Laser-induced Pressure Research Articles

Stimulated Raman scattering and enhancement mechanisms in alkali metal hydroxide aqueous solutions under laser-induced dynamic pressure

Stimulated Raman scattering (SRS) in lithium hydroxide-water/heavy water (LiOH-H2O/D2O) solutions with varying concentrations was investigated under laser-induced high-pressure conditions using an Nd: YAG laser. The spectra revealed a significant enhancement in SRS signals, characterized by the emergence of low-wavenumber Raman peaks and a shift of the main SRS peak of liquid water to lower frequencies, evolving from a single peak to two or three peaks, which suggested the formation of an ice-like structure. Additionally, the normalized SRS intensity was higher than that of pure water. The enhancement of SRS signals during LiOH dissolution was attributed to the cooperative modulation between hydrogen bonding (HB, O-H:O) by OH- and Coulombic electronic interactions generated by monovalent Li+, which strengthened HB interactions, increased the Raman gain coefficient, and induced structural modifications akin to pressure effects. Besides, the increased density amplified the dynamic high-pressure effects during the laser-induced breakdown (LIB) process, further boosting the SRS signals. This work provides valuable insights into the interaction mechanisms between alkali metal hydroxides and H2O/D2O under laser-induced pressure conditions, offering a comprehensive approach to understanding and enhancing the SRS effect in aqueous solutions.

Spatial-temporal characteristics analysis of laser-induced shockwave pressure by reverse optimization with multi-island genetic algorithm

Spatial-temporal characteristics analysis of laser-induced shockwave pressure by reverse optimization with multi-island genetic algorithm

Wake of two tandem square cylinders

The wake of two tandem square cylinders of identical width (d) is experimentally studied, with a view to understanding the dependence of the flow structure, aerodynamics forces and Strouhal number on the centre-to-centre spacing ratio L/d and Reynolds number Re, where L is the distance between the cylinder centres. Extensive measurements are carried out, using hot-wire, particle imaging velocimetry, laser-induced fluorescence flow visualization, surface-oil-flow visualization and surface pressure scanning techniques, for L/d = 1.0 ~ 5.0 and Re ≡ U∞d/ν = 2.8 × (103 ~ 104), where U∞ is the free-stream velocity and ν is the kinematic viscosity of the fluid. The flow is classified into four regimes, i.e. the extended-body (L/d ≤ 1.5–2.0), reattachment (1.5–2.0 < L/d < 2.7–3.2), co-shedding (L/d ≥ 3.0–3.4) and transition (2.7 ≤ L/d ≤ 3.3) where both reattachment and co-shedding phenomena may take place. The mean drag and fluctuating drag and lift exhibit distinct features for different flow regimes, which is fully consistent with the proposed flow classification. Comparison is made between this flow and the wake of two tandem circular cylinders, which provides valuable insight into the profound effect of the flow separation point and the presence of sharp corners on the flow development and classification.

Laser-Induced Skin-like Flexible Pressure Sensor for Artificial Intelligence Speech Recognition.

Skin-like flexible pressure sensors with good sensing performance have great application potential, but their development is limited owing to the need for multistep, high-cost, and low-efficiency preparation processes. Herein, a simple, low-cost, and efficient laser-induced forming process is proposed for the first time to prepare a skin-like flexible piezoresistive sensor. In the laser-induced forming process, based on the photothermal effect of graphene and the foaming effect of glucose, a skin-like polydimethylsiloxanes (PDMS) film with porous structures and surface protrusions is obtained by using infrared laser irradiation of the glucose/graphene/PDMS prepolymer film. Further, based on the skin-like PDMS film with a graphene conductive layer, a new skin-like flexible piezoresistive sensor is obtained. Due to the stress concentration caused by the surface protrusions and the low stiffness caused by the porous structures, the flexible piezoresistive sensor realizes an ultrahigh sensitivity of 1348 kPa-1 at 0-2 kPa, a wide range of 200 kPa, a fast response/recovery time of 52 ms/35 ms, and good stability over 5000 cycles. The application of the sensor to the detection of human pulses and robot clamping force indicates its potential for health monitoring and soft robots. Furthermore, in combination with the neural network (CNN) algorithm in artificial intelligence technology, the sensor achieves 95% accuracy in speech recognition, which demonstrates its great potential for intelligent wearable electronics. Especially, the laser-induced forming process is expected to facilitate the efficient, large-scale preparation of flexible devices with multilevel structures.

