Search

Increasing demands on the reproducibility and accuracy of lab-on-a-chip systems in the life science sector require new tools for the molding of polymer films with spatially selectively functionalized surfaces. By applying local micro- and nanostructuring to the molding tools, the aim is to achieve multifunctionality of the polymer films, by setting certain wetting properties. This means that fluid-affecting properties (hydrophobic, hydrophilic) should be possible. To functionalize polymer films, the replication tools were specifically microstructured using laser-based surface functionalization. The periodic micro- and nanostructures were produced with the help of nanosecond pulsed and picosecond pulsed laser systems using Direct Laser Interference Patterning. For this purpose, complex functional microstructures in the range of 0.1 µm to 100 µm were developed based on a bionic approach. The functional properties were transferred in a one-step replicative process using UV-imprinting and were subsequently verified by determining the wetting behavior of the molded microfluidic films and the corresponding laser microstructured replication tools. This publication presents the successfully developed processes and the promising results of this series of experiments. [ABSTRACT FROM AUTHOR]

Edge-Based Online Learning (EBOL), a technique that combines the practical, hands-on approach of EBOL with the convenience of Online Learning (OL), is growing in popularity. But accurately monitoring student engagement to enhance teaching methodologies and learning outcomes is one of the difficulties of OL. To determine this challenge, this paper has put forth an Edge-Based Student Attentiveness Analysis System (EBSAAS) method, which uses a Face Detection (FD) algorithm and a Deep Learning (DL) model known as DLIP to extract eye and mouth landmark features. Images of the eye and mouth are used to extract landmarks using DLIP or Deep Learning Image Processing. Landmark Localization pre-trained models for Facial Landmark Localization (FLL) are one well-liked DL model for facial landmark recognition. The Visual Geometry Group-19 (VGG-19) learning model then uses these features to classify the student's level of attentiveness as fatigued or focused. Compared to a server-based model, the proposed model is developed to execute on an Edge Device (ED), enabling a swift and more effective analysis. The EBOL achieves 95.29% accuracy and attains 2.11% higher than existing model 1 and 4.41% higher than existing model 2. The study's findings have shown how successful the proposed method is at assisting teachers in changing their teaching methodologies to engage students better and enhance learning outcomes.

According to the Organization for Economic Cooperation and Development in Paris, the overall economic damage caused by counterfeiting worldwide is estimated at around 250 billion euros per year. In addition to the economic damage, counterfeit drugs or safety-relevant components endanger costumers. Individual forgery-proof laser marking of each product in a batch enables a digital product identification for all consumers in the future. Direct laser interference patterning, also known as laser interference structuring, can be used to apply a unique pattern matrix to surfaces, in particular, products. Each element of this matrix with diameters of an ablation spot could be provided with a defined periodic micropattern before shipment, resulting in a pattern-dependent interaction with incident light. The application on different surfaces can be realized in one process step using an automated spatial period variation under ambient conditions. In the case of two-beam interference structuring using two partial beams, line-like structures can be patterned. The spectrum of incident light, the angle of view, and the spatial periodicity in the nano- and micrometer range determine the color perception of the human eye. In this study, an ultrashort pulsed laser radiation of 10 ps and a wavelength of 532 nm are used for ablation of line-like spatial periods of 1.00, 1.25, and 1.50 μm under ambient conditions. The texture quality is analyzed by atomic force microscopy and scanning electron microscopy. Two materials with similar densities but different thermal conductivities and thermal diffusivities, stainless steel and brass, are structured and analyzed. Scanning electron microscopy images show the influence of heat-affected zones on texture. Electron backscatter diffraction analyses characterize the influence on the microstructure in the marginal regions. [ABSTRACT FROM AUTHOR]

