Changes In Local Density Research Articles

Evolution of Dendritic Austenite in Parallel With Eutectic in Compacted Graphite Iron Under Three Cooling Conditions

Shrinkage defects are common problems in industrially produced metal cast components. Local density changes occur during freezing, which demand material transport between parts of the casting, often involving flow of liquid through partially solid regions. Cast alloys typically freeze with a dendritic morphology, which large interface against the liquid restricts liquid flow. Recent research also indicates that this dendritic structure has an impact on the mechanical properties of the final material. For these reasons it is important to understand and predict the evolution of this structure through the solidification of cast alloys. In this work, the evolution of the dendritic austenite structure is investigated in a near-eutectic compacted graphite iron solidified under three different cooling conditions. The solidification was interrupted by water quenching, enabling characterization of the dendritic austenite structure at different stages of solidification. Higher cooling rate was found to promote a more coherent dendritic austenite structure which constituted a larger volume fraction. In parallel with growth of the eutectic, the amount of dendritic austenite in extra-eutectic regions continued to rise. This rise was associated with both tip growth of new dendrites and with growth by thickening of existing dendrites.

Technical note: Towards more realistic 4DCT(MRI) numerical lung phantoms.

Numerical 4D phantoms, together with associated ground truth motion, offer a flexible and comprehensive data set for realistic simulations in radiotherapy and radiology in target sites affected by respiratorymotion. We present an openly available upgrade to previously reported methods for generating realistic 4DCT lung numerical phantoms, which now incorporate respiratory ribcage motion and improved lung density representation throughout the breathingcycle. Density information of reference CTs, toget her with motion from multiple breathing cycle 4DMRIs have been combined to generate synthetic 4DCTs (4DCT(MRI)s). Inter-subject correspondence between the CT and MRI anatomy was first established via deformable image registration (DIR) of binary masks of the lungs and ribcage. Ribcage and lung motions were extracted independently from the 4DMRIs using DIR and applied to the corresponding locations in the CT after post-processing to preserve sliding organ motion. In addition, based on the Jacobian determinant of the resulting deformation vector fields, lung densities were scaled on a voxel-wise basis to more accurately represent changes in local lung density. For validating this process, synthetic 4DCTs, referred to as 4DCT(CT)s, were compared to the originating 4DCTs using motion extracted from the latter, and the dosimetric impact of the new features of ribcage motion and density correction were analyzed using pencil beam scanned proton 4D dosecalculations. Lung density scaling led to a reduction of maximum mean lung Hounsfield units (HU) differences from 45 to 12HU when comparing simulated 4DCT(CT)s to their originating 4DCTs. Comparing 4D dose distributions calculated on the enhanced 4DCT(CT)s to those on the original 4DCTs yielded 2%/2mm gamma pass rates above 97% with an average improvement of 1.4% compared to previously reportedphantoms. A previously reported 4DCT(MRI) workflow has been successfully improved and the resulting numerical phantoms exhibit more accurate lung density representations and realistic ribcagemotion.

Deformable registration with intensity correction for CESM monitoring response to Neoadjuvant Chemotherapy

This paper proposes a robust longitudinal registration method for Contrast Enhanced Spectral Mammography in monitoring neoadjuvant chemotherapy. Because breast texture intensity changes with the treatment, a non-rigid registration procedure with local intensity compensations is developed. The approach allows registering the low energy images of the exams acquired before and after the chemotherapy. The measured motion is then applied to the corresponding recombined images. The difference of registered images, called residual, makes vanishing the breast texture that did not changed between the two exams. Consequently, this registered residual allows identifying local density and iodine changes, especially in the lesion area. The method is validated with a synthetic NAC case where ground truths are available. Then the procedure is applied to 51 patients with 208 CESM image pairs acquired before and after the chemotherapy treatment. The proposed registration converged in all 208 cases. The intensity-compensated registration approach is evaluated with different mathematical metrics and through the repositioning of clinical landmarks (RMSE: 5.9 mm) and outperforms state-of-the-art registration techniques.

Stochastic effects in bacterial communication mediated by extracellular vesicles.

