Publications by authors named "Kamel Fezzaa"

43 Publications

Fast X-ray Nanotomography with Sub-10 nm Resolution as a Powerful Imaging Tool for Nanotechnology and Energy Storage Applications.

Adv Mater 2021 Apr 19:e2008653. Epub 2021 Apr 19.

Institut d'Electronique, de Microélectronique et de Nanotechnologie, Université de Lille, CNRS, Centrale Lille Institut, YNCREA-ISEN, Université Polytechnique des Hauts de France UPHF, CNRS UMR 8520-IEMN, Lille, F-59000, France.

In the last decade, transmission X-ray microscopes (TXMs) have come into operation in most of the synchrotrons worldwide. They have proven to be outstanding tools for non-invasive ex and in situ 3D characterization of materials at the nanoscale across varying range of scientific applications. However, their spatial resolution has not improved in many years, while newly developed functional materials and microdevices with enhanced performances exhibit nanostructures always finer. Here, optomechanical breakthroughs leading to fast 3D tomographic acquisitions (85 min) with sub-10 nm spatial resolution, narrowing the gap between X-ray and electron microscopy, are reported. These new achievements are first validated with 3D characterizations of nanolithography objects corresponding to ultrahigh-aspect-ratio hard X-ray zone plates. Then, this powerful technique is used to investigate the morphology and conformality of nanometer-thick film electrodes synthesized by atomic layer deposition and magnetron sputtering deposition methods on 3D silicon scaffolds for electrochemical energy storage applications.
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http://dx.doi.org/10.1002/adma.202008653DOI Listing
April 2021

Nonlinear elasticity and damping govern ultrafast dynamics in click beetles.

Proc Natl Acad Sci U S A 2021 Feb;118(5)

Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801;

Many small animals use springs and latches to overcome the mechanical power output limitations of their muscles. Click beetles use springs and latches to bend their bodies at the thoracic hinge and then unbend extremely quickly, resulting in a clicking motion. When unconstrained, this quick clicking motion results in a jump. While the jumping motion has been studied in depth, the physical mechanisms enabling fast unbending have not. Here, we first identify and quantify the phases of the clicking motion: latching, loading, and energy release. We detail the motion kinematics and investigate the governing dynamics (forces) of the energy release. We use high-speed synchrotron X-ray imaging to observe and analyze the motion of the hinge's internal structures of four specimens. We show evidence that soft cuticle in the hinge contributes to the spring mechanism through rapid recoil. Using spectral analysis and nonlinear system identification, we determine the equation of motion and model the beetle as a nonlinear single-degree-of-freedom oscillator. Quadratic damping and snap-through buckling are identified to be the dominant damping and elastic forces, respectively, driving the angular position during the energy release phase. The methods used in this study provide experimental and analytical guidelines for the analysis of extreme motion, starting from motion observation to identifying the forces causing the movement. The tools demonstrated here can be applied to other organisms to enhance our understanding of the energy storage and release strategies small animals use to achieve extreme accelerations repeatedly.
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http://dx.doi.org/10.1073/pnas.2014569118DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7865152PMC
February 2021

Critical instability at moving keyhole tip generates porosity in laser melting.

Science 2020 11;370(6520):1080-1086

Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904, USA.

Laser powder bed fusion is a dominant metal 3D printing technology. However, porosity defects remain a challenge for fatigue-sensitive applications. Some porosity is associated with deep and narrow vapor depressions called keyholes, which occur under high-power, low-scan speed laser melting conditions. High-speed x-ray imaging enables operando observation of the detailed formation process of pores in Ti-6Al-4V caused by a critical instability at the keyhole tip. We found that the boundary of the keyhole porosity regime in power-velocity space is sharp and smooth, varying only slightly between the bare plate and powder bed. The critical keyhole instability generates acoustic waves in the melt pool that provide additional yet vital driving force for the pores near the keyhole tip to move away from the keyhole and become trapped as defects.
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http://dx.doi.org/10.1126/science.abd1587DOI Listing
November 2020

A three-dimensional thalamocortical dataset for characterizing brain heterogeneity.

Sci Data 2020 10 20;7(1):358. Epub 2020 Oct 20.

School of Electrical & Computer Engineering, Georgia Institute of Technology, Atlanta, GA, USA.

