Biomimetic surface structures in steel fabricated with femtosecond laser pulses: influence of laser rescanning on morphology and wettability

The Design and Analysis of the Fabrication of Micro- and Nanoscale Surface Structures and Their Performance Applications from a Bionic Perspective

This paper comprehensively discusses the fabrication of bionic-based ultrafast laser micro-nano-multiscale surface structures and their performance analysis. It explores the functionality of biological surface structures and the high adaptability achieved through optimized self-organized biomaterials with multilayered structures. This study details the applications of ultrafast laser technology in biomimetic designs, particularly in preparing high-precision, wear-resistant, hydrophobic, and antireflective micro- and nanostructures on metal surfaces. Advances in the fabrications of laser surface structures are analyzed, comparing top-down and bottom-up processing methods and femtosecond laser direct writing. This research investigates selective absorption properties of surface structures at different scales for various light wavelengths, achieving coloring or stealth effects. Applications in dirt-resistant, self-cleaning, biomimetic optical, friction-resistant, and biocompatible surfaces are presented, demonstrating potential in biomedical care, water-vapor harvesting, and droplet manipulation. This paper concludes by highlighting research frontiers, theoretical and technological challenges, and the high-precision capabilities of femtosecond laser technology in related fields.

Influence of Heat Accumulation on Morphology Debris Deposition and Wetting of LIPSS on Steel upon High Repetition Rate Femtosecond Pulses Irradiation

The fabrication of laser-induced periodic surface structures (LIPSS) over extended areas at high processing speeds requires the use of high repetition rate femtosecond lasers. It is known that industrially relevant materials such as steel experience heat accumulation when irradiated at repetition rates above some hundreds of kHz, and significant debris redeposition can take place. However, there are few studies on how the laser repetition rate influences both the debris deposition and the final LIPSS morphology. In this work, we present a study of fs laser-induced fabrication of low spatial frequency LIPSS (LSFL), with pulse repetition rates ranging from 10 kHz to 2 MHz on commercially available steel. The morphology of the laser-structured areas as well as the redeposited debris was characterized by scanning electron microscopy (SEM) and µ-Raman spectroscopy. To identify repetition rate ranges where heat accumulation is present during the irradiations, we developed a simple heat accumulation model that solves the heat equation in 1 dimension implementing a Forward differencing in Time and Central differencing in Space (FTCS) scheme. Contact angle measurements with water demonstrated the influence of heat accumulation and debris on the functional wetting behavior. The findings are directly relevant for the processing of metals using high repetition rate femtosecond lasers, enabling the identification of optimum conditions in terms of desired morphology, functionality, and throughput.

Long-Time Persisting Superhydrophilicity on Sapphire Surface via Femtosecond Laser Processing with the Varnish of TiO2

The acquiring of superhydrophilic surfaces attracts the strong interest in self-cleaning, anti-fogging and anti-icing fields based on the unique features. However, the persistent time of superhydrophilic surfaces is still facing a big challenge because of easily adsorbing hydrophobic groups. Here, we propose a strategy to achieve a superhydrophilicity persisting for an unprecedently long time on sapphire surfaces, by compounding the femtosecond laser-induced hierarchical structures and the subsequent varnish of TiO₂. The superhydrophilic effect (with a contact angle of CA = 0°) created by our method can be well prolonged to at least 180 days, even for its storage in air without additional illumination of UV lights. Based on comprehensive investigations, we attribute the underlying mechanisms to the coordination of laser-induced metal ions on the material surface via TiO₂ doping, which not only prevents the adsorption of the nonpolar hydrocarbon groups, but also modulates the photo-response properties of TiO₂. In addition, further experiments demonstrate the excellent anti-fogging properties of our prepared samples. This investigation provides a new perspective for further enhancing the durability of superhydrophilicity surfaces.

Femtosecond Laser Fabrication of Hybrid Metal-Dielectric Structures with Nonlinear Photoluminescence

Fabrication of hybrid micro- and nanostructures with a strong nonlinear response is challenging and represents a great interest due to a wide range of photonic applications. Usually, such structures are produced by quite complicated and time-consuming techniques. This work demonstrates laser-induced hybrid metal-dielectric structures with strong nonlinear properties obtained by a single-step fabrication process. We determine the influence of several incident femtosecond pulses on the Au/Si bi-layer film on produced structure morphology. The created hybrid systems represent isolated nanoparticles with a height of 250–500 nm exceeding the total thickness of the Au-Si bi-layer. It is shown that fabricated hybrid nanostructures demonstrate enhancement of the SHG signal (up to two orders of magnitude) compared to the initial planar sample and a broadband photoluminescence signal (more than 200 nm in width) in the visible spectral region. We establish the correlation between nonlinear signal and phase composition provided by Raman scattering measurements. Such laser-induced structures have significant potential in optical sensing applications and can be used as components for different nanophotonic devices.

