Laser Induced Research Articles

Experimental study on the instability characteristics of CO2-diluted n-butane/air cellular flames on McKenna burner

Present experimental study reports on the effects of CO2 dilution on intrinsic instabilities of non-adiabatic n-butane/air cellular flames on McKenna burner at initial 300 K temperature and 0.1 MPa pressure. The Planar Laser Induced Fluorescence (OH-PLIF and CH2O-PLIF) optical diagnostic technology was utilized to probe the structure of cellular flames. The results show that the cellular flames can be obtained under the conditions with lean, stoichiometric and rich CO2 diluted n-butane/air mixtures. At lean conditions, hydrodynamic instability was dominant and the cellular flames were formed by connected wrinkles. However, at rich conditions, diffusive-thermal instability was dominant and the cellular flames were formed by separated cells. It confirmed that the CO2 dilution can enhance flame instability significantly, as well as the enhanced effect was positive correlation with CO2 dilution ratio. Furthermore, the enhanced effects of flame instability was stronger by CO2 dilution than by N2 dilution. The chemical and thermal effects by CO2 dilution were attributed to enhance flame instability.

Dissecting Collective Cell Behavior in Migrating Testis Myotubes in Drosophila.

Collective cell migration has a key role in tissue morphogenesis, wound healing, tissue regeneration, and cancer invasion. In recent years, different animal models have been established to analyze how chemical and mechanical stimuli shape the behavior of single cells into tissues and organs. At present, there are still only a few model systems that allow to genetically dissect underlying molecular mechanisms driving cell motility during tissue morphogenesis at high resolution in real time. Here, we provide a detailed protocol and toolbox for ex vivo culturing of Drosophila testes for 4D live imaging of myotube collective migration, which allows to genetically address a wide range of developmental and cell biological questions regarding modes of filopodia-based protrusion/locomotion, cell-cell adhesion, cytoskeletal modes of collective decision-making, and collective closure processes. Additionally, this protocol has been successfully used in combination with laser-induced single-cell ablation and pharmacological treatments, but it can also be used with confocal microscopy after tissue fixation.

Drive of micro-pump by Nanosecond Laser Induced Explosion

Drive of micro-pump by Nanosecond Laser Induced Explosion

Planar Laser Induced Fluorescence of OH for Thermometry in a Flow Field Based on Two Temperature Point Calibration Method

In view of the uncertainty in the calibration process of two-color plane laser-induced fluorescence (PLIF) temperature measurement, a new calibration method is proposed, in which the influence of fluorescence yield is considered. The calibration process was carried out at high and low temperature region, respectively. Then, the bias of thermometry results origin from quenching is restrained. This new calibration method is validated in a jet flame with temperature range of 1300–1800 K. Here, the temperature results from Coherent Anti-Stokes Raman scattering (CARS), single-point calibrated PLIF, and two-point calibrated PLIF are all acquired with the maximum standard errors of 13 K, 36 K, and 37 K, respectively. The temperature deviation between the average results from PLIF and Coherent Anti-Stokes Raman scattering (CARS) is 120 K and 10 K, when the two-point and one-point calibration methods are used. Therefore, the two-point calibrated PLIF is preferred in the combustion field, especially with a large temperature range and strong quenching coefficient.

Ablation threshold of GaN films for ultrashort laser pulses and the role of threading dislocations as damage precursors.

The laser-induced ablation threshold of c-plane GaN films upon exposure to ultrashort laser pulses was investigated for different wavelengths from the IR to the UV range and pulse widths between 0.34 and 10 ps. The one-pulse ablation threshold ranges between 0.15 and 3 J/cm2 and shows an increase with the wavelength and the pulse width, except for deep UV pulses. Based on a rate equation model, we attribute this behavior to the efficiency of seed carrier generation by interband absorption. In addition, the multi-pulse ablation threshold was analyzed. Accumulation effects are more prominent in case of IR than with UV pulses and are closely linked to damage precursors. By a thorough structural investigation, we demonstrate that threading dislocations, especially those with a screw component, significantly contribute to laser damage, since they provide a variety of dispersed states within the band gap.

