Figure 1. Experimental setup. Beam from femtosecond laser source (FS)
was controlled by scanning head (SH) and focused inside the vacuum
chamber on the surface of the mesh target. The polarization properties
were controlled using λ/2 or λ/4 waveplates. Two gas valves were used to
control gas pressure inside chamber or to keep vacuum conditions. Mesh
target inside chamber was fixed in a way that no material existed behind
mesh pores and no deposition from any other material occurred.
Figure 1 shows the experimental setup used for laser treatment of the
meshes in various gas atmospheres. 5 W laser beam (FS) from high-power
fiber-based laser system (AFS-UFFL-300-2000-1030-300, Active Fiber
Systems GmbH) with a central wavelength of 1030 nm, repetition rate of
50 kHz, and a pulse duration of 40 fs, was used for laser structuring of
meshes. Laser radiation was focused and raster scanned on the sample by
160 mm F-Theta lens and scan head (SH) (FARO tech. Xtreme-20) as shown
in Fig. 1. The scanning head was mounted outside the vacuum chamber and
the scanning beam was directed inside the chamber through the quartz
window. The laser surface texturing of the stainless meshes was carried
out using similar geometry by bi-directional line-by-line scanning
strategy. The distance between scan lines was set equal to 60 μm. The
focused laser beam spot diameter was measured ~100 µm.
Single laser pulse energy was equal to 100 μJ.
The samples of meshes with 2×2 cm size were textured inside the vacuum
chamber in presence of different gases, atmospheric air or in the vacuum
conditions. Gases (N2, O2,
CO2, Ar, SF6) with 99.95% purity were
used. The mesh inside chamber was fixed in a way that no material behind
mesh pores gets existed and no deposition from any other material
occurred. Scanning speed was varied from 50 to 300 mm/s. With the
estimation of the focusing spot area it gives the laser fluence on the
target area of 1.2 J/cm2. The number of overlapped
pulses was varied from 16 to 100. The polarization direction of laser
beam was controlled by λ/2 waveplate. The circular polarization was
obtained by using the λ/4 waveplate. Different vacuum pump systems were
used in our experiments with two chambers, which we designate as Chamber
#1 and Chamber #2. In Chamber #1, we used the pair of “oil-less”
pumps (Edwards nXDS 6i and Pfeiffer HiPace 30 turbopump). Chamber #2
utilized another pair of pumps, one of which (Pfeiffer DUO 2.5) used the
vacuum oil (Edwards Ultragrade 19) as a lubricant. In both chambers, the
vacuum at 2×10-4 mbar was maintained during these
experiments. Gas pressure was controlled by two gas valves. The first
valve was placed before the vacuum pumps system, and the second one
controlled the gas inlet.
Following laser treatment, the surface morphological analysis was
performed using scanning electron microscope (SEM) (TESCAN VEGA3) and
the surface chemistry was evaluated semi-quantitatively via
energy-dispersive X-ray spectroscopy (EDS) technique using the SEM X-ray
detector. For wettability characterization, water contact angle (WCA)
was measured using Drop Shape Analyzer (KRUSS). Distilled water droplets
(~5 μl volume) were used for all samples to determine
WCA.