Nucleation Events Research Articles

Particle size dependent microstructure evolution of inert gas atomized nickel base superalloy powders

Powder of an experimental Nickel base superalloy produced through Inert Gas Atomization technique is characterized for its physical properties, chemical composition, morphology, micro-segregation and orientation distribution of grains. The atomized powders exhibits near spherical morphology with average particle size of 61 µm. Micro-chemical analysis reveal, elements like Ti, Mo, Ta and Hf partition at inter-dendritic regions, irrespective of particle size.The extent of micro-segregation however is found to be intimately related to the cooling rate and particle size. The EBSD analysis reveals random texture in the particles with the number of grains per particle decrease with decreasing particle size. The nucleation event is found to be initiated on particle surfaces as evidenced from the dominant presence of the Low Angle Grain Boundaries (LAGBs) near the particle surfaces. The selective growth of favourably oriented nuclei are observed towards the core of the particle and characterized by presence of High Angle Grain Boundaries (HAGBs) in the particle interior. A mechanistic understanding correlating evolution of grain size distribution and micro-segregation with droplet solidification process, has been elucidated through the present study.

Conformational entropy limits the transition from nucleation to elongation in amyloid aggregation

The formation of β-sheet-rich amyloid fibrils in Alzheimer’s disease and other neurodegenerative disorders is limited by a slow nucleation event. To understand the initial formation of β-sheets from disordered peptides, we used all-atom simulations to parameterize a lattice model that treats each amino acid as a binary variable with β- and non-β-sheet states. We show that translational and conformational entropy give the nascent β-sheet an anisotropic surface tension that can be used to describe the nucleus with 2D classical nucleation theory. Since translational entropy depends on concentration, the aspect ratio of the critical β-sheet changes with protein concentration. Our model explains the transition from the nucleation phase to elongation as the point where the β-sheet core becomes large enough to overcome the conformational entropy cost to straighten the terminal molecule. At this point the β-strands in the nucleus spontaneously elongate, which results in a larger binding surface to capture new molecules. These results suggest that nucleation is relatively insensitive to sequence differences in coaggregation experiments because the nucleus only involves a small portion of the peptide.

Multi-eGO: An in silico lens to look into protein aggregation kinetics at atomic resolution

Protein aggregation into amyloid fibrils is the archetype of aberrant biomolecular self-assembly processes, with more than 50 associated diseases that are mostly uncurable. Understanding aggregation mechanisms is thus of fundamental importance and goes in parallel with the structural characterization of the transient oligomers formed during the process. Oligomers have been proven elusive to high-resolution structural techniques, while the large sizes and long time scales, typical of aggregation processes, have limited the use of computational methods to date. To surmount these limitations, we here present multi-eGO, an atomistic, hybrid structure-based model which, leveraging the knowledge of monomers conformational dynamics and of fibril structures, efficiently captures the essential structural and kinetics aspects of protein aggregation. Multi-eGO molecular dynamics simulations can describe the aggregation kinetics of thousands of monomers. The concentration dependence of the simulated kinetics, as well as the structural features of the resulting fibrils, are in qualitative agreement with invitro experiments carried out on an amyloidogenic peptide from Transthyretin, a protein responsible for one of the most common cardiac amyloidoses. Multi-eGO simulations allow the formation of primary nuclei in a sea of transient lower-order oligomers to be observed over time and at atomic resolution, following their growth and the subsequent secondary nucleation events, until the maturation of multiple fibrils is achieved. Multi-eGO, combined with the many experimental techniques deployed to study protein aggregation, can provide the structural basis needed to advance the design of molecules targeting amyloidogenic diseases.

Formation of extremely low-volatility organic compounds from styrene ozonolysis: Implication for nucleation

Styrene is a highly reactive compound with the dual nature of aromatics and olefins. This work presents evidence for formation of extremely low-volatility organic compounds (ELVOCs) from styrene ozonolysis. The molecules of ELVOCs were analyzed using a high-resolution orbitrap mass spectrometer. The results show that ELVOCs were oligomers characterized by stabilized Criegee radicals (SCIs) as chain units. The addition of C6H5CHOO (SCI1) or CH2OO (SCI2) can dramatically decrease the oligomers' volatility. At low relative humidity (RH), ELVOCs are mainly formed from the reaction of RO2 radical, C6H5OO·, with SCI1 and SCI2; however, ELVOCs are primarily produced by the reaction between benzoic acid and SCI1 at high RH. Ambient particles were also collected to propose the probable oligomers from styrene-SCI. Our results suggest that styrene-SCI derived ELVOCs may act as nucleating agents, potentially providing an experimental basis for nucleation events that frequently occur in urban areas.

Secondary Nucleation-Triggered Physical Cross-Links and Tunable Stiffness in Seeded Supramolecular Hydrogels.