Neural microarchitecture is heterogeneous, varying both across and within brain regions. The consistent identification of regions of interest is one of the most critical aspects in examining neurocircuitry, as these structures serve as the vital landmarks with which to map brain pathways. Access to continuous, three-dimensional volumes that span multiple brain areas not only provides richer context for identifying such landmarks, but also enables a deeper probing of the microstructures within. Here, we describe a three-dimensional X-ray microtomography imaging dataset of a well-known and validated thalamocortical sample, encompassing a range of cortical and subcortical structures from the mouse brain . In doing so, we provide the field with access to a micron-scale anatomical imaging dataset ideal for studying heterogeneity of neural structure.
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http://dx.doi.org/10.1038/s41597-020-00692-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7576781PMC
October 2020

Pressure Drop with Moving Contact Lines and Dynamic Contact Angles in a Hydrophobic Round Minichannel: Visualization via Synchrotron X-ray Imaging and Verification of Experimental Correlations.

Langmuir 2020 Sep 16;36(38):11207-11214. Epub 2020 Sep 16.

Department of Mechanical Design Engineering, Pukyong National University, Busan 48547, Republic of Korea.

In hydrophobic mini- and microchannels, slug flow with moving contact lines is typically generated under various two-phase flow conditions. There is a significant pressure drop in this flow pattern with moving contact lines, which is closely related to the dynamic contact angles. Researchers have investigated dynamic contact angles experimentally for decades, but due to the limitations of visualization techniques, these experiments have typically been conducted in low Weber number regions ( < 10). In this study, we clearly visualized the dynamic contact angles of a liquid slug in high Weber number regions (10 < <1) via synchrotron X-ray imaging with high temporal (∼1000 fps) and spatial (∼2 μm/pixel) resolutions. We precisely measured the pressure drop with moving contact lines in a hydrophobic minichannel (inner diameter = 1.018 mm). On the basis of our experimental data, we verified previous correlations for dynamic contact angles and explored the relationship between pressure drop with moving contact lines and dynamic contact angles.
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http://dx.doi.org/10.1021/acs.langmuir.0c01014DOI Listing
September 2020

Drop impact on hot plates: contact times, lift-off and the lamella rupture.

Soft Matter 2020 Sep 6;16(34):7935-7949. Epub 2020 Aug 6.

X-ray Imaging Center, Department of Materials Science and Engineering, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang, 37673, Republic of Korea.

When a liquid drop impacts on a heated substrate, it can remain deposited, or violently boil in contact, or lift off with or without ever touching the surface. The latter is known as the Leidenfrost effect. The duration and area of the liquid-substrate contact are highly relevant for the heat transfer, as well as other effects such as corrosion. However, most experimental studies rely on side view imaging to determine contact times, and those are often mixed with the time until the drop lifts off from the substrate. Here, we develop and validate a reliable method of contact time determination using high-speed X-ray imaging and total internal reflection imaging. We exemplarily compare contact and lift-off times on flat silicon and sapphire substrates. We show that drops can rebound even without formation of a complete vapor layer, with a wide range of lift-off times. On sapphire, we find a local minimum of lift-off times that is much shorter than expected from capillary rebound in the comparatively low-temperature regime of transition boiling/thermal atomization. We elucidate the underlying mechanism related to spontaneous rupture of the lamella and receding of the contact area.
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http://dx.doi.org/10.1039/d0sm00459fDOI Listing
September 2020

Time-resolved 3D imaging of two-phase fluid flow inside a steel fuel injector using synchrotron X-ray tomography.

Sci Rep 2020 May 26;10(1):8674. Epub 2020 May 26.

Energy Systems Division, Argonne National Laboratory, Lemont, IL, USA.

The multiphase flow inside a diesel injection nozzle is imaged using synchrotron X-rays from the Advanced Photon Source at Argonne National Laboratory. Through acquisitions performed at several viewing angles and subsequent tomographic reconstruction, in-situ 3D visualization is achieved for the first time inside a steel injector at engine-like operating conditions. The morphology of the internal flow reveals strong flow separation and vapor-filled cavities (cavitation), the degree of which correlates with the nozzle's asymmetric inlet corner profile. Micron-scale surface features, which are artifacts of manufacturing, are shown to influence the morphology of the resulting liquid-gas interface. The data obtained at 0.1 ms time resolution exposes transient flow features and the flow development timescales are shown to be correlated with in-situ imaging of the fuel injector's hydraulically-actuated valve (needle). As more than 98.5% of the X-ray photon flux is attenuated within the steel injector body itself, we are posed with a unique challenge for imaging the flow within. Time-resolved imaging under these low-light conditions is achieved by exploiting both the refractive and absorptive properties of X-ray photons. The data-processing strategy converted these images with a signal-to-noise ratio of ~ 10 into a meaningful dataset for understanding internal flow and cavitation in a nozzle of diameter 200 μm enclosed within 1-2 millimeters of steel.
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http://dx.doi.org/10.1038/s41598-020-65701-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7250894PMC
May 2020

Air evolution during drop impact on liquid pool.

