We demonstrate the ability to create functionalized microfluidic channels using Femtosecond Laser Surface Processing (FLSP). FLSP is an emerging advanced manufacturing technology used to modify the surface properties of materials directly and permanently by producing self-organized quasi-periodic micro- and nano-scale surface features along with surface and subsurface chemical and grain structure changes. We demonstrate on Hastelloy X that by controlling the laser fluence and pulse count, the depth of the microchannels and height of the FLSP microstructures within the microchannels can be controlled independently.
Copper substrates were functionalized using ultrashort pulsed laser surface processing to create micro- and nano-scale features to enhance the electrocatalytic nature of the surfaces for the electrochemical reduction of carbon dioxide. Post-processing of the laser-functionalized copper surfaces was carried out to increase the efficiency and stability of the electrocatalyst. A maximum current density of 55 mA/cm2 was achieved during carbon dioxide reduction reaction (CO2RR) over the laser processed copper, and the stability of the surface was enhanced by modifying the chemical and physical nature of the surfaces. The surfaces were chemically and physically analyzed before and after the reduction reaction.
Surfaces with high directional electromagnetic absorption or emission in the infrared (IR) region of the electromagnetic spectrum have numerous potential applications, however many of the relevant surfaces suffer from extremely narrow bandwidth and/or polarization dependence. Here we demonstrate broadband directional emissivity in the mid-infrared range of 7.5 to 14 μm, that is not dependent on polarization. This was achieved with angled micro-scale structures that are overlaid with nano-scale features on stainless steel 304 produced using an emerging fabrication technique known as femtosecond laser surface processing (FLSP). FLSP is an advanced surface functionalization technique that produces hierarchical micro- and nano-scale quasi-periodic surface features in a single laser processing step. Here we report a surface with peak emission for an angle of 55° using FLSP to create fin-shaped micro- and nano-scale surface features that are tilted at a 55° angle. Cross sectioning of the fin-shaped structures using focused ion beam milling was performed to understand the morphology and subsurface microstructure. Cross-sectional images and energy dispersive X-ray spectroscopy analysis show the structure consists of a thin redeposited oxide layer and the bulk of the fin structure is consistent with the original stainless-steel alloy. The emission results are verified by full-wave electromagnetic simulations which consider all the diffraction-orders performed utilizing the finite element method software, COMSOL Multiphysics, that predicts with reasonable accuracy the resulting directional emissivity of the laser processed surface.
Femtosecond laser surface processing (FLSP) is a material processing technique used to produce self-organized micro/nanostructures on metals. The hierarchal structures can improve the surface properties of materials when applied to specific applications such as enhancing heat transfer. In this paper, we demonstrate a recently developed technique termed multi-material, multi-layer FLSP (3ML-FLSP). With 3ML-FLSP, micro/nanoscale features can be produced that are composed of multiple materials by processing surfaces using traditional FLSP techniques that are layered with thin foils of different materials. We demonstrate results with three layers of different metals (304 stainless steel, copper, and aluminum) clamped together during laser processing to create structures composed of all three metals. Ion beam milling is used to cross-section structures for subsurface analysis of the microstructure. The three metals did not mix within the bulk of the microstructures indicating that the microstructures were produced primarily through preferential removal of material around the structures. However, there was mixing of all three materials within the nanoparticle layer that covers the microstructures.
We have shown recently that unique optical signatures can be observed with the measurement of ultrashort middle infrared laser pulses that have been transmitted through molecular vapors. Here, we report on an increased signal-to-noise ratio of the pulse measurements by using a cross-correlation technique with a lockin amplifier. Carbon tetrafluoride and dimethyl methylphosphonate (DMMP) cross-correlation signatures are highly discriminated using principal component analysis. A squared exponential Gaussian process regression model is used to quantitatively predict the concentration of DMMP.
