Basab Chattopadhyay

The inherent instability and risk of liquefaction associated with unstable glaciomarine clay are both scientifically intriguing and societally important. Stabilizing sensitive soils is increasingly necessary with the unfolding climate crisis, which leads to wilder and wetter weather. The relevant length scales extend from nanometer-sized pores to kilometer-sized geological features. Laboratory experiments are important for gaining a better understanding of these phenomena. Here, we present a study of the ultra-soft sandstone Saltwash South , which essentially consists of quartz particles glued together by clay, as a proxy for nanoscopically fine-grained sensitive soil to facilitate time-resolved imaging of the onset of activation and liquefaction. By employing in situ time-resolved combined X-ray and neutron computed tomography (CT), we visualize the structural deformation caused by the presence of water and salts. While neutron imaging was sensitive to the presence of normal and heavy water, simultaneous X-ray imaging was used to measure the porous structure, swelling, and initial liquefaction response of the consolidated rock. Uniform expansion was observed in regions exposed to water, reflecting clay swelling and disaggregation. In tightly confined samples, the swelling and disaggregation were suppressed. Finally, we discuss future perspectives of this promising approach to studying liquefaction phenomena in porous media.

Calcium carbonate ( Ca CO 3 ) is a widely occurring mineral that is crucial to many natural and synthetic processes. Ca CO 3 crystallizes in three anhydrous polymorphs and often serves as a model system to study nucleation and growth phenomena. In this article, we employ coherent diffraction imaging coupled with wide-angle x-ray diffraction (CXDI-WAXD) to investigate the three-dimensional (3D) morphology and crystal structure of Ca CO 3 microparticles crystallized under different reaction temperatures. We report particles containing both vaterite and aragonite, which we interpret as an intermediate growth stage between early vaterite and later aragonite formation. The results are based on the density variations observed in the 3D tomographic CXDI images, x-ray diffraction, and focussed ion beam-transmission electron microscopy study of the mixed vaterite-aragonite microparticles. A computational study based on Monte Carlo and molecular dynamics energy minimization was used to identify the small energetic variations between the different crystalline forms of Ca CO 3 . The results presented in this article provide a better understanding of the morphology-structure correlation for Ca CO 3 polymorphs, depicting a transitional stage where both vaterite and aragonite coexist.

Time-resolved X-ray computed tomography (4D-CT) enables dynamic processes within objects to be tracked over time. A key application of 4D-CT is the scientifically and societally important study of multiphase flow in porous media. Obtaining high-quality 4D-CT images is challenging because a high data acquisition rate must be combined with bespoke algorithms for faithful reconstruction. Here, we introduce NeCT, a physics-based deep learning model achieving state-of-the-art sparse-view and 4D-CT reconstructions. NeCT enables reconstruction of 4D objects using an implicit neural representation in space and time. With standard micro-CT instruments, NeCT achieves a temporal resolution approaching a few seconds. Reconstructions of benchmark static tomography datasets show that NeCT also outperforms established algorithms in 3D. We demonstrate fast imaging of liquid imbibition in sandstone, with the high spatiotemporal resolution allowing salt dissolution and pore-filling events to be directly observed. Finally, opportunities beyond classic CT with our dynamic continuous approach are discussed.

Calcium carbonate (CaCO3) is ubiquitous due to its presence in several biomineralized entities, and it often serves as a model system to study crystal nucleation and growth. CaCO3 exists in three crystalline polymorphs, namely, calcite, vaterite, and aragonite—all of which can be precipitated in the laboratory under suitable reaction conditions, e.g., temperature and concentration. The crystallization of the different polymorphs of CaCO3 can be affected by additives that inhibit or promote the formation of specific polymorphs or alter their growth and morphology. The role of one such additive, fetuin-A, a plasma carrier protein in the animal bloodstream, on the structure and morphology of CaCO3 microparticles was investigated in this article. We have utilized coherent x-ray diffraction imaging in combination with wide-angle x-ray diffraction and transmission electron microscopy to visualize the 3D morphology of CaCO3 grown under well-defined experimental conditions. The resulting CaCO3 particles crystallized in the presence of fetuin-A were primarily composed of calcite that showed unidirectional faceted growth along the (104) planes, along with minor vaterite inclusion and internal cavities containing amorphous CaCO3 (ACC). Our observation of a single particle containing ACC, vaterite, and calcite provides direct evidence of the full crystallization pathway of CaCO3 with nanoscale spatial resolution, capturing the transformation from metastable ACC to the thermodynamically stable phase, calcite.