Ion acceleration from the interaction of ultrahigh-intensity laser pulses with near-critical density, nonuniform gas targets

Using one-dimensional, long-timescale particle-in-cell simulations, we study the processes of ion acceleration from the interaction of ultraintense (1020 W cm−2), ultrashort (30 fs) laser pulses with near-critical, nonuniform gas targets. The considered initially neutral, nitrogen gas density profiles mimic those delivered by an already developed noncommercial supersonic gas shock nozzle: they have the generic shape of a narrow (20 μm wide) peak superimposed on broad (∼1 mm, ∼180 μm scale length), exponentially decreasing ramps. While keeping its shape constant, we vary its absolute density values to identify the interaction conditions leading to collisionless shock-induced ion acceleration in the gas density ramps. We find that collisionless electrostatic shocks (CES) form when the laser pulse is able to shine through the central density peak and deposit a few 10% of its energy into it. Under our conditions, this occurs for a peak electron density between 0.35 nc and 0.7 nc. Moreover, we show that the ability of the CES to reflect the upstream ions is highly sensitive to their charge state and that the laser-induced electron pressure gradients mainly account for shock generation, thus highlighting the benefit of using sharp gas profiles, such as those produced by shock nozzles.

Exploration of laser-induced drop transfer modes from wire for AM

Laser-induced ablation of drops from a metal wire enables the sequential deposition of voxels, for additive manufacturing. Striving for the goal of controlled, reproducible drop transfer, for this new technique, further trends and phenomena have been studied. Different modes of drop growth along with necking have been observed. Rather reproducible was a growing pending drop underneath the wire tip until separation for a certain size. In contrast, initial transients from the laser-induced recoil pressure can lead to a quick separation of a smaller drop. Initiation of a swinging cycle can also cause drop ablation after one cycle. For too fast wire feeding, the wire underpins the melt for a while in a spoon-like manner. Apart from the drop size, the different modes affect the scatter of the flight trajectory and landing position, as important optimization criteria, for controlled 3D-printing.

Estimating the yield stress of soft materials via laser-induced breakdown spectroscopy

Taking three typical soft samples prepared respectively by loose packings of 77-, 95-, and 109-μm copper grains as examples, we perform an experiment to investigate the energy-dependent laser-induced breakdown spectroscopy (LIBS) of soft materials. We discovered a reversal phenomenon in the trend of energy dependence of plasma emission intensity: increasing initially and then decreasing separated by a well-defined critical energy. The trend reversal is attributed to the laser-induced recoil pressure at the critical energy just matching the sample’s yield strength. As a result, a one-to-one correspondence can be well established between the samples’ yield stress and the critical energy that is easily obtainable from LIBS measurements. This allows us to propose an innovative method for estimating the yield stress of soft materials via LIBS with attractive advantages including in-situ remote detection, real-time data collection, and minimal destructive to sample.

Terahertz characterization of combined pressure-shear shock loaded aromatic polyurea

Realistic shock loading scenarios imply combined, multiaxial stress states, where the mechanically induced energy competes for failing the material intrinsically due to cohesive failures. Under shock loading, failure encompasses various modes, including spallation, shear bands, microcracking, morphological alteration, phase transformation, and plastic deformation. However, elucidating failure in high strain loading conditions challenges the state-of-the-art characterization due to the disparity interfacing different loading and measurement techniques. This research used a novel spectro-mechanical characterization setup, integrating bulk terahertz spectroscopy synchronized in real-time with mixed-mode laser-induced shock waves. The latter was used to submit polyurea elastomers to concurrent pressure and shear waves by saddling a thin polymer sheet on the hypotenuse of a glass prism. Hence, the laser-induced pressure shock waves underwent simultaneous mode conversion at the glass-polyurea interface at the hypotenuse of the glass prism, effectively submitting the sample to tunable amplitude pressure-shear shock waves. The terahertz spectroscopy, operating in the right-angle reflection configuration, was modified to probe predetermined spots on the polyurea samples at different stress states (adjusted by changing the illumination energy) in a three-step sequence: before, during, and after shock loading. The shock-loaded polyurea samples were also characterized using the optical and scanning electron microscopes, revealing compression-shear and tensile-shear failure modes, including surface depression, nucleation of micro-voids and microcracks, and formation of small planes. The induced stress state based on the illumination energy was quantified using an accompanying finite element simulation, resulting in completing the quasi-isentropic behavior of polyurea elastomers, which was found to be in good agreement with previous experimental data from the plate impact experiments and the predictions based on the Lennard-Jones potential. Time domain analysis of terahertz data, using temporal shifting, hysteresis, and dynamic time warping based on the sum of the Euclidean distances between signal pairs, affirmed the microscopy observations while shedding insights on the elastic and plastic deformations and the reversible conformational changes. Irreversible changes to the macromolecule were also detected using the terahertz data spectral analysis. The current results establish a new experimental framework for fundamentally characterizing polymers and their composites at ultrahigh strain rate loading conditions.