Analysis of the characteristics of microwave-enhanced laser-induced atmospheric air plasma and ablation plasma for Al target

The investigation and comparison of air plasma generated directly in the absence as well as in the presence of solid target along with microwave enhancement were reported. The plasma characteristics including emissions, size, duration, and temperature were measured using high-speed camera, optical emission spectroscopy, and temperature approximation through spectral fitting. The laser energy required for ablation is much lower than the air breakdown energy but, the minimum energy density for air and alumina breakdown is inversely proportional to the focal lengths. The size and volume of the air plasma generated by the microwave and ablation plasma interactions were less than those of the microwave-enhanced laser-induced air plasma but was similar with standard laser-induced air plasma. When comparing the plasma lifetimes, both air plasmas (with and without a solid target) exceeded the duration of the microwave pulse input (approximately 1 ms). As the plasma lifetime is a function of electron density, shorter lifespan with the target ∼1500 ± 150 µs compared with the lifetime of the plasma without the target at ∼1800 ± 150 µs were observed. Similar results were observed for the time-series measurements of the OH and N2 second positive (N2PS) molecular emissions. OH and N2PS were not observed within the first 300 µs in the presence of the solid target; these air plasma components were only generated as a “by-product” of the microwave and ablation plasma. The rotational and vibrational temperatures were approximated by spectral fitting of simulated OH and N2PS spectra and the experimentally acquired spectra. The microwave increased the vibrational temperature in a constant rotational temperature during the expansion of the air plasma. The same trend is observed with the presence of the alumina target.

Machinability improvement in milling of SiCf/SiC composites based on laser controllable ablation pretreatment

The poor machinability of SiCf/SiC composites greatly limits its application and promotion. The laser-induced ablation products of SiCf/SiC composites are powdery, loose and porous. Milling of laser ablated samples demonstrated that the force and heat were almost negligible when milling ablation products. Accordingly, a laser ablation pretreatment milling (LAPM) process of SiCf/SiC composites was proposed. Under the LAPM process, after the laser ablation treatment with controllable depth, the cutting allowance could be achieved in only one pass, which greatly improved the machining efficiency compared with the conventional milling process. The material removal rate was greatly improved on the premise of ensuring the machining quality. Taking the milling of tensile specimens as an example, compared with conventional milling, the total processing time of the specimen was reduced by 31.29 % by LAPM process. Therefore, LAPM provides a potential feasible process scheme for greatly improving the machinability and machining efficiency of SiCf/SiC composites.

Femtosecond Laser Induced Lattice Deformation in KTN Crystal.

In recent years, many novel optical phenomena have been discovered based on perovskite materials, but the practical applications are limited because of the difficulties of device fabrication. Here, we propose a method to directly induce localized lattice modification inside the potassium tantalate niobate crystal by using the femtosecond laser. This selective modification at the processed regions and the surrounding areas is characterized by two-dimensional Raman spectrum mapping. The spectrum variations corresponding to specific lattice vibration modes demonstrate the lattice structure deformation. In this way, the lattice expansion at the femtosecond laser irradiated regions and the lattice compression at the surrounding areas are revealed. Furthermore, surface morphology measurement confirms this lattice expansion and suggests the extension of lattice structure along the space diagonal direction. Moreover, the existence of an amorphization core is revealed. These modifications on the sample lattice can induce localized changes in physicochemical properties; therefore, this method can realize the fabrication of both linear diffraction and nonlinear frequency conversion devices by utilizing the novel optical responses of perovskite materials.

Combined effects of polarization and secondary ablation on precision machining of microgrooves by laser-induced microjet-assisted ablation.

In this study, we propose a polarization-modulated laser-induced microjet-assisted ablation method for machining microgrooves with controllable cross-sections. A novel mathematical model is presented to accurately predict the cross-sections by considering the combined effects of polarization and secondary ablation. The simulation and experimental results reveal that the effect of secondary ablation becomes more obvious when steeper grooves are ablated with higher repetition frequency and larger pulse energy. The polarization effect and secondary ablation of target material result in asymmetric ablation of linearly polarized laser beam. To avoid the asymmetric ablation, we present the cylindrical vector beams to achieve scanning-path independent laser micromachining with various cross-sections. Based on the prediction model, the cross-sectional characteristics are precisely designed and fabricated by tuning laser processing parameters. Our work provides a reliable approach for the controllable fabrication of microgrooves at the scale of tens of micrometers.

Simulation of surface regolith gardening and impact associated melt layer production under ns-pulsed laser ablation