Frost heave is the common phenomenon in cold climates of soil being elevated by freezing. Frost heave is caused by ice lenses forming in a spatially repeated pattern downward into the ground, driven by a temperature gradient, and requiring access to unfrozen water. Here, we present an in situ combined x-ray and neutron computed tomography study of ice lens formation and frost heave in silt. A bespoke sample environment was developed to control a vertical temperature gradient across the sample while maintaining contact between the freezing silt and an external water reservoir. Our results demonstrate that detailed and quantitative time-resolved volumetric images (“4D-CT”) can be retrieved, showing the initiation and growth of ice lenses. Finally, we discuss briefly the future possibilities our approach opens for studying phase transitions.

Being able to image the microstructure of growth cartilage is important for understanding the onset and progression of diseases such as osteochondrosis and osteoarthritis, as well as for developing new treatments and implants. Studies of cartilage using conventional optical brightfield microscopy rely heavily on histological staining, where the added chemicals provide tissue-specific colours. Other microscopy contrast mechanisms include polarization, phase- and scattering contrast, enabling non-stained or ‘label-free’ imaging that significantly simplifies the sample preparation, thereby also reducing the risk of artefacts. Traditional high-performance microscopes tend to be both bulky and expensive. Computational imaging denotes a range of techniques where computers with dedicated algorithms are used as an integral part of the image formation process. Computational imaging offers many advantages like 3D measurements, aberration correction and quantitative phase contrast, often combined with comparably cheap and compact hardware. X-ray microscopy is also progressing rapidly, in certain ways trailing the development of optical microscopy. In this study, we first briefly review the structures of growth cartilage and relevant microscopy characterization techniques, with an emphasis on Fourier ptychographic microscopy (FPM) and advanced x-ray microscopies. We next demonstrate with our own results computational imaging through FPM and compare the images with hematoxylin eosin and saffron (HES)-stained histology. Zernike phase contrast, and the nonlinear optical microscopy techniques of second harmonic generation (SHG) and two-photon excitation fluorescence (TPEF) are explored. Furthermore, X-ray attenuation-, phase- and diffraction-contrast computed tomography (CT) images of the very same sample are presented for comparisons. Future perspectives on the links to artificial intelligence, dynamic studies and in vivo possibilities conclude the article.

Slow multiphase flow in porous media is intriguing because its underlying dynamics is almost deterministic, yet depends on a hierarchy of spatiotemporal processes. There has been great progress in the experimental study of such multiphase flows, but three-dimensional (3D) microscopy methods probing the pore-scale fluid dynamics with millisecond resolution have been lacking. Yet, it is precisely at these length and time scales that the crucial pore-filling events known as Haines jumps take place. Here, we report four-dimensional (4D) (3D + time) observations of multiphase flow in a consolidated porous medium as captured in situ by stroboscopic X-ray micro-tomography. With a total duration of 6.5 s and 2 kHz frame rate, our experiments provide unprecedented access to the multiscale liquid dynamics. Our tomography strategy relies on the fact that Haines jumps, although irregularly spaced in time, are almost deterministic, and therefore repeatable during imbibition-drainage cycling. We studied the time-dependent flow pattern in a porous medium consisting of sintered glass shards. Exploiting the repeatability, we could combine the radiographic projections recorded under different angles during successive cycles into a 3D movie, allowing us to reconstruct pore-scale events, such as Haines jumps, with a spatiotemporal resolution that is two orders of magnitude higher than was hitherto possible. This high resolution allows us to explore the detailed interfacial dynamics during drainage, including fluid-front displacements and velocities. Our experimental approach opens the way to the study of fast, yet deterministic mesoscopic processes also other than flow in porous media.

