Time-resolved Shadowgraph Imaging Research Articles

Effects and mechanism of focusing condition on machining quality and bubble behaviors in femtosecond laser micro-grooving of silicon wafer in liquid

In this paper, the effect and mechanism of how varying focusing conditions would affect the machining quality of microgrooves and bubble behaviors (cavitation bubbles (CB) and persistent bubbles (PB)) in liquid-assisted femtosecond laser micro-grooving of silicon wafer were investigated. Firstly, the width and depth of the microgroove machined under negative defocusing condition (NDC) were considerably uniform, which showed an increase by 100 % and 125 % compared with that machined under positive defocusing condition (PDC). Secondly, the temporal evolution images of CB were captured by a time-resolved shadowgraph imaging system. Compared with PDC, the radius and lifetime of CB induced under NDC increased by 71 % and 100 %, and the time evolution curve of the radius of CB exhibited a high consistence with the Rayleigh distribution, while that induced under PDC fluctuated irregularly. Thirdly, the PB induced under NDC showed a backward flush and contributed to the formation of tendentious liquid flow, while that induced under PDC occupied the laser transmission path and caused disturbances on subsequent laser pulses. Finally, the differences in laser transmission mechanism under both the negative and positive defocusing conditions were discussed and the NDC was confirmed as the stable and efficient processing method. Through the comprehensive study of the bubble behaviors, the correlation between focusing conditions and machining quality of liquid-assisted laser cutting was established.

Shock physics and shadowgraphic measurements of laser-produced cerium plasmas.

Shadowgraphic measurements are combined with theory on gas-dynamics to investigate the shock physics associated with nanosecond laser ablation of cerium metal targets. Time-resolved shadowgraphic imaging is performed to measure the propagation and attenuation of the laser-induced shockwave through air and argon atmospheres at various background pressures, where stronger shockwaves characterized by higher propagation velocities are observed for higher ablation laser irradiances and lower pressures. The Rankine-Hugoniot relations are also employed to estimate the pressure, temperature, density, and flow velocity of the shock-heated gas located immediately behind the shock front, predicting larger pressure ratios and higher temperatures for stronger laser-induced shockwaves.

Influence of focus positions on underwater femtosecond laser dicing of silicon wafer

This paper investigates the effects of laser-induced bubbles on the surface morphology of silicon wafer at three focus positions: above the focus (positive-defocus), at the focus and below the focus (negative-defocus). Firstly, the behaviors of the bubbles are observed from both the side view and top view of the laser ablation process at the three focus positions. Next, time-resolved shadowgraph images were used to examine evolution of the cavitation bubbles. The experimental results show that laser positive-defocus machining was accompanied by many bubbles, which aggregated to form a large bubbles barrier on the silicon wafer surface and disturbed the subsequent laser beam, and resulted in poor surface morphology of the groove. Compared to the laser machining at focus, laser positive-defocus machining reduced the groove depth by 77.77 %, groove aspect ratio by 80.98 %, cavitation bubbles size by 52.08 %, and cavitation bubbles lifetime by 58.82 %. Laser negative-defocus machining produced few bubbles that moved in the opposite direction of laser scanning. The bubbles did not interfere with the subsequent laser pulses and showed little effect on the surface morphology of the machined groove. Compared to the laser machining at focus, the laser negative-defocus machining increased the groove depth by 88.61 %, groove aspect ratio by 45.65 %, cavitation bubbles size by 65.64 %, and cavitation bubbles lifetime by 80.08 %. The results show that the negative-defocus machining method has application prospects in underwater laser machining.