The effect of surface regolith gardening and melt layer production produced by space weathering (SW) (owing to micrometeorite bombardment) of surficial regolith layers of airless planetary surfaces was investigated in an experimental setup by using laser-induced ablation of powdered analog material (synthetic Fo100) under vacuum with a ns-pulsed infrared laser. The investigated analog pellets were prepared from the fine fraction (< 1 μm) up to a grain size of 280 μm, which resembles the uppermost regolith surface of many airless planetary bodies. The Fo-powder was pressed into shape to form a pellet. We focused here on nanometer-sized structural modifications that are induced in the relocated grains, sputtered off ejecta material and melt sprinkles that formed away from the craters caused by laser irradiation of the pressed pellet surface. The ejecta particles were redistributed over the entire pellet surface and beyond. The forming sputter film, melt sprinkles and ballistically ejected grains were caught on carbon film grids positioned nearby the craters. The grids were investigated with a transmission electron microscope (TEM) to discern between the distinct deposition types that were formed by ejecta condensate and partially molten ejected nanometer-size analog grains. Apart from a heavily modified pellet surface, we found that deposited droplets are mostly amorphous with minor nanocrystalline subdomains. Eight out of ten droplets show distinct incipient crystallization stages. This indicates at a relatively high amount of amorphous regolith material at the incipient stage of SW for airless bodies, if the regolith is altered via micrometeorite bombardment.

Nanosecond Laser Induced Surface Structuring of Cadmium after Ablation in Air and Propanol Ambient.

In the present study KrF Excimer laser has been employed to irradiate the Cadmium (Cd) targets for various number of laser pulses of 500, 1000, 1500 and 2000, at constant fluence of 3.6 J cm-2. Scanning Electron Microscopy (SEM) analysis was utilized to reveal the formation of laser induced nano/micro structures on the irradiated target (Cd) surfaces. SEM results show the generation of cavities, cracks, micro/nano wires/rods, wrinkles along with re-deposited particles during irradiation in air, whereas subsurface boiling, pores, cavities and Laser Induced Periodic Surface Structures (LIPSS) on the inner walls of cavities are revealed at the central ablated area after irradiation in propanol. The ablated volume and depth of ablated region on irradiated Cd targets are evaluated for various number of pulses and is higher in air as compared to propanol ambient. Fast Fourier Transform Infrared spectroscopy (FTIR), Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD) analyses show the presence of oxides and hydro-oxides of Cd after irradiation in propanol, whereas the existence of oxides is observed after irradiation in air ambient. Nano-hardness tester was used to investigate mechanical modifications of ablated Cd. It reveals an increase in hardness after irradiation which is more pronounced in propanol as compared to air.

High-Precision and Low-Damage Microchannel Construction via Magnetically Assisted Laser-Induced Plasma Ablation for Micro-Thermoelectric Devices.

Thermoelectric devices are developing toward high density and miniaturization with a large filling factor for new applications in chip thermal management and microenergy harvesting. Pulsed laser etching has become one of the most effective tools for the patterning construction of highly integrated micro-thermoelectric devices. However, the laser spot size and Gaussian laser energy distribution restrict the processing size and accuracy of microchannels. Moreover, the rapid temperature rise caused by laser energy injection would also raise serious problems such as element volatilization, cracks, and recast layers. Herein, a liquid-assisted nanosecond laser ablation technology with magnetically controlled plasma is proposed to etch microchannels on thermoelectric thick films. By evaluating the size and shape of microchannels, we theoretically investigated the influence of cavitation bubbles on the laser optical path and surface roughness in laser-induced plasma ablation. In addition, the energy criterion for high-precision ablation is revealed, and the effect of magnetic field on ablation threshold is explained by magnetic constraint on energy and kinetic properties of the laser-induced charged plasma plume. Finally, the high-precision and low-damage microchannels are achieved on Bi2Te3 thermoelectric thick films with a minimum line width of 19.12 μm and a small sidewall inclination degree of tan θ = 0.085. This work provides a promising alternative for the fabrication of high-density three-dimensional (3D) patterning in semiconductor microdevices.

Subaquatic indirect laser ablation technique for glass processing.

Subaquatic indirect Laser-Induced Plasma-Assisted Ablation (SLIPAA) is proposed as a laser-based technique for glass processing. In this configuration, a water layer is added between a metallic target and a soda-lime glass substrate, so the processing of the glass is due to a combination of the ablation mechanism, the shock waves, and the cavitation bubbles. Thus, this method makes it possible to produce higher depth structures than those performed up to now by other standard laser techniques based on ablation, achieving structures in glass with rectangular cross-sectional profiles. Channels of 1 mm width are fabricated, reaching an average maximal depth value of almost 1400 µm at 30 passes of the laser beam while keeping the focal position fixed. Furthermore, the difference between processing the material with and without the addition of the water layer is presented. The influence of the processing parameters on the shape and quality of the fabricated structures is studied by optical and confocal microscopy, microcomputed tomography, and scanning electron microscopy. Compositional analysis of the glass is performed by energy dispersive X-ray technique to assess the transference of material from the metallic target to the fabricated channels. Deeper and more complex structures are obtained by refocusing the laser beam on the target and adding a pulsed flowing water film.