Quorum sensing (QS) allows bacterial cells to sense changes in local cell density and, hence, to regulate multicellular processes, including biofilm formation, regulation of virulence, and horizontal gene transfer. While, traditionally, QS was thought to involve the exchange of extracellular signal molecules free in solution, recent experiments have shown that for some bacterial systems a substantial fraction of signal molecules are packaged and delivered in extracellular vesicles. How the packaging of signal molecules in extracellular vesicles influences the ability of cells to communicate and coordinate multicellular behaviors remains largely unknown. We present here a stochastic reaction-diffusion model of QS that accounts for the exchange of both freely diffusing and vesicle-associated signal molecules. We find that the delivery of signal molecules via extracellular vesicles amplifies local fluctuations in the signal concentration, which can strongly affect the dynamics and spatial range of bacterial communication. For systems with multiple bacterial colonies, extracellular vesicles provide an alternate pathway for signal transport between colonies, and may be crucial for long-distance signal exchange in environments with strong degradation of free signal molecules.

Local density changes and carbonate rotation enable Ba incorporation in amorphous calcium carbonate.

Incorporation of a Ba impurity in amorphous calcium carbonate (ACC) is shown with ab initio molecular dynamics simulations to have a long-range effect on its atomic-level structure and to be energetically favoured relative to incorporation in crystalline calcium carbonate polymorphs. The ability of carbonate ions to rotate and of ACC to undergo local density changes explain ACC's propensity for incorporating divalent metal impurities with a wide range of ionic radii. These findings provide an atomic-level basis for understanding the significant effects of low concentrations of impurities on the structure of ACC.

Competing effects of buoyancy-driven and electrothermal flows for Joule heating-induced transport in microchannels

Ionic fluids subjected to externally applied electric fields experience Joule heating, which increases with the increased electric field and ionic conductivity of the medium. Temperature gradients induced by Joule heating can create buoyancy-driven flows produced by local density changes, as well as electrothermal transport due to the temperature-dependent variations in fluid permittivity and conductivity. This manuscript considers Joule heating-induced transport in microchannels by a pair of electrodes under alternating current electric fields. Resulting buoyancy-driven and alternating current electrothermal (ACET) flows are investigated theoretically, numerically and experimentally. Proper normalizations of the governing equations led to the ratio of the electrothermal and buoyancy velocities, as a new non-dimensional parameter, which enabled the construction of a phase diagram that can predict the dominance of ACET and buoyancy-driven flows as a function of the channel size and electric field. Numerical results were used to verify the phase diagram in various height microchannels for different ionic conductivity fluids and electric fields, while the numerical results were validated using the micro-particle-image velocimetry technique. The results show that ACET flow prevails when the channel dimensions are small, and the electric potentials are high, whereas buoyancy-driven flow becomes dominant for larger channel heights. The present study brings insights into Joule heating-induced transport phenomena in microfluidic devices and provides a pathway for the design and utilization of ACET-based devices by properly considering the co-occurring buoyancy-driven flow.

EVOLUTION OF THE FIELD OF DISTRIBUTED DEFECTS IN A CRYSTAL DURING CONTACT INTERACTION WITH A SYSTEM OF RIGID STAMPS

The article discusses the mathematical modeling for the evolution of the stress-strain state and fields of defects in crystals during their contact interaction with a system of rigid punches. From a macroscopic point of view, the redistribution of defects is characterized by inelastic (viscoplastic) deformation, and therefore the processes under study can be classified as elastic-viscoplastic. Elastic and inelastic deformations are assumed to be finite. To take into account inelastic deformations, it is proposed to use a differential-geometric approach, in which the evolution of the fields of distributed defects is completely characterized by measures of incompatible deformations and quantified by material connection invariants. This connection is generated by a non-Euclidean metric, which, in turn, is given by a field of symmetric linear mappings that define (inconsistent) deformations of the crystal. Since the development of local deformations depends both on thecontact interaction at the boundary and on the distribution of defects in the bulk of the crystal, the simulation problem turns out to be coupled. It is assumed that the local change in the defect density is determined by the first-order Alexander Haasen Sumino evolutionary law, which takes into account the deviatoric part of the stress field. An iterative algorithm has been developed to find coupled fields of local deformations and defects density. The numerical analysis for the model problem was provided for a silicon crystal in the form of a parallelepiped, one face of which is rigidly fixed, and a system of rigid stamps acts on the opposite face. The three-constant Mooney Rivlin potential was used to model the local elastic response.