Sci Rep 2020 Apr 1;10(1):5790. Epub 2020 Apr 1.

X-ray Imaging Center, Department of Materials Science and Engineering, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang, 37673, South Korea.

We elucidate the evolution of the entrained air in drop impact on a wide range of liquids, using ultrafast X-ray phase-contrast imaging. We elaborate the retraction mechanism of the entrapped air film in terms of liquid viscosity. We found the criterion for deciding if the entrapped air evolves into single or double bubbles, as determined by competition among inertia, capillarity, and viscosity. Low viscosity and low surface tension induce a small daughter droplet encapsulated by a larger air shell bubble, forming an antibubble. We demonstrate a phase diagram for air evolution regarding hydrodynamics.
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http://dx.doi.org/10.1038/s41598-020-62705-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7113293PMC
April 2020

Physiological responses to gravity in an insect.

Proc Natl Acad Sci U S A 2020 01 13;117(4):2180-2186. Epub 2020 Jan 13.

Department of Biomedical Engineering and Mechanics, Virginia Tech, Blacksburg, VA 24061.

Gravity is one of the most ubiquitous environmental effects on living systems: Cellular and organismal responses to gravity are of central importance to understanding the physiological function of organisms, especially eukaryotes. Gravity has been demonstrated to have strong effects on the closed cardiovascular systems of terrestrial vertebrates, with rapidly responding neural reflexes ensuring proper blood flow despite changes in posture. Invertebrates possess open circulatory systems, which could provide fewer mechanisms to restrict gravity effects on blood flow, suggesting that these species also experience effects of gravity on blood pressure and distribution. However, whether gravity affects the open circulatory systems of invertebrates is unknown, partly due to technical measurement issues associated with small body size. Here we used X-ray imaging, radio-tracing of hemolymph, and micropressure measurements in the American grasshopper, , to assess responses to body orientation. Our results show that during changes in body orientation, gravity causes large changes in blood and air distribution, and that body position affects ventilation rate. Remarkably, we also found that insects show similar heart rate responses to body position as vertebrates, and contrasting with the classic understanding of open circulatory systems, have flexible valving systems between thorax and abdomen that can separate pressures. Gravitational effects on invertebrate cardiovascular and respiratory systems are likely to be widely distributed among invertebrates and to have broad influence on morphological and physiological evolution.
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http://dx.doi.org/10.1073/pnas.1915424117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6994993PMC
January 2020

Publisher Correction: Pore elimination mechanisms during 3D printing of metals.

Nat Commun 2019 Sep 30;10(1):4506. Epub 2019 Sep 30.

Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO, 65409, USA.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.
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http://dx.doi.org/10.1038/s41467-019-12432-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6768849PMC
September 2019

Pore elimination mechanisms during 3D printing of metals.

Nat Commun 2019 Jul 12;10(1):3088. Epub 2019 Jul 12.

Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO, 65409, USA.

Laser powder bed fusion (LPBF) is a 3D printing technology that can print metal parts with complex geometries without the design constraints of traditional manufacturing routes. However, the parts printed by LPBF normally contain many more pores than those made by conventional methods, which severely deteriorates their properties. Here, by combining in-situ high-speed high-resolution synchrotron x-ray imaging experiments and multi-physics modeling, we unveil the dynamics and mechanisms of pore motion and elimination in the LPBF process. We find that the high thermocapillary force, induced by the high temperature gradient in the laser interaction region, can rapidly eliminate pores from the melt pool during the LPBF process. The thermocapillary force driven pore elimination mechanism revealed here may guide the development of 3D printing approaches to achieve pore-free 3D printing of metals.
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http://dx.doi.org/10.1038/s41467-019-10973-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6625989PMC
July 2019

High-speed X-ray visualization of dynamic crack initiation and propagation in bone.

Acta Biomater 2019 05 26;90:278-286. Epub 2019 Mar 26.

School of Aeronautics and Astronautics, Purdue University, 701 West Stadium Avenue, West Lafayette, IN 47907, USA; School of Materials Engineering, Purdue University, 701 West Stadium Avenue, West Lafayette, IN 47907, USA.