Behavior of child droplets during micro-explosion and puffing of suspension fuel droplets: The impact of the component mixing sequence

The article provides experimental research findings on the behavior of micro-explosion and puffing of two-liquid droplets in different droplet formation schemes: liquid combustible component (rapeseed oil) as the shell with liquid incombustible component (water) as the core and vice versa. The experiments were conducted on fuel compositions that had not been pre-mixed; no coal particles had been added into them. These conditions aimed to reflect industrial production conditions with separate injection and blow-in of components into combustion chambers. Shadow Photography and Planar Laser Induced Fluorescence techniques were used in the experiments. An alcohol burner was used for heating droplets. The temperature of the heating medium was varied in the range of 850–1050 K by adjusting the vertical distance of the droplet from the burner bottom to match the conditions typical for industrial applications, specifically, composite fuel creation technologies and thermal/flame purification of liquids. Conditions have been determined under which droplets of combustible and non-combustible components are atomized during fragmentation thus producing polydisperse and monodisperse aerosols. Effects have been discovered whereby the fragmentation of combustible and noncombustible liquid components produces droplets of different sizes. Correlations between the size and component composition of child droplets with the heating temperature and mixing sequence of the liquids in the parent droplet have been established.

Transparent superhydrophobic glass prepared by laser-induced plasma-assisted ablation on the surface

Transparent superhydrophobic glass prepared by laser-induced plasma-assisted ablation on the surface

One-step in-situ low damage etching of SiC/SiC composites by high-temperature chemical-assisted laser processing

SiC/SiC composites have a promising application prospect in the fields of aero-engine turbine blades and other hot-end components due to their excellent properties. It is difficult to machine SiC/SiC composites by traditional mechanical processing because of the ultra-high hardness and anisotropic character. The routinely laser direct processing in the air produce thermal effect damage, resulting in heat-affected zones and microcracks. Here high-temperature chemical-assisted nanosecond laser processing was investigated to realize one-step in-situ fast etching without causing damage to the structure of SiC/SiC composites. Firstly, the chemical wet etching only occurred in the laser-induced local high-temperature region. There was no additional etching damage to the substrate material in the region where the temperature was relatively low. Secondly, chemical liquid served as a transparent constraint layer, the ablation pressure on the material increased in a liquid confined environment. In comparison with laser direct etching in the air, the exposed fiber was not a needle-like structure and tubular structure but turned into a thin and flat tongue-like structure. The etching behavior of 0° fiber layer, 90° fiber layer, and crossing regions of fibers of different directions at low and high laser intensities were studied. Further, the hardness and friction coefficient of SiC/SiC composites after laser direct etching in the air and high-temperature chemical-assisted laser etching were compared. The chemical elementary composition before and after etching was measured to reveal the oxidized and carbonized characteristics. It was concluded that the action mechanism was the combined result of high-temperature decomposition, laser-induced ablation pressure in the liquid confined region, and local chemical wet etching. The results demonstrate that high-temperature chemical-assisted laser etching has some advantages over traditional laser beam machining and is a potential processing method for SiC/SiC composites.

High-Speed Imaging And Temperature Measurements Of Microwave Enhanced Laser Induced Plasma

Air plasma is typically generated by laser-induced breakdown using a focused high-energy laser beam. In microwaveenhanced laser-induced plasmas, the initial spark produced by laser-induced ablation of a nearby target may be utilized to generate air plasma by microwaves. Air plasma was therefore produced with and without a nearby target. Characterization of the air plasma was done by high speed-imaging and time-series emission spectral measurements. The evolution of the plasma temperature was measured by SPECAIR simulations of the OH and N2+ positive systems. The microwave-enhanced plasma exhibit unequal values for the vibrational and rotational temperatures indicating a nonequilibrium state of the air plasma. The estimated temperatures of vibrational temperatures were considered lower estimates of the electron temperature and rotational temperatures as higher estimates of the gas temperature. The increase in vibrational temperatures of the air plasma with and without the target both indicated the heating of the electrons due to the addition of microwaves. However, the constant value of the rotational temperature for both cases indicated that the gas temperature is not affected by the microwave.

