Structural analysis of disordered dimer packings

Jammed disordered packings of non-spherical particles show significant variation in the packing density as a function of particle shape for a given packing protocol. Rotationally symmetric elongated shapes such as ellipsoids, spherocylinders, and dimers, e.g., pack significantly denser than spheres over a narrow range of aspect ratios, exhibiting a characteristic peak at aspect ratios of αmax ≈ 1.4–1.5. However, the structural features that underlie this non-monotonic behaviour in the packing density are unknown. Here, we study disordered packings of frictionless dimers in three dimensions generated by a gravitational pouring protocol in LAMMPS. Focusing on the characteristics of contacts as well as orientational and translational order metrics, we identify a number of structural features that accompany the formation of maximally dense packings as the dimer aspect ratio α is varied from the spherical limit. Our results highlight that dimer packings undergo significant structural changes as α increases up to αmax manifest in the reorganisation of the contact configurations between neighbouring dimers, increasing nematic order, and decreasing local translational order. Remarkably, for α > αmax our metrics remain largely unchanged, indicating that the peak in the packing density is related to the interplay of structural rearrangements for α αmax

Structural Analysis and Optimisation of Jammed Granular Particles

Granular matter consisting of maximally dense random particle packings has been studied in science and industry. However, understanding the relationship between particle shape and random packing density has been challenging, so the question of "what shape packs with the highest density?" remains unanswered. While experiments and theory are restricted to investigating a few common shapes, numerical simulations allow studying the properties of packings of a diverse range of shapes. This thesis utilises simulations and machine learning methods to investigate particle geometry's role in packing behaviour by exploring a high-dimensional shape space. The first two parts of the thesis generate disordered packings of monodisperse and binary mixtures of frictionless dimers in three dimensions by a gravitational pouring protocol in LAMMPS and analyse their structural properties by packing density, contact statistics, and several order metrics. The results show that monodisperse dimers exhibit a non-monotonic behaviour in the packing density with the aspect ratio $\alpha$ and undergo significant structural rearrangements up to the characteristic peak at $\alpha_{\rm max} \approx 1.4-1.5$. This unique density maximum also exists in the binary case, irrespectively of the variation in shape or mixture composition, accompanied by similar microscopic rearrangements. The outcomings indicate that the packing density of binary mixtures is independent of the segregation state by holding an ideal mixing law. The final section of the thesis applies a Random forest regressor to a dataset of the packing density represented as a function of particle shape obtained by overlapping spheres to predict novel dense packing shapes. The regression model is applied to the data represented in the lower-dimensional space by Principal component analysis (PCA) and Kernel PCA separately. The findings illustrate that the regressor predicts dense packing shapes, and their novelty depends on the dimensionality reduction method. While it optimises the trimer that already exists in the dataset with the highest density for PCA, Kernel one predicts a distinct shape

Machine learning approaches for the optimization of packing densities in granular matter.

The fundamental question of how densely granular matter can pack and how this density depends on the shape of the constituent particles has been a longstanding scientific problem. Previous work has mainly focused on empirical approaches based on simulations or mean-field theory to investigate the effect of shape variation on the resulting packing densities, focusing on a small set of pre-defined shapes like dimers, ellipsoids, and spherocylinders. Here we discuss how machine learning methods can support the search for optimally dense packing shapes in a high-dimensional shape space. We apply dimensional reduction and regression techniques based on random forests and neural networks to find novel dense packing shapes by numerical optimization. Moreover, an investigation of the regression function in the dimensionally reduced shape representation allows us to identify directions in the packing density landscape that lead to a strongly non-monotonic variation of the packing density. The predictions obtained by machine learning are compared with packing simulations. Our approach can be more widely applied to optimize the properties of granular matter by varying the shape of its constituent particles

The diagnostic accuracy and cost-effectiveness of magnetic resonance spectroscopy and enhanced magnetic resonance imaging techniques in aiding the localisation of prostate abnormalities for biopsy : a systematic review and economic evaluation

