Figure 6. R11 and R12: EDS analysis of the relative weight variation of C (black bars) and O (cyan bars) in the case of laser-treated and untreated surfaces. Here R11 is a ratio of elements in the case of treated sample prepared in ambient air at 50 mm/s scan speed and stored for 1 hour in air after laser texturing. R12 is the same as R11, but at the scan speed 300 mm/s. R21 is the relation for sample prepared at 50 mm/s and stored for 12 hours in ambient air, R22 same as R21, but at 300 mm/s. R31 is the relation for sample prepared at 50 mm/s and stored for 12 hours in vacuum inside Chamber#1, R32 is the same as R31, but at 300 mm/s. R41 is the sample prepared at 50 mm/s and stored for 12 hours in vacuum inside Chamber#2, R42 is same as R41, but at 300 mm/s.
Thus, one can conclude that vacuum condition with hydrocarbons contamination can strongly change the surface chemistry of laser-treated samples. The short chained non-polar hydrocarbons replaced the –OH group formed immediately after laser ablation at an accelerated rate under vacuum storage. Thus, a faster transition in the wetting state from superhydrophilic to superhydrophobic state is observed after vacuum ageing in contaminated chamber. By this reason all SEM and EDS measurements were carried out for separately prepared samples. For further studies of different gas environment we used the Chamber #1.
Atmospheric air produced samples’ ageing
Figure 7 shows the results of long-term ageing of samples produced in ambient air and later stored in air at normal atmospheric pressure. Here, wettability dependence on scanning speed, morphology produced by different polarizations (designated by horizontal polarization (HP), vertical polarization (VP) and circular polarization (CP)) and number of days are plotted. Again, the brown line depicts WCA for untreated surface (128°). 30 days ageing (blue solid lines) was enough for the most of samples to increase WCA above 140°. HP-, VP- and CP-treated samples irradiated using scanning speeds from 50 to 250 mm/s exceeded 145° and were close to the superhydrophobic limits, while some of them demonstrated WCA>150°. Whereas, improved water contact angle for the samples processed using lower scanning speeds (50 and 150 mm/s) was observed which can be attributed to the deeper and sharper surface morphology, since application of low scanning velocities results in rougher surface, with expressed ripples and additional structures and redeposition, while at high scanning speeds the surface remains less deformed. Rougher surface’s absorbance probably becomes higher, which can attract the hydrocarbons presented in ambient air.