The initiation and propagation of physiological cracks in porcine cortical and cancellous bone under high rate loading were visualized using high-speed synchrotron X-ray phase-contrast imaging (PCI) to characterize their fracture behaviors under dynamic loading conditions. A modified Kolsky compression bar was used to apply dynamic three-point flexural loadings on notched specimens and images of the fracture processes were recorded using a synchronized high-speed synchrotron X-ray imaging set-up. Three-dimensional synchrotron X-ray tomography was conducted to examine the initial microstructure of the bone before high-rate experiments. The experimental results showed that the locations of fracture initiations were not significantly different between the two types of bone. However, the crack velocities in cortical bone were higher than in cancellous bone. Crack deflections at osteonal cement lines, a prime toughening mechanism in bone at low rates, were observed in the cortical bone under dynamic loading in this study. Fracture toughening mechanisms, such as uncracked ligament bridging and bridging in crack wake were also observed for the two types of bone. The results also revealed that the fracture toughness of cortical bone was higher than cancellous bone. The crack was deflected to some extent at osteon cement line in cortical bone instead of comparatively penetrating straight through the microstructures in cancellous bone. STATEMENT OF SIGNIFICANCE: Fracture toughness is with great importance to study for crack risk prediction in bone. For those cracks in bone, most of them are associated with impact events, such as sport accidents. Consequently, we visualized, in real-time, the entire processes of dynamic fractures in notched cortical bone and cancellous bone specimens using synchrotron X-ray phase contrast imaging. The onset location of crack initiation was found independent on the bone type. We also found that, although the extent was diminished, crack deflections at osteon cement lines, a major toughening mechanism in transversely orientated cortical bone at quasi-static rate, were still played a role in resisting cracking in dynamically loaded specimen. These finding help researchers to understand the dynamic fracture behaviors in bone.
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http://dx.doi.org/10.1016/j.actbio.2019.03.045DOI Listing
May 2019

Real time observation of binder jetting printing process using high-speed X-ray imaging.

Sci Rep 2019 Feb 21;9(1):2499. Epub 2019 Feb 21.

X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL, 60439, USA.

A high-speed synchrotron X-ray imaging technique was used to investigate the binder jetting additive manufacturing (AM) process. A commercial binder jetting printer with droplet-on-demand ink-jet print-head was used to print single lines on powder beds. The printing process was recorded in real time using high-speed X-ray imaging. The ink-jet droplets showed distinct elongated shape with spherical head, long tail, and three to five trailing satellite droplets. Significant drift was observed between the impact points of main droplet and satellite droplets. The impact of the droplet on the powder bed caused movement and ejection of the powder particles. The depth of disturbance in the powder bed from movement and ejection was defined as interaction depth, which is found to be dependent on the size, shape, and material of the powder particles. For smaller powder particles (diameter less than 10 μm), three consecutive binder droplets were observed to coalesce to form large agglomerates. The observations reported here will facilitate the understanding of underlying physics that govern the binder jetting processes, which will then help in improving the quality of parts manufactured using this AM process.
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http://dx.doi.org/10.1038/s41598-019-38862-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6385361PMC
February 2019

Keyhole threshold and morphology in laser melting revealed by ultrahigh-speed x-ray imaging.

Science 2019 Feb;363(6429):849-852

Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA, USA.

We used ultrahigh-speed synchrotron x-ray imaging to quantify the phenomenon of vapor depressions (also known as keyholes) during laser melting of metals as practiced in additive manufacturing. Although expected from welding and inferred from postmortem cross sections of fusion zones, the direct visualization of the keyhole morphology and dynamics with high-energy x-rays shows that (i) keyholes are present across the range of power and scanning velocity used in laser powder bed fusion; (ii) there is a well-defined threshold from conduction mode to keyhole based on laser power density; and (iii) the transition follows the sequence of vaporization, depression of the liquid surface, instability, and then deep keyhole formation. These and other aspects provide a physical basis for three-dimensional printing in laser powder bed machines.
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http://dx.doi.org/10.1126/science.aav4687DOI Listing
February 2019

High-speed X-ray imaging of the Leidenfrost collapse.

Sci Rep 2019 Feb 7;9(1):1598. Epub 2019 Feb 7.

Department of Mechanical Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL, 60208, United States.