Oscillatory characteristics of cavities in supersonic flow

We examine the flow structures, acoustic emissions, and oscillation types for supersonic flow past a baseline cavity and cavity with a bump. To study these, time-resolved shadowgraph images and unsteady pressure measurements are used. According to the results, the baseline cavity oscillates at various dominant frequencies. By adding a bump inside the cavity, many of its dominant oscillations are inactivated which are also corroborated by cross-correlation. Using time-resolved images, we can show the differences in wave propagation mechanisms between the two cavities. Through pressure spectra, it is demonstrated that the baseline cavity displays both fluid-dynamic and fluid-resonant types of oscillations, while the cavity with bump undergoes only the fluid-resonant type. The Rossiter prediction for the baseline cavity is very accurate but greatly deviates when applied to the cavity with a bump. As well, the wavelet analysis reveals that only the baseline cavity undergoes mode-switching phenomena. Additionally, the shear layer large-scale structures influencing the cavity oscillations are identified by using proper orthogonal decomposition (POD) and spectral proper orthogonal decomposition (SPOD) techniques. Furthermore, an analogy is established between POD and SPOD mode shapes along with the shadowgraph images for dominant modes of cavity oscillation. Finally, the results demonstrate that these modal decomposition techniques complement each other.

Effects of bubble behaviors in femtosecond laser machining of silicon wafer in liquids

In this article, the effects and mechanism of how bubble behaviors would affect liquid-assisted femtosecond laser cutting of silicon wafer are investigated. There are two kinds of bubbles generated during laser ablation in liquids, including the cavitation bubbles and the persistent bubbles. First, evolutions of the laser-induced cavitation bubbles are recorded by time-resolved shadowgraph imaging. The maximum height of the cavitation bubbles is shown to be 42 μm at the pulse energy of 18.15 μJ. The cavitation bubbles are shown to cause the defocusing effect on subsequent pulses, resulting in non-uniform ablated microgrooves. To improve the microgrooves morphology, the time interval between adjacent pulses is adjusted on basis of lifetime of the cavitation bubbles. The defocusing effect becomes alleviative when the time interval exceeds 100 μs. Next, the persistent bubbles are induced after the collapse of the cavitation bubbles and whose behaviors are studied. The results show that drastic explosions occur once the laser interacts with the persistent bubbles covered on the microgroove, and cause detrimental effects on surface morphology. The ethanol solution is chosen to improve surface wettability on basis of the dwell property of the persistent bubbles. Finally, the shielding effect of the persistent bubbles for incoming laser pulses is studied, which shows a decrease in the microgroove depth by 53 % in the 2 wt% ethanol solution. This work illustrates how the bubbles affect the cutting of silicon wafer under liquid, and how uniform microgrooves with fine surface morphology are achieved by utilizing the bubble behaviors.

Investigation of the blast pressure following laser ablation at a solid–fluid interface using shock waves dynamics in air and in water

Simple and universal relations between the laser pulse energy of the third harmonic of a Nd:YAG laser source (5 ns) and the pressure at the ablation point are investigated for the ablation of a solid target in air and in water. Shockwaves propagation are reconstructed using time-resolved shadowgraph imaging. The propagation in air is well-described by Taylor–von Neumann–Sedov’s theory for over-pressure at the shock front larger than ten times the atmospheric pressure, but the energy distribution appears to be anisotropic and mostly directed in the normal direction to the target’s surface. We compare the pressure at the shock front deduced from its velocity using Rankine–Hugoniot’s relation with the pressure deduced from Taylor’s model only related to the laser pulse energy. Rankine–Hugoniot’s relation appears more appropriate to evaluate the early pressure in the plasma plume. The shockwaves velocity in water is hyper-sonic during 50 ns and then reaches the sound velocity. Pressure values at the shock front are compared using two different approaches accounting for different state’s equations, namely the Rice and Walsh’s equation and Tait’s equation. In both cases, the ablation pressure is found out to reach several GPa and increases as the square root of the laser intensity.

