Figure 5. Results of the ageing of laser-textured meshes in air (red solid line with filled circles), in Chamber #1 (green solid line with filled triangles) and Chamber #2 (blue solid line with filled triangles). Reference samples measured after 1 hour after laser texturing are shown by black solid line with filled squares. On the right side the images of WCA recorded by Drop Shape Analyzer are shown.
Surface micro-nanostructures formed on the metal meshes by laser texturing is crucial and primary requirement for altering the wetting properties of the materials. WCA measurement of the freshly laser structured stainless steel meshes in ambient air conditions demonstrated hydrophilic behavior due to high generation of the metal oxides. The unsaturated cations-anions formed after laser ablation stabilize themselves by heterolytic dissociative adsorption of H2O molecules from the atmosphere, giving birth to hydroxylated layer over metal-oxides layer [24]. The hydroxylated (-OH) layer has high affinity to adsorb water molecules through hydrogen bonding that explains the hydrophilic nature of freshly laser textured surfaces.
Surface chemistry was semi-quantitatively analyzed using EDS measurements. Figure 6 shows the variation of the weight percentage (wt%) of carbon (black bars) and oxygen (cyan bars) elements in the case of laser treated and untreated surfaces. A significant growth of O and decrease of C on the freshly prepared mesh microstructures is observed (R11 and R12 bars, Fig. 6). This effect related to the material removal by ablation process can lead to surface cleaning and on the other hand to the oxidation of the surface material with atmospheric oxygen. Also note that, with decreasing scanning speed (from 300 to 50 mm/s), the amount of oxygen increases due to the growing number of the overlapped laser pulses on the target surface (compare blue bars R11 and R12). After ageing in air and Chamber #1 for 12 hours (bars from R21 to R32) the relative content of C and O remain approximately the same, which explains the similar results for WCA in Fig. 5. In contrast, the ageing inside Chamber #2 (R41 and R42 bars in Fig. 6) led to the notable increase of carbon with regard to oxygen.