In machining, tool wear is one of the main influence factors for the resulting quality of the product. Several wear mechanisms have to be addressed to prevent unwanted deterioration of the tool integrity. For aluminium alloys, adhesion of the workpiece material on the cutting tool is one of the most challenging wear mechanisms. Engineering surface microtopographies has been proved as a convenient strategy to tackle this issue. Particularly, the direct laser interference patterning (DLIP) technique enables the integration of periodic structures on the micrometer or sub-micrometer scale. In this study the structuring of tungsten carbide with different cobalt content is presented. The interference of two laser beams leads to periodic line-like structures with a spatial period of 5.5 µm. A maximum structure depth of 2.2 µm is reached by controlling the processing parameters. Moreover, the wettability of the structured samples was analyzed by contact angle measurements with selected cooling lubricants, revealing a hydrophilic behavior with a decreased contact angle of 10°. This work gives an insight into the possibilities of structuring tungsten carbide materials with a picosecond laser source in combination with an innovative beam shaping setup from the point of view of an analysis to explore the formation of structured surfaces and their wetting behavior with cooling lubricants. [ABSTRACT FROM AUTHOR]

Direct Laser Interference Patterning (DLIP) is used to texture current collector foils in a roll-to-roll process using a high-power picosecond pulsed laser system operating at either fundamental wavelength of 1064 nm or 2nd harmonic of 532 nm. The raw beam having a diameter of 3 mm @ 1/ e ^2 is shaped into an elongated top-hat intensity profile using a diffractive so-called FBS®-L element and cylindrical telescopes. The shaped beam is split into its diffraction orders, where the two first orders are parallelized and guided into a galvanometer scanner. The deflected beams inside the scan head are recombined with an F-theta objective on the working position generating the interference pattern. The DLIP spot has a line-like interference pattern with about 15 μ m spatial period. Laser fluences of up to 8 J cm ^−2 were achieved using a maximum pulse energy of 0.6 mJ. Furthermore, an in-house built roll-to-roll machine was developed. Using this setup, aluminum and copper foil of 20 μ m and 9 μ m thickness, respectively, could be processed. Subsequently to current collector structuring coating of composite electrode material took place. In case of lithium nickel manganese cobalt oxide (NMC 622) cathode deposited onto textured aluminum current collector, an increased specific discharge capacity could be achieved at a C -rate of 1 °C. For the silicon/graphite anode material deposited onto textured copper current collector, an improved rate capability at all C -rates between C /10 and 5 °C was achieved. The rate capability was increased up to 100% compared to reference material. At C -rates between C /2 and 2 °C, the specific discharge capacity was increased to 200 mAh g ^−1 , while the reference electrodes with untextured current collector foils provided a specific discharge capacity of 100 mAh g ^−1 , showing the potential of the DLIP technology for cost-effective production of battery cells with increased cycle lifetime.

Recently, process monitoring emerges as a breakthrough technology in industrial laser machines applications to enhance process stability and economic efficiency while ensuring high-quality processing parts and significantly reducing scrap rate. Furthermore, the latest advances in monitoring systems open a broad range of new opportunities to increase the capabilities of laser surface structuring. In this study, stainless steel and aluminum substrates are structured with a line-like geometry by Direct Laser Interference Patterning. A high-speed infrared camera is used to detect the thermal effects throughout the laser process. Simultaneously, a diffraction measurement system is implemented to analyze the quality of the fabricated periodic patterns by comparing the diffraction order characteristics. This specific combination of the systems allows a remarkably high-performance process monitoring and quality assurance. The obtained results reveal a correlation between the signals detected by the infrared camera and the intensity of the diffraction orders recorded with the quality of the surface reached. [ABSTRACT FROM AUTHOR]