Experiments on atomization and spray characteristics of an effervescent strut injector

The atomization and the spray characteristics from an effervescent/aerated strut injector were studied experimentally using water as the liquid and nitrogen gas as the secondary fluid. The spray flow features were obtained through various experimental tools such as instantaneous Mie scattering technique, phase Doppler particle analyzer, and time-resolved shadowgraph images for different Weber numbers, momentum flux ratios, and gas to liquid ratios. A proper orthogonal decomposition method was used for an in-depth analysis of the spray structures. The instantaneous Mie scattering images showed the formation of the liquid clusters and the dominant two-phase flow interaction with increasing momentum flux and gas to liquid ratios in addition to the increased spray spread area. The statistical analysis of the Mie images revealed high fluctuating locations in the spray. It was observed that by increasing the momentum flux or gas to liquid ratios, the droplet diameter rapidly decreases due to the flow transition to the annular flow regime. Beyond a certain limit of momentum flux or gas to liquid ratios, no significant change in the diameter was observed. Furthermore, the spatio-temporal fluid dynamic features and the two-phase flow transition inside the spray region were brought out through the proper orthogonal decomposition. The power spectral density revealed the ligament breakup and the droplet formation below ≤500 Hz. Regardless of the momentum flux and gas to liquid ratios, a peak was observed between 2500 Hz and 3000 Hz due to the flow transition where the liquid fragment cluster formation and the flapping were found to be almost constant.

Shock-wave propagation in supercritical CO2 induced by nanosecond-pulsed arc plasma

Shock waves generated by arc plasma in supercritical (SC) CO2 have the potential to create novel reaction fields. However, there have been few studies of shock-wave characteristics in SC-CO2. This study provides the results of visualization of shock-wave propagation in SC-CO2 generated by nanosecond-pulsed arc plasma. A propagating cylindrical shock wave originating from a discharge channel was observed using time-resolved shadowgraph imaging. The shock wave separated from the cylindrical dark-region induced by pulsed arc plasma within 64 nanoseconds. The Mach numbers of the shock waves were investigated against the medium density of the CO2 ranging from a high-pressure gas phase to the SC phase. The Mach number reached a local maximum at the critical CO2 density level. The anomaly in the Mach number can be explained by the local maximum of the specific heat capacity ratio of the CO2 at close to the critical condition of SC-CO2.

Liquid jet disintegration memory effect on downstream spray fluctuations in a coaxial twin-fluid injector

This paper intends to investigate the influence of unsteadiness in the liquid jet disintegration process on downstream fluctuations of spray characteristics in a coaxial twin-fluid injector. Time-resolved high-speed shadowgraphic imaging of the spray was obtained for different axial locations downstream of the injector exit at z = 0, 8Dl, and 30Dl, where Dl is the central liquid tube diameter. The primary jet breakup unsteadiness close to the injector exit was characterized by measuring both shear-driven Kelvin–Helmholtz (KH) instability and flapping instability in addition to jet breakup length fluctuations. Downstream of the liquid jet core region, the liquid shedding rate (fshed) was measured at z = 8Dl. The power spectrum of time series data of instantaneous volume mean diameter (VMD) measured at z = 30Dl indicated periodic variation of the droplet size. The corresponding frequency (fVMD) was obtained. It was found that for lower range of gas-to-liquid momentum flux ratio (M < 4), both fshed and fVMD are larger than the frequency of KH instability. Also, for such conditions, larger temporal variation of the droplet size is realized, and this leads to higher fluctuations of the local liquid mass flux. Proper orthogonal decomposition analysis of the shadowgraph images for different axial locations identified similar topology of the dominant mode that corresponds to flapping instability. The results suggest that even far downstream of the injector exit, some memory of the upstream unsteady jet breakup process is retained, which strongly influences spatio-temporal evolution of droplet characteristics, thereby contributing to local spray fluctuations.