The Leidenfrost layer is characterized by an insulating vapor film between a heated surface and an ambient liquid. The collapse of this film has been canonically theorized to occur from an interfacial instability between the liquid and vapor phases. The interfacial instability alone, however, is insufficient to explain the known influence of the surface on the film collapse process. In this work, we provide visual evidence for two key mechanisms governing the film collapse: the interfacial instability, and the nucleation of vapor upon multiple non-terminal liquid-solid contacts. These results were obtained by implementing high-speed X-ray imaging of the film collapse on a heated sphere submerged in liquid-water. The X-ray images were synchronized with a second high-speed visible light camera and two thermocouples to provide insight into the film formation and film collapse processes. Lastly, the dynamic film thickness was quantified by analysis of the X-ray images. This helped assess the influence of surface roughness on the disruption of the film. The results of this work encourage further investigation into non-linear stability theory to consolidate the role of the surface on the liquid-vapor interface during the film collapse process.
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http://dx.doi.org/10.1038/s41598-018-36603-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6367412PMC
February 2019

X-ray tomography of extended objects: a comparison of data acquisition approaches.

J Opt Soc Am A Opt Image Sci Vis 2018 Nov;35(11):1871-1879

The penetration power of x rays allows one to image large objects, while their short wavelength allows for high spatial resolution. As a result, with synchrotron sources, one has the potential to obtain tomographic images of centimeter-sized specimens with sub-micrometer pixel sizes. However, limitations on beam and detector size make it difficult to acquire such data of this sort in a single take, necessitating strategies for combining data from multiple regions. One strategy is to acquire a tiled set of local tomograms by rotating the specimen around each of the local tomogram center positions. Another strategy, sinogram oriented acquisition, involves the collection of projections at multiple offset positions from the rotation axis followed by data merging and reconstruction. We have carried out a simulation study to compare these two approaches in terms of radiation dose applied to the specimen, and reconstructed image quality. Local tomography acquisition involves an easier data alignment problem, and immediate viewing of subregions before the entire dataset has been acquired. Sinogram oriented acquisition involves a more difficult data assembly and alignment procedure, and it is more sensitive to accumulative registration error. However, sinogram oriented acquisition is more dose efficient, involves fewer translation motions of the object, and avoids certain artifacts of local tomography.
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http://dx.doi.org/10.1364/JOSAA.35.001871DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6994859PMC
November 2018

Revealing particle-scale powder spreading dynamics in powder-bed-based additive manufacturing process by high-speed x-ray imaging.

Sci Rep 2018 Oct 10;8(1):15079. Epub 2018 Oct 10.

Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO, 65409, USA.

Powder spreading is a key step in the powder-bed-based additive manufacturing process, which determines the quality of the powder bed and, consequently, affects the quality of the manufactured part. However, powder spreading behavior under additive manufacturing condition is still not clear, largely because of the lack of particle-scale experimental study. Here, we studied particle-scale powder dynamics during the powder spreading process by using in-situ high-speed high-energy x-ray imaging. Evolution of the repose angle, slope surface speed, slope surface roughness, and the dynamics of powder clusters at the powder front were revealed and quantified. Interactions of the individual metal powders, with boundaries (substrate and container wall), were characterized, and coefficients of friction between the powders and boundaries were calculated. The effects of particle size on powder flow dynamics were revealed. The particle-scale powder spreading dynamics, reported here, are important for a thorough understanding of powder spreading behavior in the powder-bed-based additive manufacturing process, and are critical to the development and validation of models that can more accurately predict powder spreading behavior.
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http://dx.doi.org/10.1038/s41598-018-33376-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6180046PMC
October 2018

Ultrafast X-ray imaging of laser-metal additive manufacturing processes.

J Synchrotron Radiat 2018 Sep 14;25(Pt 5):1467-1477. Epub 2018 Aug 14.

X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA.

The high-speed synchrotron X-ray imaging technique was synchronized with a custom-built laser-melting setup to capture the dynamics of laser powder-bed fusion processes in situ. Various significant phenomena, including vapor-depression and melt-pool dynamics and powder-spatter ejection, were captured with high spatial and temporal resolution. Imaging frame rates of up to 10 MHz were used to capture the rapid changes in these highly dynamic phenomena. At the same time, relatively slow frame rates were employed to capture large-scale changes during the process. This experimental platform will be vital in the further understanding of laser additive manufacturing processes and will be particularly helpful in guiding efforts to reduce or eliminate microstructural defects in additively manufactured parts.
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http://dx.doi.org/10.1107/S1600577518009554DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6544194PMC
September 2018

Data and videos for ultrafast synchrotron X-ray imaging studies of metal solidification under ultrasound.