Fuel-rich hetero-/homogeneous combustion of C3H8/O2/N2 mixtures over rhodium

The catalytic (heterogeneous) and gas-phase (homogeneous) combustion of C3H8/O2/N2 mixtures over rhodium was investigated experimentally and numerically at 5 bar and at fuel-rich equivalence ratios φ = 2.0-3.5 relevant to propane Catalytic Partial Oxidation (CPO). In situ spatially-resolved Raman measurements of major gas-phase species concentrations and Planar Laser Induced Fluorescence (PLIF) of formaldehyde were applied in an optically accessible channel-flow reactor to monitor the catalytic and gas-phase processes, respectively, while accompanying 2D simulations were carried out with detailed hetero-/homogeneous chemical reaction mechanisms. Due to the high gas-phase reactivity of propane, homogeneous chemistry could not be ignored over most of the reactor's oxidation zone length (upstream zone where the deficient reactant oxygen is not fully consumed). The presence of gas-phase chemistry deteriorated the otherwise high catalytic syngas (H2 and CO) selectivities over the oxidation zone. Raman measurements of major gas-phase species concentrations over the restricted oxidation zone length without appreciable gas-phase chemistry showed that the catalytic reaction mechanism slightly underpredicted (overpredicted) the H2 (CO) formation. The same behavior was also attested over the remaining length of the oxidation zone where combined catalytic and gas-phase chemistry was present. The production of considerable amounts of H2 at the highest investigated equivalence ratio of 3.5 accelerated the onset of homogeneous ignition and the formation of strong flames. The discrepancies between measured and predicted homogeneous ignition distances were less than 6.8% in all cases, illustrating the validity of the employed hetero-/homogeneous kinetic schemes. Contrary to past methane CPO studies, the contribution of gas-phase chemistry and the formation of strong flames in propane CPO was detrimental to syngas production.

Gold Nanostar@Polyaniline Theranostic Agent with High Photothermal Conversion Efficiency for Photoacoustic Imaging-Guided Anticancer Phototherapy at a Low Dosage.

Due to the strong and tunable photothermal effect, metallic nanoparticles are of enormous interest in light-activated biomedical applications, such as photoacoustic imaging (PAI) and photothermal therapy (PTT). However, the photothermal conversion efficiency (PCE) of existing metallic photothermal agents is still unsatisfactory. Herein, we develop an efficient photothermal theranostic agent based on a gold nanostar@polyaniline core-shell nanocomposite with high PCE for PAI-guided PTT at a low dosage. After optimizing the relative composition of polyaniline (PANI) and gold nanostars (AuNSs), this nanocomposite eventually empowers an outstanding PCE of up to 78.6%, which is much better than AuNSs or PANI alone and most of the existing photothermal theranostic agents. Besides, the nanocomposite can act as a targeted probe for tumors by hyaluronic acid (HA) modification without compromising the photothermal performance. The obtained nanoprobes named AuNSPHs exhibit promising biocompatibility and great performance of PAI-guided PTT to treat triple-negative breast cancer both in vitro and in vivo. More importantly, a single injection of AuNSPHs significantly suppresses tumor growth with a low dosage of Au (0.095 mg/kg), which is attributed to the high PCE of AuNSPHs. Taking advantage of the exhilarating photothermal conversion ability, this theranostic agent can safely potentiate the antitumor therapeutic efficacy of laser-induced ablation and holds great potential for future medical applications.

Experimental study of flow and mass transport in the near-wake region of a rigid planar metal net panel

Aquaculture cages constructed from rigid material have become increasingly popular in recent years due in part to the continuous expansion of the aquaculture industry offshore, where cages need to withstand much harsher ocean conditions. Most previous studies of hydrodynamic interactions with aquaculture cages have explored the impacts of more traditional flexible cage net panels on wake flow fields and mass transport properties. In the current experimental study, Particle Image Velocimetry and Planar Laser Induced Fluorescence techniques were used to measure flow and concentration fields in the near wake region of a steady current through a rigid metal net panel to analyze both the flow patterns and mass transport processes. The results indicated that, similar to a flexible polyethylene net panel, the time-average velocities and turbulence characteristics in the near wake region were highly inhomogeneous and anisotropic, with the turbulent intensity following a short-range, power-law decay downstream of the panel. In direct comparison with a flexible net panel of similar net solidity, the rigid net panel was found to slightly enhance the lateral spreading of the plume, with cross-sectional profiles of the mean concentration and concentration fluctuation shown to exhibit self-similar Gaussian distributions. The rigid net panel also resulted in a lower flow blockage, as measured by a velocity reduction coefficient, with the corresponding turbulent intensity also reduced compared to the flexible panel due to an overall reduction in the drag force. In addition, the rigid panel tended to create larger wake eddies than the flexible panel, leading to enhanced scalar mixing and, hence, faster rates of concentration decay in the near-wake region. These experimental findings have potential implications for cage-based aquaculture operations, where rigid fish cages are comparatively more likely to generate favorable environments for fish growth, as well as helping to reduce negative impacts on the surrounding aquatic environment.