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.