High-speed Imaging Technique Research Articles

Observed Oscillating Anodic Plasma Plume Phenomena in High Current Vacuum Arcs

The impact of current density on the anode attachment characteristics of high current vacuum arcs was investigated using high-speed imaging techniques. The current density determines the occurrence of attachment transition into an anode spot type 2. Electrodes were made of Cu/Cr 50%/50%. In combination with the adequate amplitude of current, the anode spot type 2 was accompanied by first observed oscillating anodic plasma plumes by the higher current densities. Resulting electrode overheating processes, which depend on the instantaneous power delivered, determine the formation of a plasma region in front of the anode. Image processing indicates that this region interacts actively with the cathode, displacing the radiation intensity region, and influencing the behavior of its arc spots. In addition, increased high-frequency contents on the measured voltage signals accompanied the observed oscillations of the anodic plume. An explanation through a double-layer effect for the anode spot type 2 is proposed.

Prebreakdown negative streamers in liquid nitrogen: propagation characteristics and their influence on microsecond breakdown

Further clarification of streamer-induced breakdowns in liquid nitrogen has been attracting increasing attention owing to their potential applications in superconducting systems and cryoplasmas. In this work, negative streamers in microsecond discharges were investigated using a high-speed multiple-frame imaging technique. Different external voltages, gap lengths, hydrostatic pressures, and pin tips were employed to initiate various streamers, namely bush-like streamers, single pinecone-like streamers, dendritic streamers, and filamentary streamers. Furthermore, abundant hybrid streamers involving streamer transition and combination were observed. The diversity of negative streamers may be related to the pin morphology. Generally, it was found that thinner streamers, especially filamentary streamers, could take place more frequently with higher breakdown probability under stronger filed and lower pressure. However, bush-like streamers gradually played a more dominant role upon increasing the pressure, and they were unable to cause breakdown. Moreover, several intermittent weak discharges were recorded during the growth of streamers, which might be responsible for the streamer transition and propagation. Additionally, given the evolution of streamers branches after breakdown and the influence of pressure on streamers characteristics, it was speculated that high-pressure gas could be involved inside the streamers. The possible mechanism of streamer propagation and resultant breakdown were further explored based on the analysis of streamer protrusions.

A Complete and Low-Cost Cardiac Optical Mapping System in Translational Animal Models.

Clinicians, biologists, physicists, engineers, and computer scientists are coming together to better understand heart disease, which is currently the leading cause of death globally. Optical mapping, a high-speed fluorescence imaging technique that visualizes and measures key cardiac parameters such as action potentials, cytosolic calcium transients, and fibrillation dynamics, is a core research tool that has arisen from such interdisciplinary collaborations. In an effort to broaden its use, especially among clinical scientists and students, we developed a complete and low-cost optical mapping system, including a constant-flow Langendorff perfusion system, which minimizes the economic threshold to widespread use of this powerful tool in cardiac electrophysiology research. The system described here provides high spatiotemporal resolution data about action potentials, intracellular calcium transients and fibrillation wave dynamics in isolated Langendorff-perfused hearts (pigs and rabbits), relevant for translational research. All system components and software elements are fully disclosed with the aim of increasing the use of this affordable and highly versatile tool among clinicians, basic scientists and students wishing to tackle their own research questions with their own customizable systems.

Phase stable swept-source optical coherence tomography with active mode-locking laser for contrast enhancements of retinal angiography

Swept-source optical coherence tomography (SS-OCT) is an attractive high-speed imaging technique for retinal angiography. However, conventional swept lasers vary the cavity length of the laser mechanically to tune the output wavelength. This causes sweep-timing jitter and hence low phase stability in OCT angiography. Here, we improve an earlier phase-stabilized, akinetic, SS-OCT angiography (OCTA) method by introducing coherent averaging. We develop an active mode-locking (AML) laser as a high phase-stable akinetic swept source for the OCTA system. The phase stability of the improved system was analyzed, and the effects of coherent averaging were validated using a retina phantom. The effectiveness of the coherent averaging method was further confirmed by comparing coherently and conventionally averaged en face images of human retinal vasculature for their contrast-to-noise ratio, signal-to-noise ratio, and vasculature connectivity. The contrast-to-noise ratio was approximately 1.3 times larger when applying the coherent averaging method in the human retinal experiment. Our coherent averaging method with the high phase-stability AML laser source for OCTA provides a valuable tool for studying healthy and diseased retinas.

