Multicomponent reactions

Multicomponent Reactions appear to be ideal for any form of synthesis, because of their numerous advantages in terms of sustainability and selectivity in building up complex molecular architectures, with high molecular diversity. This Special Issue collects seven contributions which expand our knowledge about Multicomponent Reactions, providing a good overview about innovative reactivities and applications

Cascades and wall-normal fluxes in turbulent channel flows

The present work describes the multidimensional behaviour of scale-energy production, transfer and dissipation in wall-bounded turbulent flows. This approach allows us to understand the cascade mechanisms by which scale energy is transmitted scale-by-scale among different regions of the flow. Two driving mechanisms are identified. A strong scale-energy source in the buffer layer related to the near-wall cycle and an outer scale-energy source associated with an outer turbulent cycle in the overlap layer. These two sourcing mechanisms lead to a complex redistribution of scale energy where spatially evolving reverse and forward cascades coexist. From a hierarchy of spanwise scales in the near-wall region generated through a reverse cascade and local turbulent generation processes, scale energy is transferred towards the bulk, flowing through the attached scales of motion, while among the detached scales it converges towards small scales, still ascending towards the channel centre. The attached scales of wall-bounded turbulence are then recognized to sustain a spatial reverse cascade process towards the bulk flow. On the other hand, the detached scales are involved in a direct forward cascade process that links the scale-energy excess at large attached scales with dissipation at the smaller scales of motion located further away from the wall. The unexpected behaviour of the fluxes and of the turbulent generation mechanisms may have strong repercussions on both theoretical and modelling approaches to wall turbulence. Indeed, actual turbulent flows are shown here to have a much richer physics with respect to the classical notion of turbulent cascade, where anisotropic production and inhomogeneous fluxes lead to a complex redistribution of energy where a spatial reverse cascade plays a central role

Volcanic ash ice-nucleating activity can be enhanced or depressed by ash-gas interaction in the eruption plume

Volcanic ash can trigger ice nucleation when immersed in supercooled water. This will impact several processes (e.g., electrification, aggregation, precipitation) in the eruption plume and cloud and in the wider atmosphere upon ash dispersal. Previous studies show that ash bulk properties, reflecting the chemistry and phase state of the source magma, likely contribute to the ice-nucleating activity (INA) of ash. However, it remains unexplored how interaction with magmatic gases in the hot eruption plume, which inevitably leads to altered ash surface properties, affects the ash INA. Here we demonstrate that the INA of tephra is raised by exposure to H2O(g) mixed with SO2(g) at both 800 and 400 °C, but is substantially reduced by exposure to H2O(g) alone or mixed with HCl(g) at the same temperatures. In contrast, the INA of K-feldspar and quartz is reduced by all three eruption plume processing treatments. The decrease in INA of all silicates after heating with H2O(g) might relate to a loss of ice-active sites by surface dehydroxylation and/or oxidation. In the presence of HCl(g) or SO2(g), respectively, metal chloride or sulphate salts form on the tephra surfaces only. While NaCl and CaCl2 seem to have no effect on the tephra INA, CaSO4 is inferred to create ice-active sites, potentially through a particular combination of surface chemistry and topography. Overall, our findings suggest a complex interplay of bulk mineralogy and surface alteration in influencing ice nucleation by volcanic ash, and highlight the general sensitivity (enhancement or depression) of ash INA to interaction with magmatic gases in the eruption plume

Natural function and structural modification of climacostol, a ciliate secondary metabolite

The review highlights the main results of two decades of research on climacostol (5-[(2Z)-non-2-en-1-yl]benzene-1,3-diol), the resorcinolic lipid produced and used by the ciliated protozoan Climacostomum virens for chemical defense against a wide range of predators, and to assist its carnivorous feeding. After the first studies on the physiological function of climacostol, the compound and some analogues were chemically synthesized, thus allowing us to explore both its effect on different prokaryotic and eukaryotic biological systems, and the role of its relevant structural traits. In particular, the results obtained in the last 10 years indicate climacostol is an effective antimicrobial and anticancer agent, bringing new clues to the attempt to design and synthesize additional novel analogues that can increase or optimize its pharmacological properties

High-speed imaging of Strombolian explosions: The ejection velocity of pyroclasts

Explosive volcanic eruptions are defined as the violent ejection of gas and hot fragments from a vent in the Earth's crust. Knowledge of ejection velocity is crucial for understanding and modeling relevant physical processes of an eruption, and yet direct measurements are still a difficult task with largely variable results. Here we apply pioneering high-speed imaging to measure the ejection velocity of pyroclasts from Strombolian explosive eruptions with an unparalleled temporal resolution. Measured supersonic velocities, up to 405 m/s, are twice higher than previously reported for such eruptions. Individual Strombolian explosions include multiple, sub-second-lasting ejection pulses characterized by an exponential decay of velocity. When fitted with an empirical model from shock-tube experiments literature, this decay allows constraining the length of the pressurized gas pockets responsible for the ejection pulses. These results directly impact eruption modeling and related hazard assessment, as well as the interpretation of geophysical signals from monitoring networks

Rheology of magmas with bimodal crystal size and shape distributions: insights from analog experiments