N/S co-doped microporous zeolite-templated carbon for efficient CO2 adsorption and separation

Heteroatomic doping of porous carbon materials can give rise to variations in the electronic structure and properties, and has been used to enhance the adsorption and separation of CO2. Herein, N and S doped zeolite-templated carbon (ZTC) are synthesized to study their CO2 adsorption and selective from CO2/N2. ZTC-N, ZTC-S and ZTC-NS exhibit comparatively high CO2 adsorption of 6.12, 7.73 and 9.32 mmol g−1 at 0 °C, which are more than three times than that of ZTC (1.98 mmol g−1). The CO2 adsorption capacity for ZTC-NS at 25 °C and 1 bar can reach as high as 6.03 mmol g−1, which can still occupy approximately 96% after 5 cycles. The obtained carbon materials ZTC-NS with the smallest BET surface area show the highest CO2 adsorption capacity, which indicate an obvious synergistic enhancement effect between sulfur and nitrogen doping. The detailed theoretical calculations indicate N and S co-doping into porous carbon materials assists the change of electrostatic surface potential and local electronic density, thus enhanced the CO2 adsorption.

Unravelling Disorder Effects on Thermoelectric Properties of Semicrystalline Polymers in a Wide Range of Doping Levels.

Thermoelectric (TE) performance of a specific semicrystalline polymer is studied experimentally only in a limited range of doping levels with molecular doping methods. The doping level is finely controlled via in situ electrochemical doping in a wide range of carrier concentrations with an electrolyte ([PMIM]+ [TFSI]- )-gated organic electrochemical transistor system. Then, the charge generation/transport and TE properties of four p-type semicrystalline polymers are analyzed and their dynamic changes of crystalline morphologies and local density of states (DOS) during electrochemical doping are compared. These polymers are synthesized based on poly[(2,5-bis(2-alkyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophene-2-yl)benzo[c][1,2,5]thiadiazole)] by varying side chains: With oligoethylene glycol (OEG) substituents, facile p-doping is achieved because of easy penetration of TFSI- ions into the polymer matrix. However, the charge transport is hindered with longer OEG chains length because of the enhanced insulation. Therefore, with the shortest OEG substituents the electrical conductivity (30.1 S cm-1 ) and power factor (2.88 µW m-1 K-2 ) are optimized. It is observed that all polymers exhibit p- to n-type transition in Seebeck coefficients in heavily doped states, which can be achieved by electrochemical doping. These TE behaviors are interpreted based on the relation between the localized DOS band structure and molecular packing structure during electrochemical doping.

Automated Plume Sentry Observations During International Space Station Thermal Control System Venting

Contamination from outgassed materials, venting, leaks, and impinging thruster plumes can cause deleterious effects to sensitive experiments on the International Space Station. In low Earth orbit, intermolecular collisions between neutral contaminants and ions in the ambient ionospheric environment result in an elevation in the local charged particle density through a process called charge exchange (CEX). Of particular concern to spacecraft engineers is the acceleration of these ionized neutrals to sensitive negatively charged spacecraft surfaces. The increase in plasma density due to CEX can be readily measured using plasma diagnostics such as electrostatic energy analyzers. These instruments are capable of monitoring charged particle flux and are sensitive enough to measure small changes in local density, thus allowing for the detection of CEX contaminants. The automated plume sentry (APS) is one such instrument, and it has successfully detected CEX plasma generated by the planned ammonia venting of the external active thermal control system. The data collected by the APS will support the development of improved computational models to predict the propagation of outgassed materials and plumes in the ionosphere and complex gas–surface interactions resulting in spacecraft surface contamination.

Bond Bundle Analysis of Ketosteroid Isomerase.