Metallic samples with unique micro- and nano-scale surface structures can easily be fabricated with Direct Laser Interference Patterning. Like in all laser processes, the material interacts with the laser radiation and as a result, thermal effects occur. These effects have a significant influence on the resulting quality of the surface patterns. In this study, the thermal effects occurring during Direct Laser Interference Patterning of stainless steel and aluminum sheets are investigated. The used experimental setup consisted of a picosecond pulsed laser source operating at 532 nm wavelength, combined with a two-beam interference optical head. An infrared camera in an off-axis position is used to detect the resulting thermal radiation of the laser process varying different process parameters such as laser power and repetition rate. The obtained results reveal a correlation between the recorded signal by the infrared camera and the reached surface quality. They show an impact of the thermal effects on the quality of the surfaces and the amount of solidified material on the resulting line-like pattern. Threshold values of the detected infrared signal detected are determined to classify the obtained surface conditions. [ABSTRACT FROM AUTHOR]

Hybrid films of poly (methacrylic acid) (PMAA) and silver fluorescent nanoclusters were structured by the Direct Laser Interference Patterning technique (DLIP) using few nanosecond laser pulses (355 nm) to obtain an arranged and well-defined periodic surface pattern. It is shown that the films of neat PMAA are not structured by irradiation at 355 nm due to its low absorbance at this wavelength. However, the silver nanoclusters act as chromophores increasing the absorbance of the film and allowing the surface patterning. Also, the interaction of the laser beam with a film of PMAA doped with Ag+ induces the formation of fluorescent silver nanoclusters and the subsequent surface patterning. The patterned films maintain the luminescent properties of the PMAA/Ag nanoclusters in solution. The structured and unstructured films were characterized by optical, atomic force and spectral laser confocal scanning microscopy showing highly oriented lines patterning already after just one laser pulse. Unlabelled Image • UV laser interference patterning of silver nanocluster-containing films was induced. • Laser interference induces photoreduction and patterning on silver cation-containing films. • Fluorescent properties of silver cluster-containing films are preserved upon patterning. [ABSTRACT FROM AUTHOR]

Refractive index modification by laser micro-structuration of diffractive optical devices in ophthalmic polymers has recently been applied for refractive correction in the fields of optics and ophthalmology. In this work, Safrofilcon-A hydrogel, used as soft contact lenses, was processed by direct laser interference patterning (DLIP) to fabricate linear periodic patterns on the surface of the samples. Periodic modulation of the surface was attained under two-beam interference by using a Q-switched laser source with emission at 263 nm and 4 ns pulse duration. Features of processed areas were studied as a function of both the interference spatial period and the laser fluence. Optical confocal microscopy used to evaluate the topography of the processed samples showed that both structured height and surface roughness increased with laser fluence. Static water contact angle (WCA) measurements were carried out with deionized water droplets on the structured areas to evaluate the hydration properties of DLIP structures. It was observed that the laser structured areas induced a delay in the hydration process. Finally, microstructural changes induced in the structured areas were assessed by confocal micro-Raman spectroscopy showing that at low laser fluences the polymer structure remained almost unaltered. In addition, Raman spectra of hydrated samples recovered the original shape of areas structured at low laser fluence.

[Display omitted] • Fabrication of multisymmetric 2D periodic structures using 2 beams interferences. • 2D Bravais lattice and quasicrystal-like patterns can be obtained. • Isotropic reflectivity is approached by increasing the number of symmetry axis. • Decrease of specular reflectivity at 520 nm and 2 µm by a factor 105. A novel method based on Direct Laser Interference Patterning with 2 beams allowing to pattern surfaces with 2 dimensional periodicity and presenting several symmetry axes has been developped. It consists in the picoseond laser writing of the same 1D pattern for different sample orientations. The resulting final morphology depends on the rotation angle between the patterning steps and the number of scans. This way, various 2D short and long range periodic structures can be obtained. We show that by increasing the number of symmetry axes, the sample orientation dependance of the reflectivity, measured at 2 different wavelengths, is reduced, approaching an isotropic behaviour beyond 3 axes. Moreover, as the number of symmetry axes increases, the reflectivity decreases reaching ∼10−3 %, which is explained by the increase of roughness. [ABSTRACT FROM AUTHOR]

Microtextures generated by pulsed lasers allow for changing the surface properties of a wide palette of materials by replicating nature's most effective topographies. In the case of laser-induced microtextures, the surface's wetting properties evolve over time and eventually stabilize. The size of the fabricated features and the initial surface roughness strongly influence this transition and play a key role in the determination of the final wetting state. This work aims to study the wettability of textured stainless-steel with two different surface finishes. Nanosecond Direct Laser Interference Patterning was applied to fabricate a wide range of dot-like microtextures that were evaluated in terms of surface roughness. The water contact angle was monitored for up to 90 days, showing a transition from hydrophilic to hydrophobic. Applying the Wenzel model, the wettability transition was analyzed in regard to surface roughness, and the transition of the average Young contact angle could be extrapolated. In the steady-state, the textured surfaces exhibited the rose-petal effect, where contact angles up to 154.4° were attributed to the microtextures, while a simultaneous high drop adhesion could be related to the initial surface finish. Measurements with water and diiodomethane showed that the textures were both hydrophobic and oleophilic in the steady-state. The surface free energy was estimated and decreased on all textures compared to the untextured reference. [Display omitted] • Fabrication of two-dimensional microtextures in one process step on rough surfaces • Process parameters correlated to Wenzel roughness factor • Average Young contact angle over time estimated through fit to Wenzel equation • Surface in steady-state are (super-)hydrophobic and (super-)oleophilic. • Surfaces show rose-petal effect; High drop adhesion comes from initial roughness. [ABSTRACT FROM AUTHOR]

[Display omitted] • Highly regular homogeneous structuring can be obtained by using interferences. • Periodicity can be reduced to 0.45λ and LIPSS present a low orientation dispersion. • Regular structures with half or a third of the interference period can be obtained. • This nanostructuring depends on the polarization and on the interference period. Direct Laser Interference Patterning (DLIP) and Laser Induced Periodic Surface Structures (LIPSS) are two distinct, key technologies for texturing of surfaces with structures in the range comprised between some hundreds of nm and few µm. Based on different physical phenomena, they enable structures often showing distinct morphologies and pitches. It has been shown the possibility of superposing LIPSS over DLIP with a final multi-scale, hierarchical morphology where the two structures coexist. Here, we report an experimental study showing that it is possible to turn from a hierarchical to mutual interacting texturing where DLIP and LIPSS give rise to a novel, uniform texturing having a homogeneous morphology with highly regular periodicity and low orientation dispersion. Moreover, we show that our approach makes possible an unexpected, significant reduction of the LIPSS period from the conventional values (roughly comprised between 0.7λ – 0.9 λ) to ∼0.45λ. In addition, for all the tests an industrial set-up has been used including an IR, ps laser, an f-theta lens with relatively high NA and a galvo scanner making possible the extension of the process over large area. Results are discussed in the light of different LIPSS generation models. We believe that our results represent a significant step forward through the further miniaturisation of laser induced morphologies opening the possibility for innovative surface functionalisations (anti-reflective, wide spectrum antimicrobial activity) to be exported in an industrial environment. [ABSTRACT FROM AUTHOR]

• Direct laser interference patterning to generate the hierarchical micro-nano structure on SiC. • The aspect ratio control capability of USP-DLIP is higher than that of a single Gaussian beam. • The peak of crystalline SiC decreased more than single-beam irradiation. This study used the ultrashort pulsed direct laser interference patterning (USP-DLIP) and a single Gaussian beam process to generate the surface structures on 4H-SiC. The diffractive optical element separated the single beam into two beams and then interfered on the 4H-SiC surface for material processing. The experimental results obtained by USP-DLIP and a single Gaussian beam process were compared. USP-DLIP uses the superposition of two femtosecond laser beams to re-adjust the laser energy distribution. It is characterized by simultaneously producing hierarchical surface structures, e.g., DLIP structure, high spatial frequency LIPSS (HSFL), and low spatial frequency LIPSS (LSFL). Also, the aspect ratio control capability of USP-DLIP is higher than that of a single Gaussian beam. Furthermore, after 4H-SiC was irradiated with USP-DLIP, the Raman results found that the peak of crystalline SiC decreased more than single-beam irradiation. [ABSTRACT FROM AUTHOR]

The direct laser interference patterning method was used to fabricate periodic structures on flat and cylindrical specimens of austenitic stainless steel (X5CrNi18-10) and martensitic bearing steel (100Cr6). In such laser processing the characterization of materials is a central issue. Therefore, this paper places special emphasis on the chemical, metallurgical and mechanical characterization of the laser processed metals. During the patterning process of 100Cr6, the carbides in the martensitic structure appear as elongated needles and become aligned to the direction of melt flow. The chemical composition analysis before and after laser interference patterning did not reveal any significant changes to either material. However, the surface hardness of 100Cr6 decreased when energy densities above 2.0 J/cm were applied. For cylindrical specimens, the grating period changes with position along the circumference. The observed difference, however, is less than ∼6.4% for rotation angles smaller than 20°. [ABSTRACT FROM AUTHOR]

We have studied the fabrication of hierarchical periodic microstructures on metals by means of direct laser interference patterning. A nanosecond pulsed Nd: YAG laser at 355 nm wavelength was used to produce the microstructures with grating periods ranging from 1 μm to 10 μm on stainless steel, titanium, and aluminum. The results indicate that the geometrical characteristics of the interference patterns as well as the thermal properties of the substrates determine the quality of the fabricated structures. in particular the best structures were obtained when the material at the interference minima position remained in the solid state and the temperature at the interference maxima is below the vaporization temperature. Thermal simulations by finite element method were carried out modeling photothermal interactions of the interference pattern with the metallic substrates to evaluate laser induced thermal effects, such as temperature distribution and temperature gradients and, thus, enabling us to explain the obtained results. [ABSTRACT FROM AUTHOR]

Abstract: We have developed a patterning procedure based on selective ablation using interference patterns with ns-laser pulses to fabricate periodic arrays on large areas of poly(3,4-ethylene dioxythiophene)-poly(4-styrene sulfonic acid) (PEDOT-PSS) thin films over a metallic gold–palladium layer. Single pulse laser-ablation experiments were performed to study the ablation characteristics of the thin films as a function of the film thickness. The ablation threshold fluence of the PEDOT-PSS films was found to be dependent on thickness with values ranging from 43mJ/cm2 to 252mJ/cm2. Additionally, fluences at which the PEDOT-PSS films could be ablated without inducing damage in the underlying metallic films were observed (128mJ/cm2 and 402mJ/cm2 for film thicknesses of 70nm and 825nm, respectively). Linear periodic arrays with line spacings of 7.82μm and 13.50μm were also fabricated. The surface topography of these arrays was analyzed using scanning electron and atomic force microscopy. For thicker polymeric layers, several peeled sub-layers of the conjugated polymer with average thicknesses of about 165–185nm were observed in the ablation experiments. The size and scale of structures produced by this technique could be suitable for several biomedical applications and devices in which controlling cell adhesion, promoting cell alignment, or improving biocompatibility are important. [Copyright &y& Elsevier]

In numerous fields such as aerospace, the environment, and energy supply, ice generation and accretion represent a severe issue. For this reason, numerous methods have been developed for ice formation to be delayed and/or to inhibit ice adhesion to the substrates. Among them, laser micro/nanostructuring of surfaces aiming to obtain superhydrophobic behavior has been taken as a starting point for engineering substrates with anti-icing properties. In this review article, the key concept of surface wettability and its relationship with anti-icing is discussed. Furthermore, a comprehensive overview of the laser strategies to obtain superhydrophobic surfaces with anti-icing behavior is provided, from direct laser writing (DLW) to laser-induced periodic surface structuring (LIPSS), and direct laser interference patterning (DLIP). Micro-/nano-texturing of several materials is reviewed, from aluminum alloys to polymeric substrates. [ABSTRACT FROM AUTHOR]