Data Brief 2018 Apr 8;17:837-841. Epub 2018 Feb 8.

School of Engineering & Computer Science, University of Hull, Hull, HU6 7RX, UK.

The data presented in this article are related to the paper entitled 'Ultrafast synchrotron X-ray imaging studies of microstructure fragmentation in solidification under ultrasound' [Wang et al., Acta Mater. 144 (2018) 505-515]. This data article provides further supporting information and analytical methods, including the data from both experimental and numerical simulation, as well as the Matlab code for processing the X-ray images. Six videos constructed from the processed synchrotron X-ray images are also provided.
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http://dx.doi.org/10.1016/j.dib.2018.01.110DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5842321PMC
April 2018

Synchrotron x-ray imaging visualization study of capillary-induced flow and critical heat flux on surfaces with engineered micropillars.

Sci Adv 2018 02 23;4(2):e1701571. Epub 2018 Feb 23.

Department of Mechanical Engineering, Pohang University of Science and Technology, 77, Cheongam-Ro, Nam-Gu, Pohang 37673, Republic of Korea.

Over the last several decades, phenomena related to critical heat flux (CHF) on structured surfaces have received a large amount of attention from the research community. The purpose of such research has been to enhance the safety and efficiency of a variety of thermal systems. A number of theories have been put forward to explain the key CHF enhancement mechanisms on structured surfaces. However, these theories have not been confirmed experimentally because of limitations in the available visualization techniques and the complexity of the phenomena. To overcome these limitations and elucidate the CHF enhancement mechanism on the structured surfaces, we introduce synchrotron x-ray imaging with high spatial (~2 μm) and temporal (~20,000 Hz) resolutions. This technique has enabled us to confirm that capillary-induced flow is the key CHF enhancement mechanism on structured surfaces.
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http://dx.doi.org/10.1126/sciadv.1701571DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5825216PMC
February 2018

Quantifying Mesoscale Neuroanatomy Using X-Ray Microtomography.

eNeuro 2017 Sep-Oct;4(5). Epub 2017 Oct 16.

Dept. of Neurobiology, University of Chicago, Chicago, IL, 60637.

Methods for resolving the three-dimensional (3D) microstructure of the brain typically start by thinly slicing and staining the brain, followed by imaging numerous individual sections with visible light photons or electrons. In contrast, X-rays can be used to image thick samples, providing a rapid approach for producing large 3D brain maps without sectioning. Here we demonstrate the use of synchrotron X-ray microtomography (µCT) for producing mesoscale (∼1 µm resolution) brain maps from millimeter-scale volumes of mouse brain. We introduce a pipeline for µCT-based brain mapping that develops and integrates methods for sample preparation, imaging, and automated segmentation of cells, blood vessels, and myelinated axons, in addition to statistical analyses of these brain structures. Our results demonstrate that X-ray tomography achieves rapid quantification of large brain volumes, complementing other brain mapping and connectomics efforts.
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http://dx.doi.org/10.1523/ENEURO.0195-17.2017DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5659258PMC
June 2018

Real-time monitoring of laser powder bed fusion process using high-speed X-ray imaging and diffraction.

Sci Rep 2017 06 15;7(1):3602. Epub 2017 Jun 15.

X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL, 60439, USA.

We employ the high-speed synchrotron hard X-ray imaging and diffraction techniques to monitor the laser powder bed fusion (LPBF) process of Ti-6Al-4V in situ and in real time. We demonstrate that many scientifically and technologically significant phenomena in LPBF, including melt pool dynamics, powder ejection, rapid solidification, and phase transformation, can be probed with unprecedented spatial and temporal resolutions. In particular, the keyhole pore formation is experimentally revealed with high spatial and temporal resolutions. The solidification rate is quantitatively measured, and the slowly decrease in solidification rate during the relatively steady state could be a manifestation of the recalescence phenomenon. The high-speed diffraction enables a reasonable estimation of the cooling rate and phase transformation rate, and the diffusionless transformation from β to α phase is evident. The data present here will facilitate the understanding of dynamics and kinetics in metal LPBF process, and the experiment platform established will undoubtedly become a new paradigm for future research and development of metal additive manufacturing.
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http://dx.doi.org/10.1038/s41598-017-03761-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5472560PMC
June 2017

In situ observation of fracture processes in high-strength concretes and limestone using high-speed X-ray phase-contrast imaging.