Femtosecond laser surface processing (FLSP) is a unique material processing technique that can produce self-organized micro/nanostructures on most materials including metals, semiconductors, and dielectrics. These structures have demonstrated the enhancement of surface properties such as heat transfer and broadband light absorption. The chemical composition and morphology of FLSP structures is highly dependent on processing parameters including background gas composition, pressure, laser fluence, and number of laser pulses. When the laser processing is carried out in open atmosphere, a thick oxide layer forms on the FLSP surface structures due to the high reactivity of the surface with the environmental constituents immediately after laser processing. In this work, N2 and forming gas are used during laser processing in an effort to form a metal nitride on the surface of aluminum. Aluminum nitride is a promising material for enhancing the heat transfer performance of surfaces because of its thermal conductivity, which can be as high as 285 W/m-K, whereas aluminum oxide has a low thermal conductivity (30 W/m-K). Aluminum nitride incorporation into FLSP surfaces has the potential to act as a passivation layer to decrease the oxygen content and increase the thermal conductivity of the surface. Nitrogen incorporation is studied by applying FLSP in air, N2, and a 95% N2/5% H2 mixture. The chemical composition of the FLSP surfaces is determined by X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS). Cross-sectional analysis of the FLSP microstructures is performed using ion beam milling.
The capability to produce femtosecond laser pulses with wavelengths in the atmospheric absorption window requires a new understanding of pulse propagation effects. In this work, we characterize the changes in temporal propagation of middle infrared femtosecond laser pulses by cross-correlation frequency resolved optical gating (XFROG). The temporally distorted infrared pulses are cross-correlated with 800 nm pulses by a four-wave mixing process in air. For the first time, we investigate these propagation effects through gas molecules that are not present in the atmosphere. Each molecule is shown to have a unique effect on the temporal propagation of the pulse that is wavelength dependent. We verify our experimental data with simulations based on a KramersKronig transformation of spectral data from the HITRAN database. The propagation effects are similar to optical free induction decay. Multiple vibrational and rovibrational absorption lines are excited by the middle infrared pulse and constructive interference occurs at various delay times relative to the initial pulse. The constructive interference impresses a unique fingerprint onto the pulse because the spectral lines of each molecule are unique. The fingerprint behaves as a nonlinear function related to the molecular concentration. To account for this, a regression model is developed to predict the concentration of unknown gas species. The middle infrared beam is the only laser beam sensitive to the analytes. Thus, standoff detection is a possibility since the XFROG can be performed locally.
The use of self-organized micro/nanostructured surfaces formed using femtosecond laser surface processing (FLSP) techniques has become a promising area of research for enhancing surface properties of metals, with many applications including enhancing heat transfer. In this work, we demonstrate advantages of the use of dual-pulse versus single-pulse FLSP techniques to produce self-organized micro/nanostructures on copper. With the dual-pulse technique, the femtosecond pulses out of the laser (spaced 1 ms apart) are split into pulse pairs spaced < 1 ns apart and are focused collinear on the sample surface. Single-pulse FLSP techniques have been widely used to produce self-organized “mound-like” structures on a wide range of metals including a number of stainless steel alloys, aluminum, nickel, titanium, and recently on copper. Due to its high thermal conductivity, copper is used in many critical heat transfer applications and micro/nanostructured copper surfaces are desired to further improve heat transfer characteristics. Using single-pulse (pulses spaced 1 ms apart) FLSP techniques, self-organized microstructure formation on copper requires much higher pulse fluence than is commonly used for producing microstructures on other metals, which results in instabilities during laser processing (non-uniform surfaces), low processing efficiency, and limitations on the control of the types of structures produced. In this paper, we report results that demonstrate that the dual-pulse FLSP technique can be used to produce microstructures on copper more efficiently than using single-pulse FLSP, with better control of the surface structures produced. Cross-sectional subsurface microstructure analysis is also presented for single-pulse versus dual-pulse FLSP functionalized copper surfaces.
Femtosecond Laser Surface Processing (FLSP) is a versatile technique for the fabrication of a wide variety of micro/nanostructured surfaces with tailored physical and chemical properties. Through control over processing conditions such as laser fluence, incident pulse count, polarization, and incident angle, the size and density of both micrometer and nanometer-scale surface features can be tailored. Furthermore, the composition and pressure of the environment both during and after laser processing have a substantial impact on the final surface chemistry of the target material. FLSP is therefore a powerful tool for optimizing interfacial phenomena such as wetting, wicking, and phasetransitions associated with a vapor/liquid/solid interface. In the present study, we utilize a series of multiscale FLSPgenerated surfaces to improve the efficiency of vapor generation on a structured surface. Specifically, we demonstrate that FLSP of stainless steel 316 electrode surfaces in an alkaline electrolysis cell results in increased efficiency of the water-splitting reaction used to generate hydrogen. The electrodes are fabricated to be superhydrophilic (the contact angle of a water droplet on the surface is less than 5 degrees). The overpotential of the hydrogen evolution reaction (HER) is measured using a 3-electrode configuration with a structured electrode as the working electrode. The enhancement is attributed to several factors including increased surface area, increased wettability, and the impact of micro/nanostructures on the bubble formation and release. Special emphasis is placed on identifying and isolating the relative impacts of the various contributions.