Peer reviewedPublisher PD

Seismotectonics and rupture process of the MW 7.1 2011 Van reverse-faulting earthquake, eastern Turkey, and implications for hazard in regions of distributed shortening

The 2011 October 23 MW 7.1 Van earthquake in eastern Turkey caused ~600 deaths and caused widespread damage and economic loss. The seismogenic rupture was restricted to 10-25 km in depth, but aseismic surface creep, coincident with outcrop fault exposures, was observed in the hours to months after the earthquake. We combine observations from radar interferometry, seismology, geomorphology and Quaternary dating to investigate the geological slip rate and seismotectonic context of the Van earthquake, and assess the implications for continuing seismic hazard in the region. Transient post-seismic slip on the upper Van fault started immediately following the earthquake, and decayed over a period of weeks; it may not fully account for our long-term surface slip-rate estimate of ≥ 0.5 mm yr-1. Post-seismic slip on the Bostaniçi splay fault initiated several days to weeks after the main shock, and we infer that it may have followed the MW 5.9 aftershock on the 9th November. The Van earthquake shows that updip segmentation can be important in arresting seismic ruptures on dip-slip faults. Two large, shallow aftershocks show that the upper 10 km of crust can sustain significant earthquakes, and significant slip is observed to have reached the surface in the late Quaternary, so there may be a continuing seismic hazard from the upper Van fault and the associated splay. The wavelength of folding in the hanging wall of the Van fault is dominated by the structure in the upper 10 km of the crust, masking the effect of deeper seismogenic structures. Thus, models of subsurface faulting based solely on surface folding and faulting in regions of reverse faulting may underestimate the full depth extent of seismogenic structures in the region. In measuring the cumulative post-seismic offsets to anthropogenic structures, we show that Structure-from-Motion can be rapidly deployed to create snapshots of postseismic displacement.We also demonstrate the utility of declassified Corona mission imagery (1960s-1970s) for geomorphic mapping in areas where recent urbanization has concealed the geomorphic markers

Desarrollo de una tormenta ionosférica esencialmente positiva a lo largo de la red sudamericana de sondadores

Un mecanismo híbrido ha sido sugerido por diferentes autores para la dinámica de la región F ionosférica por el cual, posteriormente al depósito de energía durante la iniciación de una tormenta magnética, predomina un proceso de transporte meridional de ionización (fase positiva) sobre un mecanismo de cambio de la relación (O/N₂) (fase negativa), generalmente durante el primer día perturbado. Sin embargo, la tormenta del 4 de abril de 1967 que se comenta en el presente trabajo muestra dos aspectos interesantes: -1) una fase positiva significativa que se extiende por más de dos días a lo largo de la red americana de sondadores; y -2) una posible reversión del mecanismo híbrido, debido al hecho de que excursiones negativas, que aunque menos significativas son aún importantes, siguen a cambios en la actividad magnética. Las excursiones negativas correlacionan bastante bien con ascensos de la capa F, hecho aparentemente en conflicto con las ideas acerca de la generación de la fase positiva, indicando una predominancia de los cambios en (0/N₂) al menos durante las pocas horas que siguen a cada incremento de la actividad magnética.Different authors have suggested a hybrid mechanism for the dynamics of the F region by which, following the deposit of energy during the initiation of a magnetic storm, a meridional ionization transport process (positive phase) predominates generally over the mechanism of change of the (O/N₂) ratio (negative phase), during the first perturbed day. Nevertheless, the storm of the 4 of April 1967, coimnented in this paper, shows two interesting features: -1) a significant positive phase extending for more than two days along the american sounders network; and —2) a possible reversion of the hybrid mechanism, because of the fact that less significant but still important negative excursions follow changes in magnetic activity. The. negative excursions correlate quite well with ascents of the F-layer, this fact bein apparently in conflict with the ideas about the generation of the positive phase, indicanting a predominance of the (O/N₂) changes at least during a few hours following each increase in magnetic activity.Asociación Argentina de Geofísicos y Geodesta

Brønsted acid catalyzed Prins-Ritter reaction for selective synthesis of terpenoid-derived 4-amidotetrahydropyran compounds