Electrically tunable lenses - eliminating mechanical axial movements during high-speed 3D live imaging.

The successful investigation of photosensitive and dynamic biological events, such as those in a proliferating tissue or a dividing cell, requires non-intervening high-speed imaging techniques. Electrically tunable lenses (ETLs) are liquid lenses possessing shape-changing capabilities that enable rapid axial shifts of the focal plane, in turn achieving acquisition speeds within the millisecond regime. These human-eye-inspired liquid lenses can enable fast focusing and have been applied in a variety of cell biology studies. Here, we review the history, opportunities and challenges underpinning the use of cost-effective high-speed ETLs. Although other, more expensive solutions for three-dimensional imaging in the millisecond regime are available, ETLs continue to be a powerful, yet inexpensive, contender for live-cell microscopy.

High-speed laser speckle imaging to unravel picoliter drop-on-demand to substrate interaction.

Understanding phenomena such as evaporation and imbibition of picoliter droplets into porous substrates is crucial in printing industry to achieve a higher printing quality and print speed. After printing, the residual pigment must remain fixed at the desired location on a substrate and be of a desired volume to yield a high resolution and vibrantly printed page that has become the expectation of modern printing technology. Current research entails not only chemical composition of the ink but also how this links to the dynamics and interactions that occur between the ink and the substrate at every stage of the printed spot formation, including evaporation, wetting, and imbibition. In this paper, we present an instrument that can print on-demand picoliter volume droplets of ink onto substrates and then immediately record on evolution of the resulting dynamics when these two materials interact. This high-speed laser speckle imaging (HS-LSI) technique has been developed to monitor nanometer displacement of the drying and imbibing ink droplet at a high frame rate, up to 20000Hz, given the short timescales of these interactions. We present the design of the instrument, discuss the related challenges and the theory underlying the LSI technique, specifically how photons non-evasively probe opaque objects in a multiple scattering regime, and show how this technique can unravel the dynamics of drying and imbibition. We will finish giving a validation on the instrument and an example of its usage.

Pool boiling enhancement and a method of bubble diameter determination on surfaces with deep minichannels

Pool boiling heat transfer experiments were conducted with ethanol, Novec-649 and FC-72 at atmospheric pressure on surfaces with deep minichannels. The minichannels 5.5, 6.0 and 10 mm deep and 0.5–1.2 mm wide were uniformly spaced with a pitch of 2.0 mm on the base surfaces. Heat transfer coefficients obtained with ethanol were similar for all surfaces tested. Novec-649 and FC-72 produced the highest coefficients at heat fluxes above 100 kW/m2 in 1 mm wide minichannels. All the working fluids provided a significant increase in the maximum (critical) heat flux. The images were recorded using high speed imaging techniques across the entire surface of the samples. Boiling visualization aimed to study vapor bubble diameter changes during the growth period and measure the bubble departure diameter.The bubble diameter at departure was also determined analytically according to force balance static and dynamic variants. The dynamic variant provided a high consistency between the theoretical and experimental data for bubble departure diameter with FC-72 and Novec-649.