Bond bundle analysis is used to investigate enzymatic catalysis in the ketosteroid isomerase (KSI) active site. We identify the unique bonding regions in five KSI systems, including those exposed to applied oriented electric fields and those with amino acid mutations, and calculate the precise redistribution of electron density and other regional properties that accompanies either enhancement or inhibition of KSI catalytic activity. We find that catalytic enhancement results from promoting both inter- and intra-molecular electron density redistribution, between bond bundles and bond wedges within the KSI-docked substrate molecule, in the forward direction of the catalyzed reaction. Though the redistribution applies to both types of perturbed systems and is thus suggestive of a general catalytic role, we observe that bond properties (e.g., volume vs energy vs electron count) can respond independently and disproportionately depending on the type of perturbation. We conclude that the resulting catalytic enhancement/inhibition proceeds via different mechanisms, where some bond properties are utilized more by one type of perturbation than the other. Additionally, we find that the correlations between bond wedge properties and catalyzed reaction barrier energies are additive to predict those of bond bundles and atomic basins, providing a rigorous grounding for connecting changes in local charge density to resulting shifts in reaction barrier energy.

Helicity dependent photoresistance measurement vs. beam-shift thermal gradient

Optical detection techniques are among the most powerful methods used to characterize spintronic phenomena. The spin orientation can affect the light polarization, which, by the reciprocal mechanism, can modify the spin density. Numerous recent experiments, report local changes in the spin density induced by a circularly polarized focused laser beam. These effects are typically probed electrically, by detecting the variations of the photoresistance or photocurrent associated to the reversal of the light helicity. Here we show that in general, when the light helicity is modified, the beam profile is slightly altered, and the barycenter of the laser spot is displaced. Consequently, the temperature gradients produced by the laser heating will be modulated, producing thermo-electric signals that alternate in phase with the light polarization. These unintended signals, having no connection with the electron spin, appear under the same experimental conditions and can be easily misinterpreted. We show how this contribution can be experimentally assessed and removed from the measured data. We find that even when the beam profile is optimized, this effect is large, and completely overshadows the spin related signals in all the materials and experimental conditions that we have tested.

Quantitative focused laser differential interferometry with hypersonic turbulent boundary layers.

The effect of turbulent wind-tunnel-wall boundary layers on density change measurements obtained with focused laser differential interferometry (FLDI) was studied using a detailed direct numerical simulation (DNS) of the wall from the Boeing/AFOSR Mach-6 Quiet Tunnel run in its noisy configuration. The DNS was probed with an FLDI model that is capable of reading in three-dimensional time-varying density fields and computing the FLDI response. Simulated FLDI measurements smooth the boundary-layer root-mean-square (RMS) profile relative to true values obtained by directly extracting the data from the DNS. The peak of the density change RMS measured by the FLDI falls within 20% of the true density change RMS. A relationship between local spatial density change and temporal density fluctuations was determined and successfully used to estimate density fluctuations from the FLDI measurements. FLDI measurements of the freestream fluctuations are found to be dominated by the off-axis tunnel-wall boundary layers for lower frequencies despite spatial suppression provided by the technique. However, low-amplitude (0.05%-5% of the mean density) target signals placed along the tunnel centerline were successfully measured over the noise of the boundary layers (which have RMS values of about 12% of the mean). Overall, FLDI was shown to be a useful technique for making quantitative turbulence measurements and to measure finite-width sinusoidal signals through turbulent boundary layers, but may not provide enough off-focus suppression to provide accurate freestream noise measurements, particularly at lower frequencies.

A haplodiploid mite adjusts fecundity and sex ratio in response to density changes during the reproductive period.

Population density is one of the main socio-environmental factors that have critical impacts on reproduction of animals. Consequently, they need to adjust their reproductive strategies in response to changes of local population density. In this study we used a haplodiploid spider mite, Tetranychus ludeni Zacher (Acari: Tetranychidae), to test how population density dynamics during the reproductive period altered female reproductive performance. We demonstrate that females produced fewer eggs with a significantly higher female-biased sex ratio in dense populations. Reducing fecundity and increasing daughter production in a dense environment could be an advantageous strategy to minimise the intensity of local food competition. However, females also reduced their fecundity after arrival in a new site of larger area from a dense population, which may be associated with higher web production costs because females need to produce more webs to cover the larger area. There was no trade-off between egg number and size, and egg size had little impact on reproductive fitness. Therefore, T. ludeni females could adapt to the shift of population density during their reproductive period by manipulating the fecundity and offspring sex ratio but not the egg size.