A number of SO3H-functionalized solids (biochar, montmorillonites, carbon and halloysite nanotubes) has been studied as catalysts in the cascade Prins-Ritter reaction of (-)-isopulegol with benzaldehyde and acetonitrile for synthesis of octahydro-2H-chromene amides (as 4R- and 4S-isomers). A high selectivity to these products at 30 °C in the presence of H2O was observed on catalysts modified with chlorosulfonic acid (CSA) reaching 84% (4R/4S of 5.7) in the case of biochar, while a relatively large amount of octahydro-2H-chromenols (up to 31%), products of Prins condensation, was formed on the materials functionalized by 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (CSP). Although Prins condensation proceeds efficiently on weak acid sites, the Prins-Ritter reaction requires sulfated materials with strong (0.33 – 5.8 mmol/g) Brønsted acidity. Catalysts functionalized by CSP were stable, while for the materials modified with chlorosulfonic acid, leaching of -SO3H groups was observed. Nonetheless, on resistant Bioсhar-CSP, selectivity to the amides at 30 °C (67%) was higher than that with the commercial Amberlyst-15 (47%), and triflic acid at − 25 °C (62%). Similar selectivity to the desired products on Biochar-CSA (-SO3H groups) and H2SO4 (81–84%) as well as on Biochar-CSP (-PhSO3H) and with p-toluenesulfonic acid (67–70%) was observed. DFT calculations and experimental results showed that at 30 °C formation of 4S-amide thermodynamically is more beneficial than of alcohols and dehydration products. However, addition of water results in a sharp increase in the reaction rate and 4R-amide selectivity due to a change to the kinetic control, leading eventually to both high yields and stereoselectivity. The proposed reaction pathways also were confirmed by kinetic modelling.This work is part of the scientific activity of the Institute of Chemistry of New Materials, funded by the National Academy of Sciences of Belarus. Julián E. Sánchez thanks to Pontificia Universidad Javeriana for providing computational powder and to Universidad Jaume I (Pla de Promoció de la Investigació de la Universitat Jaume I) for the Post Doctoral Fellowship. Part of this work (synthesis and charaterization of sulfonic-acid catalysts presented in Fig. 2) was funded by the Portuguese funds through Fundação para a Ciência e a Tecnologia (FCT/MCTES) in the framework of the projects UIDB/50006/2020, UIDP/50006/2020. A.F.P. is also grateful to FCT for funding through the Individual Call to Scientific Employment Stimulus 2020.01614.CEECIND/CP1596/CT0007

Evidencia de la presencia de la garganta de densidad electrónica en los registros de los sondadores de la red sudamericana

La aparición de la garganta de latitud media en la concentración electrónica ionosférica, ha sido generalmente observada con registros de la ionosfera de tope. El análisis de los datos proporcionados por la red sudamericana de sondadores de superficie, lleva en forma evidente a la conclusión de que esa garganta también se presenta a latitudes menores que la de Puerto Stanley, correspondientes a valores del parámetro de cana L menores que los encontrados para la garganta "vista" desde satélites en otras regiones del globo terrestre. Asimismo, es clara la presencia de un "pedestal" de incremento de ionización, más allá de la garganta, pero a latitudes aún aleja das del óvalo auroral. Se atribuyen ambos procesos y su discrepancia con otros resultados, en cuanto a ubicación, a la presencia de la anomalía magnética sudamericana.The appearance of the middle latitude trough on the ionospheric electron concentration, has been generally observed with topside ionosphere records. The analysis of data provided by the South American network of surface sounders suggests, in an evident way, that the trough is also present at lower latitudes than Pt. Stanley cor responding to L-shell parameter values smaller than those found for the trough "seen" from satellite heights, for other regions of the terrestrial globe. Likewise, the presence of a "pedestal" of increased ionization is clear, beyond the trough, but at latitudes still somewhat distant from the auroral oval. Both effects, the trough and the pedestal at lower latitudes than "normal", could be justified by the existence of the South American magnetic anomaly.Asociación Argentina de Geofísicos y Geodesta