Vapor flux and recrystallization during dry snow metamorphism under a steady temperature gradient as observed by time-lapse micro-tomography

Dry snow metamorphism under an external temperature gradient is the most common type of recrystallization of snow on the ground. The changes in snow microstructure modify the physical properties of snow, and therefore an understanding of this process is essential for many disciplines, from modeling the effects of snow on climate to assessing avalanche risk. We directly imaged the microstructural changes in snow during temperature gradient metamorphism (TGM) under a constant gradient of 50 K m<sup>−1</sup>, using in situ time-lapse X-ray micro-tomography. This novel and non-destructive technique directly reveals the amount of ice that sublimates and is deposited during metamorphism, in addition to the exact locations of these phase changes. We calculated the average time that an ice volume stayed in place before it sublimated and found a characteristic residence time of 2–3 days. This means that most of the ice changes its phase from solid to vapor and back many times in a seasonal snowpack where similar temperature conditions can be found. Consistent with such a short timescale, we observed a mass turnover of up to 60% of the total ice mass per day. The concept of hand-to-hand transport for the water vapor flux describes the observed changes very well. However, we did not find evidence for a macroscopic vapor diffusion enhancement. The picture of {temperature gradient metamorphism} produced by directly observing the changing microstructure sheds light on the micro-physical processes and could help to improve models that predict the physical properties of snow

A 4-D dataset for validation of crystal growth in a complex three-phase material, ice cream

Four dimensional (4D, or 3D plus time) X-ray tomographic imaging of phase changes in materials is quickly becoming an accepted tool for quantifying the development of microstructures to both inform and validate models. However, most of the systems studied have been relatively simple binary compositions with only two phases. In this study we present a quantitative dataset of the phase evolution in a complex three-phase material, ice cream. The microstructure of ice cream is an important parameter in terms of sensorial perception, and therefore quantification and modelling of the evolution of the microstructure with time and temperature is key to understanding its fabrication and storage. The microstructure consists of three phases, air cells, ice crystals, and unfrozen matrix. We perform in situ synchrotron X-ray imaging of ice cream samples using in-line phase contrast tomography, housed within a purpose built cold-stage (-40 to +20oC) with finely controlled variation in specimen temperature. The size and distribution of ice crystals and air cells during programmed temperature cycling are determined using 3D quantification. The microstructural evolution of three-phase materials has many other important applications ranging from biological to structural and functional material, hence this dataset can act as a validation case for numerical investigations on faceted and non-faceted crystal growth in a range of materials

Artifacts in X-ray Dark-Field Tomography

Grating-based x-ray imaging provides three principle kinds of contrast: absorption, phase, and dark-field. Due to the availability of tomographic reconstruction algorithms for the dark-field contrast, it is now possible to take advantage of quantitative scatter information. However, the published algorithm is based on several assumptions that might be violated in reality. We use numerical simulations in order to identify artifacts in the reconstructions, which is crucial for the interpretation of experimental data

Measuring the specific surface area of snow with X-ray tomography and gas adsorption: comparison and implications for surface smoothness

Chemical and physical processes, such as heterogeneous chemical reactions, light scattering, and metamorphism occur in the natural snowpack. To model these processes in the snowpack, the specific surface area (SSA) is a key parameter. In this study, two methods, computed tomography and methane adsorption, which have intrinsically different effective resolutions – molecular and 30 μm, respectively – were used to determine the SSA of similar natural snow samples. Except for very fresh snow, the two methods give identical results, with an uncertainty of 3%. This implies that the surface of aged natural snow is smooth up to a scale of about 30 μm and that if smaller structures are present they do not contribute significantly to the overall SSA. It furthermore implies that for optical methods a voxel size of 10 μm is sufficient to capture all structural features of this type of snow; however, fresh precipitation appears to contain small features that cause an under-estimation of SSA with tomography at this resolution. The methane adsorption method is therefore superior to computed tomography for very fresh snow having high SSA. Nonetheless, in addition to SSA determination, tomography provides full geometric information about the ice matrix. It can also be advantageously used to investigate layered snow packs, as it allows measuring SSA in layers of less than 1 mm