• WO 3 sensors only-annealed and processed by DLIP after annealing have been fabricated. • Two-dimensional DLIP generates stripes with a hole structure on the WO 3 surface. • Tetragonal crystal structure is shown for the two types of WO 3 layers. • DLIP processed sensing layers present great selectivity towards NO2 with a LOD of 10 ppb. • NO 2 detection mechanism for the two types of sensors is discussed and analyzed. In this paper two kind of sensors based on WO 3 sputtered by magnetron sputtering and annealed at 600 °C have been studied. The first kind was processed by two-dimensional direct laser interfering patterning (DLIP) and the second one without any additional treatment. Morphological and structural characterization have shown a hole structure in a periodic line-pattern for the DLIP-processed sensors and a flat surface for the only-annealed sensors, both with a tetragonal WO 3 phase. TOF-SIMS analysis has revealed that the first WO 3 layers are reduced for both samples, which could improve sensing performance. Promising response enhancement of DLIP-processed sensors has been observed for low concentrations of NO 2 (from 0.5 ppm to 5 ppm) at 200 °C, lowering the limit of detection (LOD) to 10 ppb, half of the LOD of the only-annealed sensors (20 ppb). Cross sensitivity to CO and HCHO have been investigated and the sensing mechanisms discussed. [ABSTRACT FROM AUTHOR]

Refractive index modification by laser micro-structuration of diffractive optical devices in ophthalmic polymers has recently been applied for refractive correction in the fields of optics and ophthalmology. In this work, Safrofilcon-A hydrogel, used as soft contact lenses, was processed by direct laser interference patterning (DLIP) to fabricate linear periodic patterns on the surface of the samples. Periodic modulation of the surface was attained under two-beam interference by using a Q-switched laser source with emission at 263 nm and 4 ns pulse duration. Features of processed areas were studied as a function of both the interference spatial period and the laser fluence. Optical confocal microscopy used to evaluate the topography of the processed samples showed that both structured height and surface roughness increased with laser fluence. Static water contact angle (WCA) measurements were carried out with deionized water droplets on the structured areas to evaluate the hydration properties of DLIP structures. It was observed that the laser structured areas induced a delay in the hydration process. Finally, microstructural changes induced in the structured areas were assessed by confocal micro-Raman spectroscopy showing that at low laser fluences the polymer structure remained almost unaltered. In addition, Raman spectra of hydrated samples recovered the original shape of areas structured at low laser fluence.

Objective The aim of this work is to generate micrometric linear patterns with different topography on dental grade zirconia by means of UV laser interference and to assess the quality of the produced surface, both in term of the geometry produced and of the surface damage induced in the material. Methods The third harmonic of a Q-switched Nd:YAG laser (355 nm, pulse duration of 10 ns and repetition rate of 1 Hz) was employed to pattern the surface of 3Y-TZP with micrometric-spaced lines. The resulting topography was characterized with White Light Interferometry and Scanning electron microscopy: pattern depth ( H ), amplitude roughness parameters ( S a , filtered- S a ), Fourier spatial analysis and collateral damages were related to laser fluence and number of pulses employed. Results With our experimental setup, line-patterning of zirconia surfaces can be achieved with periodicities comprised within 5 and 15 μm. Tuning laser parameters allows varying independently pattern depth, overall roughness and surface finish. Increasing both fluence and number of pulses allows producing deeper patterns (maximum achievable depth of 1 μm). However, increasing the number of pulses has a detrimental effect on the quality of the produced lines. Surface damage (intergranular cracking, open porosity and nano-droplets formation) can be generated, depending on laser parameters. Significance This work provides a parametric analysis of surface patterning by laser interference on 3Y-TZP. Best conditions in terms of quality of the produced pattern and minimum material damage are obtained for low number of pulses with high laser fluence. With the employed method we can produce zirconia materials with controlled topography that are expected to enhance biological response and mechanical performance of dental components. [ABSTRACT FROM AUTHOR]