Ionisation processes and laser induced periodic surface structures in dielectrics with mid-infrared femtosecond laser pulses

Irradiation of solids with ultrashort pulses and laser processing in the mid-Infrared (mid-IR) spectral region is a yet predominantly unexplored field with a large potential for a wide range of applications. In this work, laser driven physical phenomena associated with processes following irradiation of fused silica (SiO2) with ultrashort laser pulses in the mid-IR region are investigated in detail. A multiscale modelling approach is performed that correlates conditions for formation of perpendicular or parallel to the laser polarisation low spatial frequency periodic surface structures for low and high intensity mid-IR pulses (not previously explored in dielectrics at those wavelengths), respectively. Results demonstrate a remarkable domination of tunneling effects in the photoionisation rate and a strong influence of impact ionisation for long laser wavelengths. The methodology presented in this work is aimed to shed light on the fundamental mechanisms in a previously unexplored spectral area and allow a systematic novel surface engineering with strong mid-IR fields for advanced industrial laser applications.

Preferential Growth of ZnO Micro- and Nanostructure Assemblies on Fs-Laser-Induced Periodic Structures

In this work, we demonstrate the use of laser-induced periodic surface structures (LIPSS) as templates for the selective growth of ordered micro- and nanostructures of ZnO. Different types of LIPSS were first produced in Si-(100) substrates including ablative low-frequency spatial (LSF) LIPSS, amorphous-crystalline (a-c) LIPSS, and black silicon structures. These laser-structured substrates were subsequently used for depositing ZnO using the vapor-solid (VS) method in order to analyze the formation of organized ZnO structures. We used scanning electron microscopy and micro-Raman spectroscopy to assess the morphological and structural characteristics of the ZnO micro/nano-assemblies obtained and to identify the characteristics of the laser-structured substrates inducing the preferential deposition of ZnO. The formation of aligned assemblies of micro- and nanocrystals of ZnO was successfully achieved on LSF-LIPSS and a-c LIPSS. These results point toward a feasible route for generating well aligned assemblies of semiconductor micro- and nanostructures of good quality by the VS method on substrates, where the effect of lattice mismatch is reduced by laser-induced local disorder and likely by a small increase of surface roughness.

High-Rate Laser Surface Texturing for Advanced Tribological Functionality

This article features with the enhancement of the static coefficient of friction by laser texturing the contact surfaces of tribological systems tested under dry friction conditions. The high-rate laser technology was applied for surface texturing at unprecedented processing rates, namely using powerful ultrashort pulses lasers in combination with ultrafast polygon-mirror based scan systems. The laser textured surfaces were analyzed by ion beam slope cutting and Raman measurements, showing a crystallographic disordering of the produced microscopic surface features. The laser induced self-organizing periodic surface structures as well as deterministic surface textures were tested regarding their tribological behavior. The highest static coefficient of friction was found of µ20 = 0.68 for a laser textured cross pattern that is 126% higher than for a fine grinded reference contact system. The line pattern was textured on a shaft-hub connection where the static coefficient of friction increased up to 75% that demonstrates the high potential of the technology for real-world applications.

An investigation on the drag reduction performance of bioinspired pipeline surfaces with transverse microgrooves

A novel surface morphology for pipelines using transverse microgrooves was proposed in order to reduce the pressure loss of fluid transport. Numerical simulation and experimental research efforts were undertaken to evaluate the drag reduction performance of these bionic pipelines. It was found that the vortex 'cushioning' and 'driving' effects produced by the vortexes in the microgrooves were the main reason for obtaining a drag reduction effect. The shear stress of the microgrooved surface was reduced significantly owing to the decline of the velocity gradient. Altogether, bionic pipelines achieved drag reduction effects both in a pipeline and in a concentric annulus flow model. The primary and secondary order of effect on the drag reduction and optimal microgroove geometric parameters were obtained by an orthogonal analysis method. The comparative experiments were conducted in a water tunnel, and a maximum drag reduction rate of 3.21% could be achieved. The numerical simulation and experimental results were cross-checked and found to be consistent with each other, allowing to verify that the utilization of bionic theory to reduce the pressure loss of fluid transport is feasible. These results can provide theoretical guidance to save energy in pipeline transportations.