High Temporal–spatial Distribution of Soot Temperature and Volume Fraction in Single Coal Combustion Flame

ABSTRACT In this study, a high-speed imaging technique for recording spectral radiative images at high temporal–spatial resolutions were proposed to visualize the combustion process of a single coal particle. A Shenhua (SH) single coal particle was burned on a McKenna flat-flame burner, and its combustion process was recorded using a high-speed imaging system. The radiative images of the SH single coal particle captured by the imaging system indicated that the particle mainly experienced the release and combustion of volatiles during the luminescence combustion process (lasting more than 62.5 ms). In addition, spectral radiative intensity images at 604 and 808 nm were also obtained from the radiative images, which were shown to enable the calculation of soot temperature and concentration. Thereafter, the temporal and spatial evolution of the soot temperature and volume fraction in the envelope flame of a single coal particle were obtained from the two spectral radiative intensity images. The results revealed that the soot in the single coal flame experienced a temperature increase in the initial stage, and then remained at a high temperature. A large amount of soot was produced in the middle stage of combustion, whereas small amounts of soot were produced in the initial and later stages.

Dynamic mode decomposition of elliptical liquid jets in crossflow

The dynamics of round and elliptical liquid jets in subsonic crossflow was studied using high-speed imaging technique. The experiments were performed at constant gaseous Weber number and liquid–gas momentum flux ratios of 6.45 and 17.87, respectively, with orifices of different aspect ratios (AR) having an equivalent diameter of 0.43 mm. All of the cases were carried out inside an open-loop subsonic wind tunnel with a test section of 100 mm × 100 mm × 750 mm. For each case, dynamic modes were generated directly from the snapshots using a variant of the Arnoldi method known as the dynamic mode decomposition (DMD). The DMD results indicate that elliptical liquid jets have more small-scaled patterns with higher frequencies compared with the round liquid jets. As the first attempt to investigate the dynamics of elliptical liquid jets in crossflow, this study captures the dominant spatiotemporal structures. We also found that the orifice AR can significantly alter the jet wavelengths. The data from this study contribute to understanding the behavior of elliptical liquid jets exposed to gas crossflow in an enhanced capillary breakup regime.

Impact of gas film thickness on the performance of RM-ECDM process during machining of glass

ABSTRACT Nowadays, the extensive use of glass-based materials for biomedical, micro-fluidic, and MEMS applications grabbed great attention. The use of glass substrate for the above-said applications required subtractive processing to fabricate essential microstructures. Electrochemical discharge machining (ECDM) is an emerging hybrid micromachining process employed to machine the glass work-material. In ECDM, the thermal energy released by the tool electrode in the form of electrochemical discharges due to the electrical breakdown of hydrogen gas film takes away the material from the work surface. The thickness of hydrogen gas film plays a crucial role in deciding the discharge characteristics and subsequent thermal energy amount. Thus, in the present research work, an experimental investigation on hydrogen gas film thickness during the triplex hybrid process of rotary mode (RM) ECDM has been carried out. A high-speed imaging technique was utilized to capture the images of the gas film. These captured images were processed through Matlab’s edge detection toolbox to measure its thickness. Central composite design (CCD) of response surface methodology (RSM) was used to design the experimental plan. Applied voltage, tool rotation rate, and electrolyte concentration were taken as input parameters. To investigate the influence of gas film thickness on energy channelization behavior during RM-ECDM process, a material removal rate (MRR) and L/D ratio (diameter to depth ratio of machined holes) were taken as the response characteristics. A desirability approach was used to optimize the input parameters.

Tip Vortex Cavitation and Induced Noise Characteristics of Hydrofoils

Tip vortex cavitation is one of the most classical themes in fluid mechanics. Although many experimental and theoretical studies have been performed, unsolved problems still remain. In particular, the trailing vortices at the tip of the hydrofoil directly affects the hydrodynamic and acoustic performance of submerged objects such as the marine propeller, rudder and various foil-shaped appendages of the ship. In this study, the experimental results from the measurements of the vortex cavitation from the tip of two different three-dimensional hydrofoils are presented. Experiments have been carried out in Chungnam National University-Cavitation Tunnel (CNU-CT). By high speed imaging technique, the development process of vortex cavitation is observed in detail. Based on the high-speed images, physical features of the cavity inception and the swirling motion of the tip vortex cavity flow are examined. In addition, the induced noise characteristics in the vortex development process are examined by unsteady pressure measurements. The forces exerted on the hydrofoil were also measured using a dynamometer with a view to verify the scaling relation between the inception cavitation number and the non-dimensional parameters namely, the coefficient of lift, CL and the Reynolds number, Re. The results further shed light on the cause of the intense noise induced by tip vortex cavitation.