Practical Considerations for Laser-Induced Graphene Pressure Sensors Used in Marine Applications.

Small, low-power, and inexpensive marine depth sensors are of interest for a myriad of applications from maritime security to environmental monitoring. Recently, laser-induced graphene (LIG) piezoresistive pressure sensors have been proposed given their rapid fabrication and large dynamic range. In this work, the practicality of LIG integration into fieldable deep ocean (1 km) depth sensors in bulk is explored. Initially, a design of experiments (DOEs) approach evaluated laser engraver fabrication parameters such as line length, line width, laser speed, and laser power on resultant resistances of LIG traces. Next, uniaxial compression and thermal testing at relevant ocean pressures up to 10.3 MPa and temperatures between 0 and 25 °C evaluated the piezoresistive response of replicate sensors and determined the individual characterization of each, which is necessary. Additionally, bare LIG sensors showed larger resistance changes with temperature (ΔR ≈ 30 kΩ) than pressure (ΔR ≈ 1-15 kΩ), indicating that conformal coatings are required to both thermally insulate and electrically isolate traces from surrounding seawater. Sensors encapsulated with two dip-coated layers of 5 wt% polydimethylsiloxane (PDMS) silicone and submerged in water baths from 0 to 25 °C showed significant thermal dampening (ΔR ≈ 0.3 kΩ), indicating a path forward for the continued development of LIG/PDMS composite structures. This work presents both the promises and limitations of LIG piezoresistive depth sensors and recommends further research to validate this platform for global deployment.

Molten pool air-liquid interface in high-power laser manufacturing: Evolution law and energy absorption characteristics

During modern high-power laser manufacturing, the laser beam-heat energy conversion mainly occurs in the air-liquid interface of laser-induced molten pool evaporation. The molten pool air-liquid interface's evolution law and laser energy coupling characteristics were performed by using in-situ optical observations and numerical simulation in this study. They revealed the four stages of this evolution process: preheating, expansion, oscillation, and contraction. In the expansion stage of the molten pool, the molten metal will flow from the center of the molten pool to the periphery due to the joint action of surface tension and laser-induced evaporation recoil pressure, and the air-liquid interface exhibits a middle depression. When the laser is turned off, the evaporative recoil pressure drop causes the molten pool's oscillation, which amplitude and duration positively correlate with laser power. Due to the effects of the solid state and the air-liquid interface depression after melting, the absorptivity first drops and then grows with the increase of laser power. Eventually, the melting efficiency increases with laser power.

Investigation on auto-ignition and chemical energy release characteristics of pilot hydrogen in supersonic combustion flow

This study investigates auto-ignition and heat release characteristics of pilot hydrogen chemical energy in a scramjet combustor equipped with a single cavity. Experiments are conducted in a direct-connected facility simulating Mach 6.0 flight conditions with a total temperature of 1350 K and total pressure of 1.75 MPa. Data are obtained from schlieren imaging, hydroxyl planar laser-induced fluorescence, flame emission, and 10-kHz static pressure transducers. The present investigation extends the pilot hydrogen ignition delay experimental dataset and clarifies the instabilities present in the ignition process. The results show that the supersonic internal flow of a confined cavity exhibits self-oscillating behavior with a dominant frequency of approximately 141.3 Hz. The primary chemical reaction occurs at mid-cavity, where the chemical energy of the pilot hydrogen begins to be converted into heat energy, then approaches the cavity ramp before finally being distributed across the whole cavity. The combustion mode is the cavity-stabilized scramjet mode. The distribution of hydroxyl radicals varies significantly because the combustion in the cavity is unsteady. The ignition delay time increases as the injection pressure rises. However, an injection pressure of 4.0 MPa produces an ignition delay of 24.7 ms, which is apparently shorter than the delay under an injection pressure of 3.5 MPa and similar to that under an injection pressure of 3.0 MPa. The injection of pilot hydrogen under high pressures induces greater heat release and more intense blockage effects, thus enhancing the probability of successful ignition and stable combustion.

Photoacoustic remote sensing elastography.

