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We examined three descending positive leaders at distances of 5–11 km and three descending negative leaders at distances of 6–7 km, all simultaneously imaged by high‐speed framing cameras operating in the visible and UV ranges. UV images (290–370 nm) of the positive leaders each exhibited a strong embellishment at the lower channel end, which was not observed in the corresponding visible images (480–800 nm). In contrast, none of the negative leaders exhibited channel embellishment in the UV range and their morphology in UV was similar to that in the visible. Additionally, no embellishment was seen in four negative leaders imaged in UV only. The observed UV embellishment, which is likely to be the streamer zone at the positive‐leader tip, appeared to undergo expansion‐contraction cycles. We attributed the lack of detectable streamer‐zone emission in the UV range in negative leaders to a much lower streamer generation rate compared to positive leaders. Plain Language Summary Lightning is usually imaged in the visible (400–800 nm) range. In this study, we compare, for the first time, ultraviolet (UV, 290–370 nm) images of positive and negative lightning leaders with the simultaneously recorded visible images. The key findings include the discovery of a pulsating streamer zone at the positive leader tip in UV, which was not detectable in the visible. We did not observe any streamer zone at the negative leader tip in either UV or visible ranges. We explained the observed disparity between positive and negative leaders in terms of different streamer generation rates and offered a hypothetical mechanism of the positive streamer zone pulsation. Key Points UV images of descending positive leaders exhibited a strong and pulsating embellishment at their lower end, not seen in the visible range In contrast, images of negative leaders in the UV range were similar to those in the visible range (no UV embellishment at the tip) The observed polarity asymmetry is likely to be related to very different streamer production rates at positive and negative leader tips

As the energy density of lithium‐ion cells and batteries increases, controlling the outcomes of thermal runaway becomes more challenging. If the high rate of gas generation during thermal runaway is not adequately vented, commercial cell designs can rupture and explode, presenting serious safety concerns. Here, ultra‐high‐speed synchrotron X‐ray imaging is used at >20 000 frames per second to characterize the venting processes of six different 18650 cell designs undergoing thermal runaway. For the first time, the mechanisms that lead to the most catastrophic type of cell failure, rupture, and explosion are identified and elucidated in detail. The practical application of the technique is highlighted by evaluating a novel 18650 cell design with a second vent at the base, which is shown to avoid the critical stages that lead to rupture. The insights yielded in this study shed new light on battery failure and are expected to guide the development of safer commercial cell designs. Thermal runaway of 18650 lithium‐ion cells can result in a wide range of failure mechanisms, from controlled release of gases to catastrophic bursting and generation of projectiles. Here, the structural dynamics associated with venting and rupture of five different commercial 18650 cell designs are captured and explored using high‐speed X‐ray imaging at up to 20 272 frames per second.

Green laser sources are advantageous in the processing of copper due to the increase of absorptivity compared with more commonly available infrared lasers. Laser metal deposition of copper with a green laser onto various substrate metals namely copper, aluminium, steel and titanium alloy was carried out and observed through high-speed imaging. The effects of process parameters such as laser power, cladding speed and powder feed rate, and material attributes such as absorptivity, surface conditions and thermal conductivity are tied together to explain the size and geometry of the melt pool as well as the fraction of the power used for melting material. The copper substrate has the smallest melt pool with a high angle, followed by aluminium, steel and titanium alloy. The incorporation times for powder grains in the melt pools vary based on the substrate materials. Its dependency on material properties, including surface tension forces, melting temperatures and material density, is discussed. Oxide skins present on melt pools can affect powder incorporation, most significantly on the aluminium substrate. The lower limits of the fraction of power irradiated on the surface used purely for melting were calculated to be 0.73%, 2.94%, 5.95% and 9.78% for the copper, aluminium, steel and titanium alloy substrates, respectively, showing a strong dependence on thermal conductivity of the substrate material. For a copper wall built, the fraction was 2.66%, much higher than a single clad on a copper substrate, due to reduced workpiece heating. The results of this paper can be transferred to other metals with low absorptivity such as gold.

Ultra‐high‐speed synchrotron‐based hard X‐ray (i.e. above 10 keV) imaging is gaining a growing interest in a number of scientific domains for tracking non‐repeatable dynamic phenomena at spatio‐temporal microscales. This work describes an optimized indirect X‐ray imaging microscope designed to achieve high performance at micrometre pixel size and megahertz acquisition speed. The entire detector optical arrangement has an improved sensitivity within the near‐ultraviolet (NUV) part of the emitted spectrum (i.e. 310–430 nm wavelength). When combined with a single‐crystal fast‐decay scintillator, such as LYSO:Ce (Lu2−xYxSiO5:Ce), it exploits the potential of the NUV light‐emitting scintillators. The indirect arrangement of the detector makes it suitable for high‐dose applications that require high‐energy illumination. This allows for synchrotron single‐bunch hard X‐ray imaging to be performed with improved true spatial resolution, as herein exemplified through pulsed wire explosion and superheated near‐nozzle gasoline injection experiments at a pixel size of 3.2 µm, acquisition rates up to 1.4 MHz and effective exposure time down to 60 ps. This study introduces an optimized indirect X‐ray microscope capable of achieving micrometre pixel size and megahertz acquisition speed, leveraging enhanced sensitivity in the near‐ultraviolet spectrum and single‐crystal fast‐decay scintillators. This development enables high‐resolution imaging for dynamic phenomena, exemplified by experiments with pulsed wire explosion and superheated near‐nozzle gasoline injection.