The formation of calcium carbonate (CaCO₃) nanoparticles (NPs) in the presence of polystyrene sulfonate (PSS) as an additive was examined by time-resolved small-angle X-ray scattering (SAXS) in a flow system that mimics experimental conditions used at home facilities where the precipitation can be achieved in a beaker. The experiments were carried out at low concentrations to remain in the dilute regime. A model-independent analysis was performed using the Porod invariant which defines the scale factor, leaving only the distribution of radii as the adjustable parameter. The presence of the PSS additive strongly retards the precipitation of CaCO₃ NPs. The formation of NPs reaches a state of equilibrium after a few minutes. Here, it is shown that the concentration of precursors at a fixed PSS concentration plays a key role in determining the size of the NPs obtained. A full analysis of the SAXS patterns was carried out using the Hurd–Flower model to account for the weaker intensity decay than the classical Porod behaviour. The temporal evolution of the particle radii was determined. Wide-angle X-ray scattering experiments carried out simultaneously show that the particles formed have the structure of vaterite with growth consistent with the evolution of the Porod invariant.

The development of systems capable of responding to environmental changes, such as humidity, requires the design and assembly of highly sensitive and efficiently transducing elements. Such a challenge can be mastered only by disentangling the role played by each component of the responsive system, thus ultimately achieving high performance by optimizing the synergistic contribution of all functional elements. Here, we designed and synthesized a novel [1]benzothieno[3,2-b][1]benzothiophene derivative equipped with hydrophilic oligoethylene glycol lateral chains (OEG-BTBT) that can electrically transduce subtle changes in ambient humidity with high current ratios (>10⁴) at low voltages (2 V), reaching state-of-the-art performance. Multiscale structural, spectroscopical, and electrical characterizations were employed to elucidate the role of each device constituent, viz., the active material’s BTBT core and OEG side chains, and the device interfaces. While the BTBT molecular core promotes the self-assembly of (semi)conducting crystalline films, its OEG side chains are prone to adsorb ambient moisture. These chains act as hotspots for hydrogen bonding with atmospheric water molecules that locally dissociate when a bias voltage is applied, resulting in a mixed electronic/protonic long-range conduction throughout the film. Due to the OEG-BTBT molecules’ orientation with respect to the surface and structural defects within the film, water molecules can access the humidity-sensitive sites of the SiO₂ substrate surface, whose hydrophilicity can be tuned for an improved device response. The synergistic chemical engineering of materials and interfaces is thus key for designing highly sensitive humidity-responsive electrical devices whose mechanism relies on the interplay of electron and proton transport.

Shales have a complex mineralogy with structural features spanning several length scales, making them notoriously difficult to fully understand. Conventional attenuation-based X-ray computed tomography (CT) measures density differences, which, owing to the heterogeneity and sub-resolution features in shales, makes reliable interpretation of shale images a challenging task. CT based on X-ray diffraction (XRD-CT), rather than intensity attenuation, is becoming a well established technique for non-destructive 3D imaging, and is especially suited for heterogeneous and hierarchical materials. XRD patterns contain information about the mineral crystal structure, and crucially also crystallite orientation. Here, we report on the use of orientational imaging using XRD-CT to study crystallite-orientation distributions in a sample of Pierre shale. Diffraction-contrast CT data for a shale sample measured with its bedding-plane normal aligned parallel to a single tomographic axis perpendicular to the incoming X-ray beam are discussed, and the spatial density and orientation distribution of clay minerals in the sample are described. Finally, the scattering properties of highly attenuating inclusions in the shale bulk are studied, which are identified to contain pyrite and clinochlore. A path forward is then outlined for systematically improving the structural description of shales.