Philos Trans A Math Phys Eng Sci 2017 Jan;375(2085)

School of Aeronautics and Astronautics, Purdue University, 701 West Stadium Avenue, West Lafayette, IN 47907-2045, USA

The mechanical properties and fracture mechanisms of geomaterials and construction materials such as concrete are reported to be dependent on the loading rates. However, the in situ cracking inside such specimens cannot be visualized using traditional optical imaging methods since the materials are opaque. In this study, the in situ sub-surface failure/damage mechanisms in Cor-Tuf (a reactive powder concrete), a high-strength concrete (HSC) and Indiana limestone under dynamic loading were investigated using high-speed synchrotron X-ray phase-contrast imaging. Dynamic compressive loading was applied using a modified Kolsky bar and fracture images were recorded using a synchronized high-speed synchrotron X-ray imaging set-up. Three-dimensional synchrotron X-ray tomography was also performed to record the microstructure of the specimens before dynamic loading. In the Cor-Tuf and HSC specimens, two different modes of cracking were observed: straight cracking or angular cracking with respect to the direction of loading. In limestone, cracks followed the grain boundaries and voids, ultimately fracturing the specimen. Cracks in HSC were more tortuous than the cracks in Cor-Tuf specimens. The effects of the microstructure on the observed cracking behaviour are discussed.This article is part of the themed issue 'Experimental testing and modelling of brittle materials at high strain rates'.
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http://dx.doi.org/10.1098/rsta.2016.0178DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5179972PMC
January 2017

Inhibition of Tafel Kinetics for Electrolytic Hydrogen Evolution on Isolated Micron Scale Electrocatalysts on Semiconductor Interfaces.

ACS Appl Mater Interfaces 2016 Sep 12;8(37):24612-20. Epub 2016 Sep 12.

X-ray Science Division, Advanced Photon Source, Argonne National Laboratory , 9700 South Cass Avenue, Argonne, Illinois 60439, United States.

Semiconductor-liquid junctions are ubiquitous in photoelectrochemical approaches to artificial photosynthesis. By analogy with the antennae and reaction centers in natural photosynthetic complexes, separating the light-absorbing semiconductor and electrocatalysts can improve catalytic efficiency. A catalytic layer can also impair the photovoltage-generating energetics of the electrode without appropriate microscopic organization of catalytically active area on the surface. Here, we have developed a method using high-speed X-ray phase contrast imaging to study in situ electrolytic bubble growth on semiconductor electrodes fabricated with isolated, micron-scale platinum electrocatalysts. X-rays are a nonperturbative probe by which gas evolution dynamics can be studied under conditions relevant to solar fuels applications. The self-limited growth of a bubble residing on the isolated electrocatalyst was measured by tracking the evolution of the gas-liquid boundary. Contrary to observations on macroscopic electrodes, bubble evolution on isolated, microscopic Pt pads on Si electrodes was insensitive to increasing overpotential. The persistence of the bubble causes mass transport limitations and inhibits the expected Tafel-like kinetics. The observed scaling of catalytic current densities with pad size implies that electrolysis is occurring predominantly on the perimeter of the active area.
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http://dx.doi.org/10.1021/acsami.6b07729DOI Listing
September 2016

HiSPoD: a program for high-speed polychromatic X-ray diffraction experiments and data analysis on polycrystalline samples.

J Synchrotron Radiat 2016 07 17;23(Pt 4):1046-53. Epub 2016 Jun 17.

X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA.

A high-speed X-ray diffraction technique was recently developed at the 32-ID-B beamline of the Advanced Photon Source for studying highly dynamic, yet non-repeatable and irreversible, materials processes. In experiments, the microstructure evolution in a single material event is probed by recording a series of diffraction patterns with extremely short exposure time and high frame rate. Owing to the limited flux in a short pulse and the polychromatic nature of the incident X-rays, analysis of the diffraction data is challenging. Here, HiSPoD, a stand-alone Matlab-based software for analyzing the polychromatic X-ray diffraction data from polycrystalline samples, is described. With HiSPoD, researchers are able to perform diffraction peak indexing, extraction of one-dimensional intensity profiles by integrating a two-dimensional diffraction pattern, and, more importantly, quantitative numerical simulations to obtain precise sample structure information.
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http://dx.doi.org/10.1107/S1600577516005804DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5315097PMC
July 2016

The structural origin of the hard-sphere glass transition in granular packing.

Nat Commun 2015 Sep 28;6:8409. Epub 2015 Sep 28.

Department of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China.