Temporal variability of quasi-linear pitch-angle diffusion

Kinetic wave-particle interactions in Earth’s outer radiation belt energize and scatter high-energy electrons, playing an important role in the dynamic variation of the extent and intensity of the outer belt. It is possible to model the effects of wave-particle interactions across long length and time scales using quasi-linear theory, leading to a Fokker-Planck equation to describe the effects of the waves on the high energy electrons. This powerful theory renders the efficacy of the wave-particle interaction in a diffusion coefficient that varies with energy or momentum and pitch angle. In this article we determine how the Fokker-Planck equation responds to the temporal variation of the quasi-linear diffusion coefficient in the case of pitch-angle diffusion due to plasmaspheric hiss. Guided by in-situ observations of how hiss wave activity and local number density change in time, we use stochastic parameterisation to describe the temporal evolution of hiss diffusion coefficients in ensemble numerical experiments. These experiments are informed by observations from three different example locations in near-Earth space, and a comparison of the results indicates that local differences in the distribution of diffusion coefficients can result in material differences to the ensemble solutions. We demonstrate that ensemble solutions of the Fokker-Planck equation depend both upon the timescale of variability (varied between minutes and hours), and the shape of the distribution of diffusion coefficients. Based upon theoretical construction of the diffusion coefficients and the results presented here, we argue that there is a useful maximum averaging timescale that should be used to construct a diffusion coefficient from observations, and that this timescale is likely less than the orbital period of most inner magnetospheric missions. We discuss time and length scales of wave-particle interactions relative to the drift velocity of high-energy electrons and confirm that arithmetic drift-averaging is can be appropriate in some cases. We show that in some locations, rare but large values of the diffusion coefficient occur during periods of relatively low number density. Ensemble solutions are sensitive to the presence of these rare values, supporting the need for accurate cold plasma density models in radiation belt descriptions.

3D Flame surface density measurements via orthogonal cross-planar mie scattering in a low-turbulence bunsen flame

Quasi-instantaneous Mie scattering measurements were conducted to determine the flame surface location using two orthogonal planes to obtain the 3D orientation and measurement of 3D flame surface density (FSD) on a stabilised piloted Bunsen burner with turbulence levels of 1 to 2.5 times the laminar flame speed. A double-pulsed 527 nm high-frequency laser (part of a high-frequency particle image velocimetry dual-head setup) was split into two separated laser beams through a polarizer to generate laser sheets at 3 kHz on a vertical plane and a horizontal plane. The vertical plane across the centerline of the Bunsen-stabilized flame was kept constant, whilst the height of the horizontal plane was adjusted from the base of the flame for different heights. The flame edge was defined as the location where droplet tracers disappear, and calculated based on the local change in the number density of the flame edge. Projections of the flame normal vector onto two measurement planes were used to calculate directing angles on each plane, and the 3D FSD was estimated based on the measured angles at the intersecting lines of two planes. A comparison of the 2D to 3D FSD magnitudes was made, for cases with and without mean angle corrections. The results show that the 2D FSD approximations are lower than the 3D measurements by a factor of 20–30% if uncorrected, and up to 28% after corrections.

LB1016 Gradual differentiation uncoupled from cell cycle exit generates heterogeneity in the epidermal stem cell layer

In the mammalian skin epidermis, it has long been unclear how molecularly heterogenous stem/progenitor cell populations fit into the complete trajectory of epidermal differentiation. Here, we combine in vivo imaging with single-cell RNA sequencing to define the early stages of epidermal stem cell differentiation at the molecular and behavioral level. We show that differentiation, from commitment to exit from the stem cell layer, is a multi-day process wherein cells transit through a gradual continuum of transcriptional changes. By tracking epidermal regions over many days, we find that differentiation-committed cells remain capable of dividing to produce daughter cells fated to further differentiate, demonstrating that differentiation is uncoupled from cell cycle exit. These cell divisions occur as a response to local density changes, and are not required as part of an obligate transit amplifying program but instead help to buffer the differentiating cell pool upon disruption of the epidermal barrier. Thus, instead of distinct contributions from multiple progenitor types, a single continuous differentiation process shaped by feedback from the local environment fuels lifelong turnover of the epidermis.