Femtosecond laser surface processing (FLSP) is a powerful technique used to create self-organized microstructures with nanoscale features on metallic surfaces. By combining FLSP surface texturing with surface chemistry changes, either induced by the femtosecond laser during processing or introduced through post processing techniques, the wetting properties of metals can be altered. In this work, FLSP is demonstrated as a technique to create superhydrophobic surfaces on grade 2 titanium and 304 stainless steel that can retain an air film (plastron) between the surface and a surrounding liquid when completely submerged. It is shown that the plastron lifetime when submerged in distilled water or synthetic stomach acid is critically dependent on the specific degree of surface micro- and nano-roughness, which can be tuned by controlling various FLSP parameters. The longest plastron lifetime was on a 304 stainless steel sample that was submerged in distilled water and maintained a plastron for 41 days, the length of time of the study, with no signs of degradation. Also demonstrated for the first time is the precise control of pulse fluence and pulse count to produce three unique classes of surface micron/nano-structuring on titanium.
Femtosecond Laser Surface Processing (FLSP) is a powerful technique for the fabrication of self-organized multiscale
surface structures on metals that are critical for advanced control over energy transfer at a liquid/solid interface in
applications such as electrolysis. The efficiency of the hydrogen evolution reaction on stainless steel 316 electrodes in a
1 molar potassium hydroxide solution is used to analyze the role of surface geometry to facilitate the phase conversion of
the liquid to a gaseous state in the vicinity of the interface. It is found that the efficiency of the electrolysis process is
directly related to the separation of micro-scale features on an electrode surface. The enhancement is attributed to the
size of the valleys between microstructures controlling the contact between an evolving vapor bubble and the electrode
surface. The results suggest an alternative pathway for the tailoring of interfacial energy transfer on structured surfaces
separate from traditional benchmarks such as surface area and contact angle.
There are a growing number of unique self-organized micro/nanostructures created using femtosecond laser surface
processing that have been demonstrated. Although researchers have provided insight into the formation processes for
distinctive morphologies on specific materials, there is a need for a broader understanding of the physics behind the
formation of a wide range of morphologies and what parameters affect their formation. In this work, the formation
processes for mound structures on 316 stainless steel (SS) with growth above the original sample surface are studied.
The formation process for the structures on 316 SS is compared to similar structures formed on nickel using the same
technique. The structures are formed using 800 nm, 50 fs laser pulses, and are self-organized, meaning the structure
dimensions are much smaller than the spot size of the pulses used to create them. The formation dynamics were studied
using a stop-motion scanning electron microscope (SEM) technique, where the same location of an irradiated sample
was imaged in the SEM at various pulse counts. The result is a series of images showing the developmental progress
with increasing pulse counts. The structures form through a combination of fluid flow of the surface melt that results
after irradiation, preferential ablation of the center of the pits between structures, and material/nanoparticle redeposition.
We describe the evolution of laser damage spots on bulk nickel generated by multipulse femtosecond laser irradiation
with a 100 μm x 100 μm square flat-top beam profile as a function of the laser fluence and the number of pulses incident
on the target. This large-area irradiation simulates conditions associated with the interaction of femtosecond laser pulses
on a remote target. The larger area laser damage sites are characterized either by a series of self-organized surface
structures with micro- and nanoscale features or a deep circular pit rather than a crater that mirrors the beam profile.
Furthermore, the ablation rate of the deepest feature sharply increases above a laser fluence of 2 J/cm2; this increase is
associated with the creation of a deep circular ablation pit generated during ablation with the first few pulses on the
sample that continuously grows upon multipulse irradiation due to the focusing of incident laser energy into the pit by
the sloped pit surfaces.
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