Full‐field ultra‐high‐speed (UHS) X‐ray imaging experiments have been well established to characterize various processes and phenomena. However, the potential of UHS experiments through the joint acquisition of X‐ray videos with distinct configurations has not been fully exploited. In this paper, we investigate the use of a deep learning‐based spatio‐temporal fusion (STF) framework to fuse two complementary sequences of X‐ray images and reconstruct the target image sequence with high spatial resolution, high frame rate and high fidelity. We applied a transfer learning strategy to train the model and compared the peak signal‐to‐noise ratio (PSNR), average absolute difference (AAD) and structural similarity (SSIM) of the proposed framework on two independent X‐ray data sets with those obtained from a baseline deep learning model, a Bayesian fusion framework and the bicubic interpolation method. The proposed framework outperformed the other methods with various configurations of the input frame separations and image noise levels. With three subsequent images from the low‐resolution (LR) sequence of a four times lower spatial resolution and another two images from the high‐resolution (HR) sequence of a 20 times lower frame rate, the proposed approach achieved average PSNRs of 37.57 dB and 35.15 dB, respectively. When coupled with the appropriate combination of high‐speed cameras, the proposed approach will enhance the performance and therefore the scientific value of UHS X‐ray imaging experiments. A deep learning‐based algorithm is developed and evaluated that demonstrates the potential to reconstruct simultaneously high‐resolution high‐frame‐rate X‐ray image sequences with high fidelity through spatio‐temporal fusion. Experimental evaluation shows that the method can significantly improve the accuracy of the reconstruction, achieving an average peak signal‐to‐noise ratio (PSNR) of more than 35 dB on two representative X‐ray image sequences with input data streams of four times lower spatial resolution and 20 times lower frame rate, respectively.

High‐speed imaging of coarse sand particles transported as bed load over a planar bed reveals that the particle activity, the solid volume of particles in motion per unit streambed area, fluctuates as particles respond to near‐bed fluid turbulence while simultaneously interacting with the bed. The relative magnitude of these fluctuations systematically varies with the size of the sampling area. The particle activity within a specified sampling area is distributed in a manner that is consistent with the existence of an ensemble of configurations of particle positions wherein certain configurations are preferentially selected or excluded by the turbulence structure, manifest as patchiness of active particles. The particle activity increases with increasing bed stress far faster than does the average particle velocity, so changes in the transport rate with changing stress are dominated by changes in the activity, not velocity. The probability density functions of the streamwise and cross‐stream particle velocities are exponential‐like and lack heavy tails. Plots of the mean squared particle displacement versus time may ostensibly indicate non‐Fickian diffusive behavior while actually reflecting effects of correlated random walks associated with intrinsic periodicities in particle motions, not anomalous diffusion. The probability density functions of the particle hop distance (start‐to‐stop) and the associated travel time are gamma‐like, which provides the empirical basis for showing that particle disentrainment rates vary with hop distance and travel time. Key Points Bed load particle activity systematically varies with scale Active particle configurations are preferentially selected by turbulence Particle velocities possess an exponential‐like distribution

To explore the effect of conduit roughness on volcanic jet dynamics and on the related seismo‐acoustic radiation we performed a series of shock‐tube experiments using pipes with variable inner surface fractal dimension D. Variable starting pressure produced subsonic to supersonic jets visualized using high‐speed shadowgraph and recorded with an array of accelerometers and microphones. At all starting pressures, increasing D increases the energy transfer from the gas to the conduit walls, decreasing the jet exit velocity (Mach number) and, for supersonic cases, the related shock‐cell spacing, and increasing the seismic to acoustic radiation amplitude ratio. The roughness‐induced changes in jet velocity and turbulence affect the dominant sources of the jet noise and modulates the spectral properties of the acoustic signals. From our study we show that conduit wall roughness is an important and yet largely neglected factor in the dynamics of explosive volcanic eruptions and their monitored geophysical signals. Plain Language Summary Volcanoes are amongst the most fascinating and mysterious subjects of science, for they allow no direct observation of what is happening within the conduit during eruptive activity. Indirect observations (such as measurements of the sound and vibration accompanying eruptions) are routinely performed for monitoring and research purposes. Laboratory studies mimicking the eruptive processes in small‐scale devices, are of great support for correctly interpreting such data. As such, we investigated the effect of the irregularity of conduit surface, amongst the most relevant and poorly known variables characterizing the eruptive processes, on volcanic jets and on their seismic and acoustic signals. We performed a series of laboratory experiments using conduits with different roughness of the internal surface and various starting pressures. Microphones and accelerometers, capable of measuring conduit sounds and vibration, respectively, in synch with high‐speed camera were used to constrain the characteristics of the generated subsonic and supersonic jets. Results show that conduit roughness controls: (a) the relative amplitude of seismic and acoustic signals; (b) the velocity, turbulence and properties of the sound of these jets. Our results will shed light on the link between observation at the surface and dynamic evolution of conduit geometry at depth. Key Points Elastic radiation of analogue experimental volcanic subsonic to supersonic jets was compared against high‐speed shadowgraph analysis We quantified variation in the spectral features, energy partitioning and jet structure due to differences in conduit roughness Different behavior in subsonic versus supersonic regime was observed due to distinct dominating noise sources at different roughness