On the Rayleigh-Taylor instability induced atomization

A new atomization method to destabilize an annular liquid sheet ejected from a gas turbine engine injector, called the Rayleigh-Taylor instability induced atomization (RTA) is proposed. In contrast to the traditional coaxial atomizers, a liquid is allowed to flow through an annular passage in the axial direction and air is allowed to impinge on the liquid sheet in the radial direction. Due to this interaction, the liquid sheet exhibits a radial acceleration that induces the azimuthal instability and forms a ligament via azimuthal modes. Experimentally, we investigated the atomizer performance using a high-speed imaging technique by varying the Bond number. Here, Bond number (Bo) is defined by the competition between the radial acceleration force and the surface tension force. We observed that the number of ligaments (N) formed from the liquid sheet is a strong function of Bo and found that N∼Bo. The experimental results agree well with the theoretical model and the proposed scaling is valid in the limit of Weber number ≤ 3.14. Here, Weber number represents the ratio of inertial force in axial direction to the surface tension force. RTA achieves a novel method of ligament formation and it can create a new avenue for the distinct classes of atomizers.

Spark effects on compression ignition of PRF95 direct injection spray in a constant volume combustion chamber

In this study, spark assisted compression ignition (SACI) combustion was investigated using PRF95 (a low reactivity high octane reference fuel of common commercial gasoline), a mixture of 95% isooctane (by volume) and 5% n-heptane (by volume) with a RON of 95 and compared with pure compression-ignition without spark. Characteristics like the cumulative heat release (CHR) and the heat release rates (HRR) were studied using pressure data from experiments conducted in a constant volume combustion chamber (CVCC). The combustion process was visualized with a high speed imaging technique. Tests were conducted to understand the effects of ambient oxygen concentration and temperatures on the heat release and combustion flame developments while controlling other factors. From the experiments it is evident that the peak heat release rate increases initially and then decreases with the increase in the ambient temperature and the highest peak heat release rate appears around 650–700 K. The peak heat release rate timing decreases with the increase of the ambient temperature or oxygen level. Under a low ambient temperature, the oxygen level plays a major role in affecting the peak heat release rate. Under lower oxygen levels, the flame becomes weaker, the ignition delay becomes longer, and the combustion process takes more time to complete. Proper spark timing can help advance the peak HRR and shorten ignition delay, but this effect becomes minor when the ambient temperature increases. For the current high octane low reactivity fuel, auto-ignition can be achieved for all the investigated ambient temperature and oxygen levels. But it is noticed that a spark is necessary for low ambient oxygen and low ambient temperature to achieve proper combustion.

In-Situ Characterization of Pore Formation Dynamics in Pulsed Wave Laser Powder Bed Fusion.

Laser powder bed fusion (LPBF) is an additive manufacturing technology with the capability of printing complex metal parts directly from digital models. Between two available emission modes employed in LPBF printing systems, pulsed wave (PW) emission provides more control over the heat input compared to continuous wave (CW) emission, which is highly beneficial for printing parts with intricate features. However, parts printed with pulsed wave LPBF (PW-LPBF) commonly contain pores, which degrade their mechanical properties. In this study, we reveal pore formation mechanisms during PW-LPBF in real time by using an in-situ high-speed synchrotron x-ray imaging technique. We found that vapor depression collapse proceeds when the laser irradiation stops within one pulse, resulting in occasional pore formation during PW-LPBF. We also revealed that the melt ejection and rapid melt pool solidification during pulsed-wave laser melting resulted in cavity formation and subsequent formation of a pore pattern in the melted track. The pore formation dynamics revealed here may provide guidance on developing pore elimination approaches.