Glass transition is accompanied by a rapid growth of the structural relaxation time and a concomitant decrease of configurational entropy. It remains unclear whether the transition has a thermodynamic origin, and whether the dynamic arrest is associated with the growth of a certain static order. Using granular packing as a model hard-sphere glass, we show the glass transition as a thermodynamic phase transition with a 'hidden' polytetrahedral order. This polytetrahedral order is spatially correlated with the slow dynamics. It is geometrically frustrated and has a peculiar fractal dimension. Additionally, as the packing fraction increases, its growth follows an entropy-driven nucleation process, similar to that of the random first-order transition theory. Our study essentially identifies a long-sought-after structural glass order in hard-sphere glasses.
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http://dx.doi.org/10.1038/ncomms9409DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4598628PMC
September 2015

Origin and dynamics of vortex rings in drop splashing.

Nat Commun 2015 Sep 4;6:8187. Epub 2015 Sep 4.

Department of Materials Science and Engineering, X-ray Imaging Center, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang 790-784, Korea.

A vortex is a flow phenomenon that is very commonly observed in nature. More than a century, a vortex ring that forms during drop splashing has caught the attention of many scientists due to its importance in understanding fluid mixing and mass transport processes. However, the origin of the vortices and their dynamics remain unclear, mostly due to the lack of appropriate visualization methods. Here, with ultrafast X-ray phase-contrast imaging, we show that the formation of vortex rings originates from the energy transfer by capillary waves generated at the moment of the drop impact. Interestingly, we find a row of vortex rings along the drop wall, as demonstrated by a phase diagram established here, with different power-law dependencies of the angular velocities on the Reynolds number. These results provide important insight that allows understanding and modelling any type of vortex rings in nature, beyond just vortex rings during drop splashing.
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http://dx.doi.org/10.1038/ncomms9187DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4569801PMC
September 2015

Angularly anisotropic correlation in granular packings.

Phys Rev E Stat Nonlin Soft Matter Phys 2014 Dec 1;90(6):062201. Epub 2014 Dec 1.

Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, USA.

We present an x-ray microtomography study of the three-dimensional structural correlations in monodisperse granular packings. By measuring an orientation-dependent pair correlation function, we find that the local structure shows an angularly anisotropic orientation correlation. The correlation is strongest along the major axis of the local Minkowski tensor of the Voronoi cell. It turns out that this anisotropic correlation is consistent with the existence of some locally favored structures. The study suggests the importance of high-order structural correlations in random granular packings.
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http://dx.doi.org/10.1103/PhysRevE.90.062201DOI Listing
December 2014

Similarity of wet granular packing to gels.

Nat Commun 2014 Sep 23;5:5014. Epub 2014 Sep 23.

Department of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China.

To date, there is still no general consensus on the fundamental principle that governs glass transition. Colloidal suspensions are ordinarily utilized as model systems to study the dynamical arrest mechanisms in glass or gels. Here, we tackle the problem using athermal granular particles. Slow dynamics and structural evolution of granular packing upon tapping are monitored by fast X-ray tomography. When the packing are wet and short-range attractive interactions exist, we find a large amount of locally favoured structures with fivefold symmetry, which bear great structural similarity to colloidal gels. In addition, these structures are almost absent in dry packing with similar packing fractions. The study leads strong support for the geometrical frustration mechanism for dynamic arrest in both thermal and athermal systems with attractive interactions. It also suggests nontrivial structural mechanism, if exists, for dynamic arrest in systems with purely repulsive interactions.
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http://dx.doi.org/10.1038/ncomms6014DOI Listing
September 2014

A dynamic synchrotron X-ray imaging study of effective temperature in a vibrated granular medium.

Soft Matter 2014 Aug 16;10(29):5398-404. Epub 2014 Jun 16.

Department of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Rd., Shanghai 200240, China.

We present a dynamic synchrotron X-ray imaging study of the effective temperature Teff in a vibrated granular medium. By tracking the directed motion and the fluctuation dynamics of the tracers inside, we obtained Teff of the system using the Einstein relationship. We found that as the system unjams with increasing vibration intensities Γ, the structural relaxation time τ increases substantially which can be fitted by an Arrhenius law using Teff. And the characteristic energy scale of structural relaxation yielded by the Arrhenius fitting is E = 0.20 ± 0.02pd(3), where p is the pressure and d is the background particle diameter, which is consistent with those from hard sphere simulations in which the structural relaxation happens via the opening up of free volume against pressure.
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http://dx.doi.org/10.1039/c4sm00602jDOI Listing
August 2014