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.