Parametric Experimental Studies on Supersonic Flow Unsteadiness over a Hemispherical Spiked Body

Experimental studies are carried out to investigate the effects of the geometrical parameters with a drag reducing spike on a hemispherical forebody in a supersonic freestream of at 0 deg angle of attack. The spike length (, 1.0, 1.5, 2.0), spike stem diameter (, 0.12, 0.18), and spike tip shapes are varied, and their influence on the time-averaged and time-resolved flowfield is examined. When increases, a significant reduction in drag is achieved at , whereas the variation in has only a minor effect. The intensity of the shock-related unsteadiness is reduced with an increase in from 0.06 to 0.18, whereas changes in have a negligible effect. The effects of spike tip geometry are studied by replacing the sharp spike tip with a hemispherical one having three different base shapes (vertical base, circular base, and elliptical base). A hemispherical spike tip with a vertical base is performing better by reducing and flow unsteadiness. The dominant spatiotemporal mode arising due to the shock-related unsteadiness is represented through modal analysis of time-resolved shadowgraph images, and the findings are consistent with the other measurements.

Laser-induced plasma in water at high pressures up to 40 MPa: A time-resolved study.

The knowledge on the laser-induced plasma emission in water at high pressures is essential for the application of laser-induced breakdown spectroscopy (LIBS) in the deep-sea. In this work, we investigate the spectral features of ionic, atomic and molecular emissions for the plasma in water at different pressures from 1 to 40 MPa. By comparing between the time-resolved spectra and shadowgraph images, we demonstrate that the dynamics of the cavitation bubble at high pressures plays a key role on the characterization of plasma emission. The initial plasma emission depends weakly on the external pressure. As time evolves, the cavitation bubble is more compressed by the higher external pressure, leading to a positive confinement effect to maintain the plasma emission. However, at very high pressures, the bubble collapses extremely fast and even earlier than the cooling of the plasma. The plasma will gain energy from the bubble collapse phase, but quench immediately after the collapse, leading to a sharp reduction in the plasma persistence. These effects caused by bubble dynamics explain well the observed spectral features and are further proved by the temporal evolutions of the plasma temperature and electron density. This work gives not only some insights into the laser-induced plasma and bubble dynamics in high pressure liquids but also better understanding for the application of underwater LIBS in the deep-sea.

Microhole Drilling by High-Intensity Focused Ultrasound-Assisted Water-Confined Laser Micromachining

Laser micromachining has several advantages such as the capability of flexibly producing very small features in both conductive and nonconductive materials. However, it may often suffer from induced defects, such as debris deposition on workpieces. To improve laser micromachining, a novel machining process, called “ultrasound-assisted water-confined laser micromachining” (UWLM), was proposed by the corresponding author. The ultrasound during UWLM can be applied through different approaches, such as an ultrasonic horn or a high-intensity focused ultrasound (HIFU) transducer, which can be called horn- and HIFU-based UWLM, respectively. This is the first paper (to the authors’ best knowledge) reporting experimental studies on microhole drilling using the novel HIFU-based UWLM process. In this study, drilled workpieces have been characterized; and in situ time-resolved shadowgraph imaging and pressure measurement during the UWLM process have been performed. Under the investigated conditions, it has been found that the microholes drilled by HIFU-based UWLM under suitable conditions appear reasonably clean without significant debris depositions often seen for a nanosecond (ns) laser ablation in air. The UWLM process can produce much larger average ablation depths per pulse than laser ablation in water without ultrasound (e.g., for copper, the former depth can be up to more than six times the latter). The study has revealed one important mechanism for the enhanced ablation depth, which is introduced in more details in the paper.