A Computational Study: Structural and Electronic Properties of Some Transition Metal Doped Bilayer Graphene Systems

Graphene, which is accepted as the main material of nanomaterials, attracts great attention thanks to its applicability in almost every field and its superior properties. The zero-band graphene gap is a problem that scientists must overcome in designing new electronics. Tailoring the electronic properties of graphene systems by making a change in the band gap allow us great advantages. In this study, Intercalation of transition metal (TM) atom to graphene systems as sandwich-like graphene|TM|graphene structures were investigated by ab initio first principle Density Functional Theory (DFT) computations. GGA with BPW91 basis set were used for DFT calculations. DFT calculations were performed on W, Re, and Os transition metal atoms intercalted between bilayer graphene (BLG). After geometry optimization of graphene|TM|graphene structures, graphene layers doped with nitrogen atoms by substitutional doping for investigation of change in electronic behavior. The electronic behaviour of metal intercalated BLG structures can be modified by type of transition metal and dopant as a result of charge transfer. Substitutional doping with nitrogen atoms to graphene structures showed a change in local density due to the charge transfer as a result of its extra one electron. Placing a transition metal atom between BLG layers leads to constriction in the band gap with a boost in conductive character.

Patterns of functional connectivity alterations induced by alcohol reflect somatostatin interneuron expression in the human cerebral cortex

Acute alcohol administration affects functional connectivity, yet the underlying mechanism is unknown. Previous work suggested that a moderate dose of alcohol reduces the activity of gamma-aminobutyric acidergic (GABAergic) interneurons, thereby leading to a state of pyramidal disinhibition and hyperexcitability. The present study aims to relate alcohol-induced changes in functional connectivity to regional genetic markers of GABAergic interneurons. Healthy young adults (N = 15, 5 males) underwent resting state functional MRI scanning prior to alcohol administration, immediately and 90 min after alcohol administration. Functional connectivity density mapping was performed to quantify alcohol-induced changes in resting brain activity between conditions. Patterns of differences between conditions were related to regional genetic markers that express the primary GABAergic cortical interneuron subtypes (parvalbumin, somatostatin, and 5-hydroxytryptamine receptor 3A) obtained from the Allen Human Brain Atlas. Acute alcohol administration increased local functional connectivity density within the visual cortex, sensorimotor cortex, thalamus, striatum, and cerebellum. Patterns of alcohol-induced changes in local functional connectivity density inversely correlated with somatostatin cortical gene expression. These findings suggest that somatostatin-expressing interneurons modulate alcohol-induced changes in functional connectivity in healthy individuals.

3D total variation denoising in X-CT imaging applied to pore extraction in additively manufactured parts

X-ray computed tomography (X-CT) plays an important role in non-destructive quality inspection and process evaluation in metal additive manufacturing, as several types of defects such as keyhole and lack of fusion pores can be observed in these 3D images as local changes in material density. Segmentation of these defects often relies on threshold methods applied to the reconstructed attenuation values of the 3D image voxels. However, the segmentation accuracy is affected by unavoidable X-CT reconstruction features such as partial volume effects, voxel noise and imaging artefacts. These effects create false positives, difficulties in threshold value selection and unclear or jagged defect edges. In this paper, we present a new X-CT defect segmentation method based on preprocessing the X-CT image with a 3D total variation denoising method. By comparing the changes in the histogram, threshold selection can be significantly better, and the resulting segmentation is of much higher quality. We derive the optimal algorithm parameter settings and demonstrate robustness for deviating settings. The technique is presented on simulated data sets, compared between low- and high-quality X-CT scans, and evaluated with optical microscopy after destructive tests.