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.