飞秒激光烧蚀石英微孔的时间分辨阴影成像

飞秒激光烧蚀石英微孔的时间分辨阴影成像

Microhole Drilling by Double Laser Pulses With Different Pulse Energies

Previous investigations on “double-pulse” nanosecond (ns) laser drilling reported in the literature typically utilize double pulses of equal or similar pulse energies. In this paper, “double-pulse” ns laser drilling using double pulses with energies differing by more than ten times has been studied, where both postprocess workpiece characterizations and in situ time-resolved shadowgraph imaging observations have been performed. A very interesting physical phenomenon has been discovered under the studied conditions: the “double-pulse” ns laser ablation process, where the low-energy pulse precedes the high-energy pulse (called “low-high double-pulse” laser ablation) by a suitable amount of time, can produce significantly higher ablation rates than “high-low double-pulse” or “single-pulse” laser ablation under a similar laser energy input. In particular, “low-high double-pulse” laser ablation at a suitable interpulse separation time can drill through a ∼0.93 mm thick aluminum 7075 workpiece in less than 200 pulse pairs, while “high-low double-pulse” or “single-pulse” laser ablation cannot drill through the workpiece even using 1000 pulse pairs or pulses, respectively. This indicates that “low-high double-pulse” laser ablation has led to a significantly enhanced average ablation rate that is more than five times those for “single-pulse” or “high-low double-pulse” laser ablation. The fundamental physical mechanism for the ablation rate enhancement has been discussed, and a hypothesized explanation has been given.

Liquid jet breakup unsteadiness in a coaxial air-blast atomizer

The aim of this paper is to experimentally characterize the liquid jet breakup unsteadiness in a coaxial air-blast atomizer. The current research focuses on the measurement of the fluctuations of the jet breakup length and the flapping instability of the liquid jet, which contribute to the downstream fluctuations of the spray characteristics. The optical connectivity technique was used to measure the instantaneous breakup length of the water jet. Also, time resolved shadowgraph images of the primary jet breakup process were captured by high-speed imaging to characterize the jet instabilities at different axial locations from the atomizer exit. Experiments were performed for a wide range of air-to-liquid momentum flux ratio ( M) and aerodynamic Weber number ( Weg) corresponding to membrane- and/or fiber breakup mode of the jet disintegration process. The mean jet breakup length was found to vary inversely with M through a power law relation in agreement with the literature, while the breakup length fluctuations were found to first decrease and then increase with M. In order to capture the unsteady dynamics of the jet breakup process, the proper orthogonal decomposition analysis of the optical connectivity images was performed. The jet flapping and the fluctuations of the jet breakup length were identified as the second and the third spatial proper orthogonal decomposition modes, respectively, for all operating conditions of the atomizer. The amplitude and the frequency of the instabilities were measured by temporal tracking of the liquid–air interface on the shadowgraph images. The disturbance close to the injector exit corresponds to the Kelvin–Helmholtz instability, while close to the jet breakup point the jet exhibits the flapping instability, which is characterized by lateral oscillation of the jet about the atomizer axis. The influence of the liquid jet Reynolds number and momentum flux ratio on the KH and the flapping instabilities are examined.

Simultaneous observation of cavitation collapse and shock wave formation in cavitating jet

In this study, the mechanism of cavitation erosion in the cavitating jet discharging from a cavitator nozzle into a still water environment was examined through simultaneous observations of cloud collapse behavior and shock wave formation with and without wall. The simultaneous observations of the cloud collapse and shock wave formation were conducted by time-resolved shadowgraph and laser schlieren methods, respectively, while the impulsive signal on the wall was synchronously detected by an acceleration sensor. The time-resolved shadowgraph and laser schlieren images indicated that some shock waves were occurred at the instant of cloud collapse during the periodic behavior of the cavitating jet, which results in the impulsive forces on the wall. To a large extent, the shock waves originated from the near-wall region, and they induced the impulsive forces on the wall; in the case without wall, shock wave formation was observed in a similar distance from the nozzle with a broad distribution from the origin along the jet centerline. These results indicate that the cavitating-jet erosion occurs due to the cloud collapse near the wall, where some shock waves are generated and results in the erosion of the wall.

First Observation of Bright Solitons in Bulk Superfluid ^{4}He.