Tailoring metal-dielectric nanocomposite materials with ultrashort laser pulses for dichroic color control

Metal-dielectric nanocomposites are multiphase material systems containing nanostructures, whose size and arrangement determine the optical properties of the material, enabling the production of new materials with custom-designed response. In this paper, we exploit a femtosecond laser-based strategy to fabricate nanocomposites based on silver nanoparticles (Ag NPs) with tunable optical spectral response. We demonstrate how the spectral response, specifically color and dichroic response, is linked to Ag NPs growth and self-organization processes that are controlled locally by the choice of the laser irradiation parameters, such as scan speed and laser light polarization. When the scan speed increases, the Ag NPs are formed at larger depths below the film surface and give rise to the formation of embedded NPs gratings. As a result, the effective optical properties of the films are strongly modified enabling the display of a broad range of solid colors in the visible region. Furthermore, the choice of the laser light polarization allows to fabricate films either with iridescent or dichroic properties (linear polarization) or with non-diffractive and non-dichroic colors (circular polarization). Finally, the high spatial control over the transformed areas achieved with the laser processing, allows the building of hybrid nanostructures by means of interlacing structures with different optical responses. These results demonstrate the high potential of fs-laser technology to process Ag-based nanocomposites to fabricate coatings with a designed reflectivity, transmission, diffraction, as well as polarization anisotropy response. The Ag nanocomposites investigated in this work hold great promise for a broad range of applications especially for coloring, for enhanced visual effects, and for smart information encoding for security applications.

Femtosecond Laser Fabrication of Stable Hydrophilic and Anti-Corrosive Steel Surfaces

We report on a novel single-step method to develop steel surfaces with permanent highly hydrophilic and anti-corrosive properties, without employing any chemical coating. It is based on the femtosecond (fs) laser processing in a saturated background gas atmosphere. It is particularly shown that the fs laser microstructuring of steel in the presence of ammonia gas gives rise to pseudoperiodic arrays of microcones exhibiting highly hydrophilic properties, which are stable over time. This is in contrast to the conventional fs laser processing of steel in air, which always provides surfaces with progressively increasing hydrophobicity following irradiation. More importantly, the surfaces subjected to fs laser treatment in ammonia exhibit remarkable anti-corrosion properties, contrary to those processed in air, as well as untreated ones. The combination of two functionalities, namely hydrophilicity and corrosion resistance, together with the facile processing performed directly onto the steel surface, without the need to deposit any coating, opens the way for the laser-based production of high-performance steel components for a variety of applications, including mechanical parts, fluidic components and consumer products.

Study on the Fabrication of Super-Hydrophobic Surface on Inconel Alloy via Nanosecond Laser Ablation

Nanosecond laser ablated metallic surfaces showed initial super-hydrophilicity, and then experienced gradual wettability conversion to super-hydrophobicity with the increase of exposing time to ambient air. Due to the presence of hierarchical structures and change of surface chemistry, the laser-induced Inconel alloy surfaces showed a stable apparent contact angle beyond 150° over 30-day air exposure. The wetting states were proposed to elucidate the initial super-hydrophilicity and the final super-hydrophobicity. The basic fundaments behind the wettability conversion was explored by analyzing surface chemistry using X-ray photoelectron spectroscopy (XPS). The results indicated that the origins of super-hydrophobicity were identified as the increase of carbon content and the dominance of C⁻C(H) functional group. The C⁻C(H) bond with excellent nonpolarity derived from the chemisorbed airborne hydrocarbons, which resulted in dramatic reduction of surface-free-energy. This study confirmed that the surface chemistry is not the only factor to determine surface super-hydrophobicity. The laser-induced super-hydrophobicity was attributed to the synergistic effect of surface topography and surface chemical compositions. In this work, the corresponding chemical reaction was particularly described to discuss how the airborne hydrocarbons were attached onto the laser ablated surfaces, which reveals the generation mechanism of air-exposed super-hydrophobic surfaces.