Experimental investigations and development of mathematical model to estimate drop diameter and jet length

The key process used in nuclear industries for the management of radiotoxicity associated with spent fuel in a closed fuel cycle is solvent extraction. An understanding of hydrodynamics and mass transfer is of primary importance for the design of mass transfer equipment used in solvent extraction processes. Understanding the interfacial phenomenon and the associated hydrodynamics of the liquid drops is essential for model-based design of mass transfer devices. In this work, the phenomenon of drop formation at the tip of a nozzle submerged in quiescent immiscible liquid phase is revisited. Previously reported force balance based models and empirical correlations are analyzed. Experiments are carried out to capture the process of drop formation using high-speed imaging technique. The images are digitally processed to measure the average drop diameter. A correlation based on the force balance model is proposed to estimate drop diameter and jet length. The average drop diameter obtained from the proposed model is in good agreement with experimental data with an average error of 6.3%. The developed model is applicable in both the necking as well as jetting regime and is validated for liquid-liquid systems having low, moderate and high interfacial tension.

Fluid experiments on the dump bailing method in the plug and abandonment of oil and gas wells

Plug and abandonment (P&A) operations prevent oil and gas leakages from hydrocarbon zones to different formations, freshwater underground resources, and the surface. Cement plugs that are used in P&A operations play a crucial role in impeding reservoir fluid migrations. The dump bailing method is one of the common methods of the cement plug placement in which a determined volume of cement is dumped by a bailer on a bridge plug in the wellbore, to replace the in-situ fluid while filling the wellbore and the annular region outside the bailer. In this paper, we experimentally investigate the placement of a heavy fluid to remove and replace an in-situ light fluid in an inclined closed-end pipe, representing the dump bailing method from a fluid mechanics perspective. The two fluids are Newtonian and miscible, and they have a density difference. We examine the effects of the injection fluid properties and flow parameters, on the fluid placement efficiency, the degree of mixing and quality of the placement. We use high-speed imaging and non-intrusive measurement techniques to provide ample placement flow characterizations. Our results show that the most efficient heavy fluid placement ensues from smaller dumping heights. Also, increasing the density and viscosity differences strengthens the efficiency of the in-situ fluid removal. In addition, the influence of a higher rate of injection is more discernible in smaller dumping heights, where it results in the removal of the light fluid promptly from the flow domain, with minimum mixing and placement time. The results are in general helpful for improving the quality of the cement plug placement in P&A operations.

Implantable photonic neural probes for light-sheet fluorescence brain imaging.

.Significance: Light-sheet fluorescence microscopy (LSFM) is a powerful technique for high-speed volumetric functional imaging. However, in typical light-sheet microscopes, the illumination and collection optics impose significant constraints upon the imaging of non-transparent brain tissues. We demonstrate that these constraints can be surmounted using a new class of implantable photonic neural probes.Aim: Mass manufacturable, silicon-based light-sheet photonic neural probes can generate planar patterned illumination at arbitrary depths in brain tissues without any additional micro-optic components.Approach: We develop implantable photonic neural probes that generate light sheets in tissue. The probes were fabricated in a photonics foundry on 200-mm-diameter silicon wafers. The light sheets were characterized in fluorescein and in free space. The probe-enabled imaging approach was tested in fixed, in vitro, and in vivo mouse brain tissues. Imaging tests were also performed using fluorescent beads suspended in agarose.Results: The probes had 5 to 10 addressable sheets and average sheet thicknesses for propagation distances up to in free space. Imaging areas were as large as in brain tissue. Image contrast was enhanced relative to epifluorescence microscopy.Conclusions: The neural probes can lead to new variants of LSFM for deep brain imaging and experiments in freely moving animals.