The mechanical properties of organisms are important indicators for clinical disputes and disease monitoring, yet most existing elastography techniques are based on contact measurements, which are limited in many application scenarios. Photoacoustic remote sensing elastography (PARSE) is the first, to the best of our knowledge, elastography modality based on acoustic pressure monitoring, where elastic contrast information is obtained by using an all-optical non-contact and non-coherent intensity monitoring method through the time-response properties of laser-induced photoacoustic pressure. To validate PARSE, sections of different elastic organs were measured and this modality was applied to differentiate between bronchial cartilage and soft tissue to confirm the validity of the elasticity evaluation. PARSE, through a mathematical derivation process, has a 9.5-times greater distinction detection capability than photoacoustic remote sensing (PARS) imaging in stained bronchial sections, expands the scope of conventional PARS imaging, and has potential to become an important complementary imaging modality.

Ultrafast measurement of laser-induced shock waves

We present measurements of laser-induced shockwave pressure rise time in liquids on a sub-nanosecond scale, using custom-designed single-mode fiber optic hydrophone. The measurements are aimed at the study of the shockwave generation process, helping to improve the effectiveness of various applications and decrease possible accidental damage from shockwaves. The developed method allows measurement of the fast shockwave rise time as close as 10 µm from an 8 µm sized laser-induced plasma shockwave source, significantly improving the spatial and temporal resolution of the pressure measurement over other types of hydrophones. The spatial and temporal limitations of the presented hydrophone measurements are investigated theoretically, with actual experimental results agreeing well with the predictions. To demonstrate the capabilities of the fast sensor, we were able to show that the shockwave rise time is linked to liquid viscosity exhibiting logarithmic dependency in the low viscosity regime (from 0.4 cSt to 50 cSt). Additionally, the shockwave rise time dependency on propagation distance close to the source in water was investigated, with shock wave rise times measured down to only 150 ps. It was found that at short propagation distances in water halving the shock wave peak pressure results in the rise time increase by approximately factor of 1.6. These results extend the understanding of shockwave behavior in low viscosity liquids.

Residual dynamic mode decomposition: robust and verified Koopmanism

Dynamic mode decomposition (DMD) describes complex dynamic processes through a hierarchy of simpler coherent features. DMD is regularly used to understand the fundamental characteristics of turbulence and is closely related to Koopman operators. However, verifying the decomposition, equivalently the computed spectral features of Koopman operators, remains a significant challenge due to the infinite-dimensional nature of Koopman operators. Challenges include spurious (unphysical) modes and dealing with continuous spectra, which both occur regularly in turbulent flows. Residual dynamic mode decomposition (ResDMD), introduced by Colbrook & Townsend (Rigorous data-driven computation of spectral properties of Koopman operators for dynamical systems. 2021. arXiv:2111.14889), overcomes such challenges through the data-driven computation of residuals associated with the full infinite-dimensional Koopman operator. ResDMD computes spectra and pseudospectra of general Koopman operators with error control and computes smoothed approximations of spectral measures (including continuous spectra) with explicit high-order convergence theorems. ResDMD thus provides robust and verified Koopmanism. We implement ResDMD and demonstrate its application in various fluid dynamic situations at varying Reynolds numbers from both numerical and experimental data. Examples include vortex shedding behind a cylinder, hot-wire data acquired in a turbulent boundary layer, particle image velocimetry data focusing on a wall-jet flow and laser-induced plasma acoustic pressure signals. We present some advantages of ResDMD: the ability to resolve nonlinear and transient modes verifiably; the verification of learnt dictionaries; the verification of Koopman mode decompositions; and spectral calculations with reduced broadening effects. We also discuss how a new ordering of modes via residuals enables greater accuracy than the traditional modulus ordering (e.g. when forecasting) with a smaller dictionary. This result paves the way for more significant dynamic compression of large datasets without sacrificing accuracy.

Analysis of uncertainties in the measurement of the space charge distribution in dielectrics

The results obtained from the commonly used space charge distribution measurement methods are frequently apparent and one-dimensional results of a localized area; moreover, random errors can be caused by various factors. In this work, three factors—sample thickness uniformity, pressure wave intensity uniformity, and space charge distribution uniformity—that may cause measurement uncertainties are examined. The analysis and evaluation of the experimental data are affected by the measurement uncertainties, which are particularly noticeable in experiments on semiconductive electrode-flat polyethylene samples that use the laser-induced pressure propagation method. These factors are investigated separately using several auxiliary experiments, including laser beam spot energy distribution measurements and space charge distribution measurements, at different measurement positions on the same uniform sample. A clear and intelligible model is developed to discuss the influence of these factors on the uncertainties in the space charge distribution measurement in dielectrics.

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.