Magmas in volcanic conduits commonly contain microlites in association with preexisting phenocrysts, as often indicated by volcanic rock textures. In this study, we present two different experiments that inves- tigate the flow behavior of these bidisperse systems. In the first experiments, rotational rheometric methods are used to determine the rheology of monodisperse and polydisperse suspensions consisting of smaller, prolate particles (microlites) and larger, equant particles (phenocrysts) in a bubble‐free Newtonian liquid (silicate melt). Our data show that increasing the relative proportion of prolate microlites to equant pheno- crysts in a magma at constant total particle content can increase the relative viscosity by up to three orders of magnitude. Consequently, the rheological effect of particles in magmas cannot be modeled by assuming a monodisperse population of particles. We propose a new model that uses interpolated parameters based on the relative proportions of small and large particles and produces a considerably improved fit to the data than earlier models. In a second series of experiments we investigate the textures produced by shearing bimodal suspensions in gradually solidifying epoxy resin in a concentric cylinder setup. The resulting textures show the prolate particles are aligned with the flow lines and spherical particles are found in well‐organized strings, with sphere‐depleted shear bands in high‐shear regions. These observations may explain the measured variation in the shear thinning and yield stress behavior with increasing solid fraction and particle aspect ratio. The implications for magma flow are discussed, and rheological results and tex- tural observations are compared with observations on natural samples

Coexistence of calc-alkaline and ultrapotassic alkaline magmas at Mounts Cimini : evidence for transition from the Tuscan to the Roman Magmatic Provinces (central Italy)

The volcanic complex of Mts. Cimini (~0.90-1.30Ma) represents the geographical and chronological transition between the Tuscan Magmatic Province (TMP) and the Roman Magmatic Province (RMP), in central Italy. Major and trace elements, and Sr, Nd and Pb isotopes of whole-rock, as well as mineral chemistry analyses, were carried out on samples representative of the different petrographic and chronological units of Mts. Cimini. In particular, we focused on the olivine-bearing latites of Mts. Cimini that are the most mafic magmas, belong to the last phase of this volcanic activity, and are heterogeneous in highly incompatible element ratios and Sr-isotope compositions. We suggest that such heterogeneity reflects the occurrence of a heterogeneous upper mantle beneath central Italy, in which different portions, e.g., the sources of both the TMP and RMP, are characterized by distinct geochemical and petrographic features. In this scenario, about 900ka ago, the olivine-bearing latites mark the progressive decline of the TMP magma production in favour of partial melting of the RMP mantle region, thus recording the coexistence of both ultrapotassic alkaline and calc-alkaline magmas in the same volcanic region

Anak Krakatau triggers volcanic freezer in the upper troposphere

Volcanic activity occurring in tropical moist atmospheres can promote deep convection and trigger volcanic thunderstorms. These phenomena, however, are rarely observed to last continuously for more than a day and so insights into the dynamics, microphysics and electrification processes are limited. Here we present a multidisciplinary study on an extreme case, where volcanically-triggered deep convection lasted for six days. We show that this unprecedented event was caused and sustained by phreatomagmatic activity at Anak Krakatau volcano, Indonesia during 22-28 December 2018. Our modelling suggests an ice mass flow rate of similar to 5x10(6)kg/s for the initial explosive eruption associated with a flank collapse. Following the flank collapse, a deep convective cloud column formed over the volcano and acted as a 'volcanic freezer' containing similar to 3x10(9)kg of ice on average with maxima reaching similar to 10(10)kg. Our satellite analyses reveal that the convective anvil cloud, reaching 16-18km above sea level, was ice-rich and ash-poor. Cloud-top temperatures hovered around -80 degrees C and ice particles produced in the anvil were notably small (effective radii similar to 20 mu m). Our analyses indicate that vigorous updrafts (>50m/s) and prodigious ice production explain the impressive number of lightning flashes (similar to 100,000) recorded near the volcano from 22 to 28 December 2018. Our results, together with the unique dataset we have compiled, show that lightning flash rates were strongly correlated (R=0.77) with satellite-derived plume heights for this event

Vesiculation and Quenching During Surtseyan Eruptions at Hunga Tonga-Hunga Ha'apai Volcano, Tonga

Surtseyan eruptions are shallow to emergent subaqueous explosive eruptions that owe much of their characteristic behavior to the interaction of magma with water. The difference in thermal properties between water and air affects the cooling and postfragmentation vesiculation processes in magma erupted into the water column. Here we study the vesiculation and cooling processes during the 2009 and 2014–2015 Surtseyan eruptions of Hunga Tonga‐Hunga Ha'apai volcano by combining 2‐D and 3‐D vesicle‐scale analyses of lapilli and bombs and numerical thermal modeling. Most of the lapilli and bombs show gradual textural variations from rim to core. The vesicle connectivity in the lapilli and bombs increases with vesicularity from fully isolated to completely connected and also increases from rim to core in transitional clasts. We interpret the gradual textural variations and the connectivity‐vesicularity relationships as the result of postfragmentation bubble growth and coalescence interrupted at different stages by quenching in water. The measured vesicle size distributions are bimodal with a population of small and large vesicles. We interpret this bimodality as the result of two nucleation events, one prefragmentation with the nucleation and growth of large bubbles and one postfragmentation with nucleation of small vesicles. We link the thermal model with the textural variations in the clasts—showing a dependence on particle size, Leidenfrost effect, and initial melt temperature. In particular, the cooling profiles in the bombs are consistent with the gradual textural variations from rim to core in the clasts, likely caused by variations in time available for vesiculation before quenching