Simultaneous high-speed spectroscopy and 2-color pyrometry analysis in an optical compression ignition engine fueled with OMEX-diesel blends

E-fuels are a very attractive way for improving the well-to-wheel emissions of CO2 in internal combustion engines. In the particular case of compression ignition engines, the Oxymethylene dimethyl ether (OMEX), an e-fuel with nearly soot-free combustion under mixing-controlled conditions, is a good candidate for the replacement of fossil fuels. However, the Lower Heating Value of OMEX is nearly half of the diesel fuel, which means that much longer injection durations are required in the real engine. In addition, the very low viscosity and lubricity of OMEX can damage the injection system if used pure, but it can be an interesting fuel when blended with conventional diesel. Thus, the main objective of this paper is to evaluate the potential of OMEX-diesel blends to bypass these OMEX limitations whilst keeping low soot formation trends. For this purpose, a single cylinder optical diesel engine at part load was employed. The soot production for the different fuel blends was analyzed by applying three different high-speed imaging techniques: natural luminosity, flame spectroscopy and 2-color pyrometry. Natural luminosity analysis showed that the flame light intensity scales with diesel fraction up to 30% of diesel in the blend. The spectroscopy analysis has revealed that soot formation of OMEX fuel is almost null. When blended with diesel at 50%, although soot formation is still lower than for pure diesel, higher soot levels are obtained in the last stages of the cycle as a consequence of the longer injections required.

Characteristics of droplet spatter behavior and process-correlated mapping model in laser powder bed fusion

The spatter behavior in the laser powder bed fusion (L-PBF) additive manufacturing is unavoidable owing to the interaction between high-energy laser beam and powder particles. And the droplet spatters generated by the laser induction not only directly reflect the steady-state of the micro-molten pool but also directly affect the processing quality of the L-PBF process. In order to further understand the mechanism of laser–powder interaction, and the generation and evolution of droplet spatter, the paper presents a droplet spatter feature information extraction algorithm and a manufacturing status classification method. Three kinds of feature information of droplet spatter were collected, extracted, and discussed by high-speed imaging, image processing, and statistical analysis techniques. The evolution mechanisms of the molten pool, the keyhole, and the droplet spatter behavior under different linear energy density inputs were discussed in combination with the single-track build. A mapping model of the droplet spatter behavior characteristics and manufacturing status was processed after that. The accuracy and reliability of the mapping model were verified and analyzed by using the AdaBoost CART classification model. The classification model not only verified the mapping mechanism, and the findings provide a basis and reference for the quality control of the L-PBF process in the future.

Quality assessment of digital speckle patterns for the single-shot speckle projection profilometry based on a visualised simulation platform

As a high-speed 3D surface imaging technique, the single-shot speckle projection profilometry (SPP) is widely applied to various fields, where the design of speckle pattern plays a critical role in the fundamental issue of correspondence matching. However, the assessment of speckle patterns in the real imaging system is a time consuming and tedious procedure and their performance might vary with the operators and testing environment. To solve this problem, a visualised simulation platform based on 3ds Max engine is proposed in the paper. Furthermore, the performance of digital speckle patterns (DSPs) on the SPP technique is comprehensively evaluated in terms of 3D reconstruction precision, algorithm efficiency and pattern uniqueness. Firstly, the influence of the noise levels of the projection and imaging processes is considered. Then, the 3D profile reconstruction using five types of speckle pattern is performed in a real experiment. Afterward, a comparative analysis with the optimal DSP is conducted. In addition, the effectiveness of the typical metric of the painted speckle pattern in the field of digital image correlation (DIC) is investigated; results demonstrate its insufficient capability in the SPP field. Moreover, the scaling model of speckle size, influence of speckle deformation and material surface are also discussed to guide the design of DSP. This study can potentially facilitate the selection of high-quality DSPs and the standardisation of the SPP technique. The virtual experiment platform executable file of 3ds Max and the recommended DSP are uploaded as reference.