Present and Future X-ray Tomographic Microscopy at TOMCAT

During its first four years of life, the TOMCAT beamline [1] at the Swiss Light Source has established itself as a state‐of‐the art hard x‐ray tomographic microscopy endstation for experiments on a large variety of samples, ranging from the fields of biology to materials science. It routinely performs absorption as well as phase‐contrast imaging with an isotropic voxel size ranging from 0.360 up to 14.8 microns. Phase contrast is obtained either with simple edge‐enhancement, a modified transport of intensity approach [2] or grating interferometry [3]. Typical acquisition times are on the order of a few minutes, depending on energy and resolution. A recently implemented automatic sample exchanger is now available for high‐throughput studies [4]. In addition to further developments in phase‐contrast imaging, current scientific activities at the beamline focus on pushing spatial and temporal resolution by a few orders of magnitude, aiming at nano‐ [5] and “real‐time” [6] tomography

Early tumor development captured through nondestructive, high resolution differential phase contrast X-ray imaging

Although a considerable amount is known about molecular dysregulations in later stages of tumor progression, much less is known about the regulated processes supporting initial tumor growth. Insight into such processes can provide a fuller understanding of carcinogenesis, with implications for cancer treatment and risk assessment. Work from our laboratory suggests that organized substructure emerges during tumor formation. The goal here was to examine the feasibility of using state-of-the-art differential phase contrast X-ray imaging to investigate density differentials that evolve during early tumor development. To this end the beamline for TOmographic Microscopy and Coherent rAdiology experimenTs (TOMCAT) at the Swiss Light Source was used to examine the time-dependent assembly of substructure in developing tumors. Differential phase contrast (DPC) imaging based on grating interferometry as implemented with TOMCAT, offers sensitivity to density differentials within soft tissues and a unique combination of high resolution coupled with a large field of view that permits the accommodation of larger tissue sizes (1 cm in diameter), difficult with other imaging modalities

3D-characterization of three-phase systems using X-ray tomography: tracking the microstructural evolution in ice cream

The microstructure of food is key to its sensorial perception, and methods to characterize the microstructure are of crucial importance in food engineering. Ice cream is a special example whose microstructure changes dramatically in response to temperature variations. Since ice cream is a multiphase material, the complex interactions among the phases and the physical mechanisms that drive the evolution of microstructure are not yet well understood. This is mostly due to the fact that observing the microstructure with traditional microscopic methods is destructive and does not allow the study of undisturbed samples. With X-ray micro-tomography, it is possible to overcome these limitations and carry out time lapse studies of the evolution of the microstructure of ice cream. Using iodine as a contrast agent, we measured the three-dimensional distribution of the three main phases (air, unfrozen sugar solution, and ice crystals) with a voxel size of 6 μm. An automated routine was developed that allows for the segmentation of the three phases. Based on the three-dimensional data we calculated the temporal evolution of air bubble sizes and ice crystal sizes during cyclic variations of temperature. Under the given temperature variations we find strong hints that for ice crystal coarsening a melt refreeze mechanism and for air microstructure coarsening coalescence are the dominating underlying mechanisms. This method—which can be applied to a plethora of soft multiphase materials—provides new insights into the coarsening mechanisms of multiphase materials and could contribute to a better understanding of complex materials

Cushing’s Disease in a 7-Month-Old Girl due to a Tumor Producing Adrenocorticotropic Hormone and Thyreotropin-Secreting Hormone

We present the case of a 7-month-old baby with Cushing’s disease due to an adrenocorticotropic hormone (ACTH)-secreting pituitary adenoma combined with cells producing thyreotropin-secreting hormone (TSH). In MRI scans, a contrast-enhancing lesion was seen inside the pituitary fossa, and it extended into the suprasellar region. On the assumption of a pituitary adenoma, surgery was performed. Corresponding with biochemical findings, histopathological evaluation revealed an ACTH- and TSH-producing tumor. Genetic analysis did not demonstrate an alteration at codon 201 (Arg) and 227 (Glu). To our knowledge, this is the first case described in a child of this age.Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich