Abstract:Femtosecond laser micromachining has emerged in recent years as a new technique for micro/nano structure fabrication because of its applicability to virtually all kinds of materials in an easy one-step process that is scalable. In the past, much research on femtosecond laser micromachining was carried out to understand the complex ablation mechanism, whereas recent works are mostly concerned with the fabrication of surface structures because of their numerous possible applications. The state-of-the-art knowledge on the fabrication of these structures on metals with direct femtosecond laser micromachining is reviewed in this article. The effect of various parameters, such as fluence, number of pulses, laser beam polarization, wavelength, incident angle, scan velocity, number of scans, and environment, on the formation of different structures is discussed in detail wherever possible. Furthermore, a guideline for surface structures optimization is provided. The authors' experimental work on laser-inscribed regular pattern fabrication is presented to give a complete picture of micromachining processes. Finally, possible applications of laser-machined surface structures in different fields are briefly reviewed.

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In this work, internal and external flows over superhydrophobic (SH) polytetrafluoroethylene (PTFE) were studied. The SH surface was fabricated by a one-step femtosecond laser micromachining process. The drag reduction ability of the textured surface was studied experimentally both in microscale and macroscale internal flows. The slip length, which indicates drag reduction in fluid flow, was determined in microscale fluid flow with a cone-and-plate rheometer, whereas a pressure channel setup was used for macroscale flow experiments. The textured PTFE surface reduced drag in both experiments yielding comparable slip lengths. Moreover, the experimentally obtained slip lengths correspond well to the result obtained applying a semianalytical model, which considers the solid fraction of the textured surface. In addition to the internal flow studies, we fabricated SH PTFE spheres to test their drag reduction abilities in an external flow experiment, where the terminal velocities of the falling spheres were measured. These experiments were conducted at three different Reynolds numbers in both viscous and inertial flow regimes with pure glycerol, a 30% glycerol solution, and water. Surprisingly, the drag on the SH spheres was higher than the measured drag on the non-SH spheres. We hypothesize that the increase in form drag outweighs the decrease in friction drag on the SH sphere. Thus, the overall drag increased. These experiments demonstrate that a superhydrophobic surface that reduces drag in internal flow might not reduce drag in external flow.

Hierarchical laser-patterned surfaces were tested for their drag reduction abilities. A tertiary level of surface roughness which supports stable Cassie wetting was achieved on the patterned copper samples by laser-scanning multiple times. The laser-fabricated micro/nano structures sustained the shear stress in liquid flow. A rheometer setup was used to measure the drag reduction abilities in term of slip lengths on eight different samples. A considerable increase in slip length (111% on a grate sample) was observed on these surfaces compared to the slip length predictions from the theoretical and the experimental models for the non-hierarchical surfaces. The increase in slip lengths was correlated to the secondary level of roughness observed on the patterned samples. The drag reduction abilities of three different arrangements of the surface features were also compared: posts in a square lattice, parallel grates, and posts in a hexagonal lattice. Although the latter facilitates a stable Cassie state, it nevertheless resulted in a lower normalized slip length compared to the other two arrangements at a similar solid fraction. Furthermore, we coated the laser-patterned surfaces with a silane to test the effect of surface chemistry on drag reduction. While the contact angles were surprisingly similar for both the non-silanized and the silanized samples, we observed higher slip lengths on the latter, which we were able to explain by measuring the respective penetration depths of the liquid-vapour interface between surface features.

Salvinia leaf and sharkskin are prime examples of nature's marvel. Salvinia leaf‐inspired superhydrophobic surfaces keep themselves clean and reduce drag in fluid flow. Sharkskin also reduces drag in turbulent flow and inhibits biofouling. Therefore, the prospect of having a drag‐reducing surface with both salvinia leaf and sharkskin properties is attractive. However, fabricating such a surface is difficult, and the current fabrication methods require at least two separate steps. In addition, the mechanisms of drag reduction of salvinia leaf and sharkskin are different, and their combined effect on the flow field is not well understood. In this study, we produced a PTFE surface that mimics sharkskin in its surface pattern and copies the superhydrophobic nature of the salvinia leaf in its microstructure. This surface was fabricated by laser machining and tested in a closed channel under turbulent flow conditions. We measured the pressure drop at different Reynolds numbers on this surface both in pre‐wet and non‐pre‐wet conditions and compared the result with pressure drop data on four other PTFE samples: two types of non‐superhydrophobic sharkskin inspired surface (riblets), a superhydrophobic surface, and a non‐machined surface. Both the non‐superhydrophobic riblets and the superhydrophobic sample reduced drag compared to the non‐machined surface. However, we observed a lack of drag reduction by the superhydrophobic riblets sample. We presented a qualitative explanation for the lack of drag reduction and concluded that the modifications of the flow field by the two drag reduction mechanisms are not beneficial for overall drag reduction in our experiment.

Technological innovations and quality control processes within blood supply organizations have significantly improved blood safety for both donors and recipients. Nevertheless, the risk of transfusion-transmitted infection remains non-negligible. Applying a nanoparticular, antibacterial coating at the surface of medical devices is a promising strategy to prevent the spread of infections. In this study, we characterized the antibacterial activity of an SiO2 nanoparticular coating (i.e., the “Medical Antibacterial and Antiadhesive Coating” [MAAC]) applied on relevant polymeric materials (PM) used in the biomedical field. Electron microscopy revealed a smoother surface for the MAAC-treated PM compared to the reference, suggesting antiadhesive properties. The antibacterial activity was tested against selected Gram-positive and Gram-negative bacteria in accordance with ISO 22196. Bacterial growth was significantly reduced for the MAAC-treated PVC, plasticized PVC, polyurethane and silicone (90–99.999%) in which antibacterial activity of ≥1 log reduction was reached for all bacterial strains tested. Cytotoxicity was evaluated following ISO 10993-5 guidelines and L929 cell viability was calculated at ≥90% in the presence of MAAC. This study demonstrates that the MAAC could prevent bacterial contamination as demonstrated by the ISO 22196 tests, while further work needs to be done to improve the coating processability and effectiveness of more complex matrices.

An experimental study was performed to investigate the dynamics of droplet shedding under the effect of various shear flow speeds on a laser micromachined surface with superhydrophobic properties. To account for the effect of liquid properties on droplet shedding, four different liquids were used in these sets of experiments, namely, distilled water, ethylene glycol, propylene glycol, and glycerol. The wetting length of the liquid droplets was measured based on the air shear speed, and three different regimes were observed based on the critical Weber and Ohnesorge numbers. In the first regime, where the Weber and Ohnesorge numbers are low, droplets deform with slight movement or rotation without detachment from the surface. Under the second regime, where the Weber number is relatively high and the Ohnesorge number is low, droplets deform and detach from the surface, and then subsequent breakup may occur. The variation of droplet detachment time with the Weber and Ohnesorge numbers is further discussed in this paper. In the third regime, where the Ohnesorge number is high, there is no droplet detachment nor are rivulets formed. Finally, empirical correlations are developed to predict the droplet behavior on laser-patterned surfaces under the effect of shear flow. This work can be used as a baseline to study the droplet dynamics on a superhydrophobic surface in cases where temperature changes the liquid properties.

Colored poly­(vinyl chloride) (PVC) was fabricated by femtosecond laser micromachining without the addition of chemical colorants, eliminating the concern of leaching dyes and pigments. We determined that the changes in surface chemistry and surface topography both contribute to the observed yellow, brown, and black color formation. X-ray photoelectron spectroscopy (XPS) on the machined samples showed that conjugated double bonds are liable for the yellow and brown colors, whereas the presence of oxidized carbon and surface topography contribute to the black color. Fourier transform infrared spectroscopy (FTIR) indicated that laser irradiation altered the material’s properties only near the surface, which left the bulk properties unaltered. Furthermore, chemical resistance tests showed that some of the samples were able to withstand the influence of aggressive chemicals and their color did not fade. Finally, we showed that the fabrication of colored PVC highly depends on its ablation energy threshold which is affected by the laser pulse duration and wavelength.