In this work, 3D polymeric atomic force microscopy (AFM) tips, referred to as 3DTIPs, are manufactured with great flexibility in design and function using two‐photon polymerization. With the technology holding a great potential in developing next‐generation AFM tips, 3DTIPs prove effective in obtaining high‐resolution and high‐speed AFM images in air and liquid environments, using common AFM modes. In particular, it is shown that the 3DTIPs provide high‐resolution imaging due to their extremely low Hamaker constant, high speed scanning rates due to their low quality factor, and high durability due to their soft nature and minimal isotropic tip wear; the three important features for advancing AFM studies. It is also shown that refining the tip end of the 3DTIPs by focused ion beam etching and by carbon nanotube inclusion substantially extends their functionality in high‐resolution AFM imaging, reaching angstrom scales. Altogether, the multifunctional capabilities of 3DTIPs can bring next‐generation AFM tips to routine and advanced AFM applications, and expand the fields of high speed AFM imaging and biological force measurements. Multipurpose polymeric atomic force microscopy (AFM) tips, named 3DTIPs, are presented with low Hamaker constant for high resolution, high speed AFM imaging. The 3DTIPs are generated with great flexibility in design, material, and function allowing better control over resonance frequency, quality factor, and spring constant. Their tip ends are further post‐processed to extend their functionality in high resolution AFM imaging.

Stabilized microbubbles were commercialized over 30 years ago for use as contrast agents in ultrasound imaging. In recent years, interest in microbubble–acoustic interactions has expanded to applications not only in ultrasound imaging but also in drug and gene delivery. To understand the interaction of a microbubble and ultrasonic field, scientists optically observe the behavior of microbubbles during acoustic excitation. Because of the fast oscillations of microbubbles in ultrasound fields, the application of ultra‐high‐speed photography is required to capture bubble behavior. This manuscript reviews the approaches, challenges, and progress in high‐speed imaging systems utilized for microbubble analysis, focusing on innovations in camera technology. Stabilized microbubbles are used as contrast agents in bubble–acoustic interactions and are applied broadly in fields ranging from ultra‐high‐speed photography to drug delivery. This manuscript reviews the approaches, challenges, and progress in high‐speed imaging systems, such as the depicted rotating mirror camera.

SARS‐CoV‐2 and its ever‐emerging variants are spread from host‐to‐host via expelled respiratory aerosols and saliva droplets. Knowing the number of virions which are exhaled by a person requires precise measurements of the size, count, velocity and trajectory of the virus‐laden particles that are ejected directly from the mouth. These measurements are achieved in 3D, at 15,000 images/s, and are applied when speaking, yelling and coughing. In this study, 33 events have been analysed by post‐processing ∼500,000 images. Using these data, the flow rates of SARS‐CoV‐2 virions have been evaluated. At high concentrations, 107 virions/mL, it is found that 136–231 virions are ejected during a single cough, where the virion flow rate peak is capable of reaching 32 virions within a millisecond. This peak can reach tens of virions/ms when yelling but reduced to only a few virions/ms when speaking. At medium concentrations, ∼105 virions/mL, those results are hundreds of times lower. The total number of virions that are ejected when yelling at 110 dB, instead of speaking at 85 dB, increases by two‐ to threefold. From the measured data analysed in this article, the flow rate of other diseases, such as influenza, tuberculosis or measles, can also be estimated. As these data are openly accessible, they can be used by modellers for the simulation of saliva droplet transport and evaporation, allowing to further advance our understanding of airborne pathogen transmission. Key points Advanced, optimized and combined laser‐based imaging techniques for temporally sizing and tracking respiratory droplets and aerosols. Understanding how pathogens are being ejected from the mouth when speaking, yelling and coughing. Quantifying and analysing the variation of SARS‐CoV‐2 flow rates emission during exhalation.