The existence of bright solitons in bulk superfluid ^{4}He is demonstrated by time-resolved shadowgraph imaging experiments and density functional theory (DFT) calculations. The initial liquid compression that leads to the creation of nonlinear waves is produced by rapidly expanding plasma from laser ablation. After the leading dissipative period, these waves transform into bright solitons, which exhibit three characteristic features: dispersionless propagation, negligible interaction in a two-wave collision, and direct dependence between soliton amplitude and the propagation velocity. The experimental observations are supported by DFT calculations, which show rapid evolution of the initially compressed liquid into bright solitons. At high amplitudes, solitons become unstable and break down into dispersive shock waves.

Application of time-resolved shadowgraph imaging and computer analysis to study micrometer-scale response of superfluid helium.

Application of inexpensive light emitting diodes as backlight sources for time-resolved shadowgraph imaging is demonstrated. The two light sources tested are able to produce light pulse sequences in the nanosecond and microsecond time regimes. After determining their time response characteristics, the diodes were applied to study the gas bubble formation around laser-heated copper nanoparticles in superfluid helium at 1.7 K and to determine the local cavitation bubble dynamics around fast moving metal micro-particles in the liquid. A convolutional neural network algorithm for analyzing the shadowgraph images by a computer is presented and the method is validated against the results from manual image analysis. The second application employed the red-green-blue light emitting diode source that produces light pulse sequences of the individual colors such that three separate shadowgraph frames can be recorded onto the color pixels of a charge-coupled device camera. Such an image sequence can be used to determine the moving object geometry, local velocity, and acceleration/deceleration. These data can be used to calculate, for example, the instantaneous Reynolds number for the liquid flow around the particle. Although specifically demonstrated for superfluid helium, the technique can be used to study the dynamic response of any medium that exhibits spatial variations in the index of refraction.

Observation of large-scale structure in flickering diffusion flame by time-resolved particle image velocimetry and shadowgraph imaging

In order to understand the flow structure of a flickering flame, time-resolved observations are carried out of the velocity and density fields in a diffusion flame in co-flow using particle image velocimetry (PIV) and shadowgraph imaging, respectively. The PIV measurement combined with invalid velocity vector analysis using proper orthogonal decomposition (POD) allows for the high spatial resolution measurement of the velocity field in the flame, while the shadowgraph images show the high-temperature contour and soot formation of the flame. These experimental results indicate a periodic variation in the flow field and soot structure of the flickering flame. The observation of the velocity field shows that the periodic inflow and outflow are generated in the outer layer of the flame near the burner, which promotes the flow acceleration in the vertical direction leading to clip-off behavior. It is found that the vorticity contour evaluated from the velocity field matches well with the high-temperature contour observed in the shadowgraph images. Further examination of the flow structure is carried out using the first two POD modes, which demonstrate the presence of counter-rotating vortices in the outer flame contributing to stretching of the flame due to the clip-off motion in the flame.

Enhancing the expansion of a plasma shockwave by crater-induced laser refocusing in femtosecond laser ablation of fused silica

The dynamics of plasma and shockwave expansion during two femtosecond laser pulse ablation of fused silica are studied using a time-resolved shadowgraph imaging technique. The experimental results reveal that during the second pulse irradiation on the crater induced by the first pulse, the expansion of the plasma and shockwave is enhanced in the longitudinal direction. The plasma model and Fresnel diffraction theory are combined to calculate the laser intensity distribution by considering the change in surface morphology and transient material properties. The theoretical results show that after the free electron density induced by the rising edge of the pulse reaches the critical density, the originally transparent surface is transformed into a transient high-reflectivity surface (metallic state). Thus, the crater with a concave-lens-like morphology can tremendously reflect and refocus the latter part of the laser pulse, leading to a strong laser field with an intensity even higher than the incident intensity. This strong refocused laser pulse results in a stronger laser-induced air breakdown and enhances the subsequent expansion of the plasma and shockwave. In addition, similar shadowgraphs are also recorded in the single-pulse ablation of a concave microlens, providing experimental evidence for the enhancement mechanism.