fig. 3, last column).
In the case of different gas environment, the resulted SEM images of the textured surfaces are shown on Fig. 4. Same, as for Fig. 3, tendency was observed. In general, there was no big difference in textured surfaces for different gases and vacuum. The resulting morphology was very similar for different gases suggesting that any observed wettability changes had no relations with roughness variations. Note that, in case of SF6, a suppression of the formation of the nano-textured microstructures due to chemical reactivity was reported [21]. We didn’t observe such effect, probably due to lower laser fluence and smaller number of overlapped pulses compared with the experimental conditions of [21].
Vacuum ageing
In this subsection, we address the effect of low-pressure vacuum ageing on the wettability transformation of metal surfaces textured in air [22,23]. It was reported that the ageing of laser-treated samples in vacuum allows increasing the hydrophobic properties for much shorter (up to a few hours) period of time, compared with the time required for long-term air ageing. This behavior was attributed to presence of hydrocarbons in form of contaminations from either oil used for lubrication of moving parts of vacuum pumps. We address this problem since in our experiments the mesh samples were treated by laser radiation inside the chamber at vacuum conditions or in gas environment under pressure (200 mbar) still lower that atmospheric one and then stored for a period of 2 hours. The goal was to clarify and separate the possible side effect from vacuum system.
For these purposes four sets of samples were prepared in atmospheric air. After preparation first set was tested for WCA after keeping sample during one hour in air. This measurement is served as a reference for three other sets. Second set of samples was stored in ambient air for 12 hours at the conditions described in the experimental section. Third and fourth sets of samples were placed in Chamber #1 and Chamber #2, respectively, for 12 hours of vacuum storage. After ageing for given period of time the WCA’s of one air set and two vacuum sets of samples were measured.
The results of this experiment are shown on Fig. 5. Black solid line with solid squares shows the dependence of WCA on the scanning speed in the case of the set kept in air within 1 hour after texturing. As it was expected, after texturing in air the sample’s wettability changed to hydrophilic (θw < 90o). WCA increased once the scanning speed increased from 50 to 300 mm/s. Second set aged in air for 12 hours (Fig. 5, red solid line with solid circles) still demonstrated the hydrophilic properties. WCA for the low scanning speeds remained almost unchanged, while for higher speeds (250-300 mm/s) it increased compare to the one tested 1 hour from laser treatment, while still remaining lower than WCA of untreated mesh surface (θw = 128o).
Meanwhile, the third and fourth sets of samples aged in vacuum inside the Chamber #1 and Chamber #2 showed the dramatic difference in WCA once we compared these two samples and air stored samples. For all samples stored in Chamber #2 (Fig. 5, blue solid line with solid triangles), WCA shifted to the superhydrophobic region (Fig.5, hatched grey area corresponding to WCA>150°), while the samples stored in Chamber #1 (Fig. 5, green solid line with solid triangles), though showing the increased WCA, still demonstrated smaller value of this parameter compared with the untreated surface. Some increase of wettability in the latter case can be explained by dehydration of samples in vacuum. Brown horizontal line in the Fig. 5 shows WCA for untreated surface of the meshes. This angle was equal ~128° and remained same under different gas phases and low vacuum ageing.