Influence of surface roughness on the transient interfacial phenomena in laser impact welding

Presented is an investigation into laser impact welding (LIW) wherein experimentally measured surface profiles of the flyer and target foils are incorporated to study their effects on the transient physical phenomena that occur during this rapid, collision-based joining process. During LIW, thermal response, plastic strains, and shear stresses evolve over a sub-microsecond timescale, which necessitates the use of computational modeling to predict the influence that surface roughness has on the material response and resulting joint morphology. White light interferometry is used to experimentally characterize the surface profiles of an aluminum 1100 flyer foil and a stainless steel 304 target foil. The profiles are mapped to material volumes within a plane strain, thermomechanical simulation employing an Eulerian framework. A spatially and temporally varying laser-induced plasma pressure is applied to the flyer foil, and the resulting transient phenomena are analyzed at timeframes ranging from initial contact through complete weld formation. To reveal effects of the rough foil surfaces, results from the same computational model using smooth foil surfaces are also obtained. When the rough surface profiles are included, it is found that significant differences in the thermal response and plastic strains are observed, and these differences are hypothesized to arise from the effects of discontinuous surface contact along the collision path. The incorporation of rough surface asperities also reveals two distinct regions prior to the weld initiation point; the first where elastic rebound occurs at a negligible collision angle, leaving interfacial voids, and the second where superficial shear deformation flattens the surfaces without mutual ablation and jetting. The work represents the first study in which measured surface profiles of the flyer and target foils are incorporated into a computational model of laser impact welding.

Experimental and numerical studies of fatigue behavior of Ti6Al4V alloy treated by laser shock peening

Laser shock peening (LSP) is applied as the laser surface treatment technology to improve the fatigue resistance of aircraft parts with complex geometric features. In this paper, the effects of laser energy inputs on residual stresses and fatigue properties for hole-structured specimens of Ti6Al4V alloy treated by LSP are studied. Changes of residual stress distributions, fatigue behaviors and fracture morphologies are analyzed with different laser-induced shock pressures. The results show the fatigue life is prolonged by 137% compared with the untreated specimens due to the introduction of compressive residual stresses. The sectional observations of fatigue fracture morphology show the inwards movement of fatigue crack initiation positions and the complexity of fracture cracks after LSP treatment. The simulation results confirm that the fatigue life with different laser energy inputs depends strongly on the three-dimensional stress distribution around the hole. Among them, high surface compressive stresses and deep stress affected layers play a decisive role in improving fatigue life. The results in this study broaden the knowledge of fatigue behavior and explore engineering potentials of improved fatigue life for complex industrial specimens.

Two-Dimensional Simulations of Laser Shock Peening of Aluminum With Water Confinement

Nowadays, advanced surface improvement techniques are required, due to continuously rising demands in aerospace and automotive productions. Laser shock peening (LSP) is a widely known modern surface enhancement method. LSP usually deals with nanosecond laser pulses with high intensities exceeding 1 GW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Due to very short time scales, all the occurring physical phenomena and processing parameters are extremely difficult to measure and study just based on experimental investigations. Therefore, multidimensional computer simulations of the processes within LSP are required for further optimization. In this work, a 2-D hydrodynamic simulation of water-confined nanosecond laser shock peening of an aluminum target is performed applying the open-source code MULTI2D. Circular laser beam with a pulse energy of 1 J, 20-ns duration, a diameter of 1 mm, and a uniform spatial distribution is used. Spatial profiles of the laser-induced plasma pressure and temperature are determined together with the shock wave parameters. It is shown that the plasma is highly confined and a temperature-affected region is very thin. Radial expansion of the surface pressure profile and the shock wave propagation can be observed, indicating that radial effects have to be considered during the industrial application of LSP. The obtained results are in good agreement with available simulations and measurements, validating the proposed simulation approach. In addition, temporal and spatial distributions of the pressure at the surface of the target material can be applied in finite-element simulations to predict residual stresses distributions.

The influence of laser-induced recoil pressure on particles speed in Directed Energy Deposition

Directed Energy Deposition is a common Additive Manufacturing technique used for its high deposition rate, but the interactions between the powder stream and the laser beam are still not completely understood. It is known that the powder particles heat up in the laser beam and some theoretical models predict that they can reach vaporization temperature and are significantly accelerated by the recoil pressure. In order to learn more about these phenomena, AlSi10Mg powder streams were observed with a high-speed camera at different laser powers and a particle-tracking software was used to measure the speed of the particles. The results show no significant increase of particle speed near the powder focus where the powder is processed, nor further below the powder focus. The high initial speed of the particles results in a short travelling time through the laser beam and in a very small effect of the recoil pressure.