Mechanistic understanding and the control of molecular self-assembly at all hierarchical levels remain grand challenges in supramolecular chemistry. Functional realization of dynamic supramolecular materials especially requires programmed assembly at higher levels of molecular organization. Herein, we report an unprecedented molecular control on the fibrous network topology of supramolecular hydrogels and their resulting macroscopic properties by biasing assembly pathways of higher-order structures. The surface-catalyzed secondary nucleation process, a well-known mechanism in amyloid fibrilization and chiral crystallization of small molecules, is introduced as a non-covalent strategy to induce physical cross-links and bundling of supramolecular fibers, which influences the microstructure of gel networks and subsequent mechanical properties of hydrogels. In addition, seed-induced instantaneous gelation is realized in the kinetically controlled self-assembled system under this study, and more importantly, the extent of secondary nucleation events and network topology is manipulated by the concentration of seeds.

Revealing the nucleation event of Mg-Al alloy induced by Fe impurity

Revealing the nucleation event of Mg-Al alloy induced by Fe impurity

Mechanisms of Exclusive Scale Formation in the High Temperature Oxidation of Alloys

The high temperature oxidation of alloys is most often considered within the continuum framework developed by C. Wagner. We argue that in order to make progress in understanding exclusive scale formation, one needs to examine the atomic-scale kinetic processes that today are amenable to a variety of experimental, computational and theoretical approaches. In our discussion, we suggest that there is an analogy between the kinetic processes required for exclusive scale formation and thin film layer-by-layer growth. In both cases, the magnitude of the Ehrlich-Schwoebel barrier sets an effective length scale for growth, that if too large, results in three-dimensional rather than lateral island or oxide growth. In this event, exclusive scale formation can only occur if there is a sufficiently large density of oxide nucleation events at the surface of the alloy. We suggest that a suitably identified, dilute alloy component, could serve as a “surfactant” that segregates to the surface and is capable of enhancing the nucleation density.

Nucleation parameters of SPC/E and TIP4P/2005 water vapor measured in NPT molecular dynamics simulations.

Nucleation rates for droplet formation in water vapor are measured in molecular dynamics (MD) simulations of SPC/E and TIP4P/2005 water by monitoring individual nucleation events. The nucleation process is simulated in the NPT ensemble to evaluate the steady-state nucleation rate in accordance with the assumptions of classical nucleation theory (CNT). Nucleation rates measured between 300 and 425 K for the SPC/E model, and between 325 and 475 K for the TIP4P/2005 model, agree with the CNT predictions roughly within the standard deviation of the MD measurements of the nucleation rates.

A kinetic model for the impact of packaging signal mimics on genome encapsulation

Inspired by recent experiments on the spontaneous assembly of virus-like particles from a solution containing a synthetic coat protein and double-stranded DNA, we put forward a kinetic model that has as main ingredients a stochastic nucleation and a deterministic growth process. The efficiency and rate of DNA packaging strongly increase after tiling the DNA with CRISPR-Cas proteins at predesignated locations, mimicking assembly signals in viruses. Our model shows that treating these proteins as nucleation-inducing diffusion barriers is sufficient to explain the experimentally observed increase in encapsulation efficiency, but only if the nucleation rate is sufficiently high. We find an optimum in the encapsulation kinetics for conditions where the number of packaging signal mimics is equal to the number of nucleation events that can occur during the time required to fully encapsulate the DNA template, presuming that the nucleation events can only take place adjacent to a packaging signal. Our theory is in satisfactory agreement with the available experimental data.

Water film-driven Mn (oxy)(hydr)oxide nanocoating growth on rhodochrosite

Minerals exposed to moist air stabilize thin water films that drive a score of chemical reactions of great importance to water-unsaturated terrestrial environments. In this study, we identified Mn (oxy)(hydr)oxide nanocoatings formed by the dissolution, oxidation and precipitation of Mn in oxygenated water films grown on rhodochrosite (MnCO3) microparticles. Nanocoatings that could be identified by vibrational spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and (scanning and transmission) electron microscopy formed in water films containing the equivalent of at least 7 monolayers (∼84 H2O/nm2). These films were formed by exposing microparticles to moist air with at least 50% relative humidity (RH). Films of neutral pH reacted up to 14% of the MnII located in the topmost ∼5 nm region of the microparticles in atmospheres of up to 90% RH for 7 d. These reactions produced MnOOH, birnessite (MnO2) and hausmannite (Mn3O4) nanoparticles of low crystallinity, while exposure to atmospheric air for 1 yr. converted only 2% of MnII in this region to MnOOH. In contrast, reactions in alkaline water films converted up to ∼75% of the MnII but only after 16 d of reaction. These films produced MnOOH and MnO2 of low crystallinity, as well as crystalline hausmannite. Kinetic modeling of the time-resolved growth of the MnO stretching vibrational bands of these nanocoatings revealed two concurrent reaction processes. A 1rst-order process was assigned to nucleation events terminating only after a few hours, and a 0-order process was assigned to the sustained growth of nanocoatings from these nuclei over longer reaction time. By identifying nanocoatings formed by water film-driven reactions on rhodochrosite, our study adds new insight into mineralogical transformations relevant to anoxic–oxic boundaries in water-unsaturated environments.

Intracellular Aβ42 Aggregation Leads to Cellular Thermogenesis.

The aggregation of Aβ42 is a hallmark of Alzheimer’s disease. It is still not known what the biochemical changes are inside a cell which will eventually lead to Aβ42 aggregation. Thermogenesis has been associated with cellular stress, the latter of which may promote aggregation. We perform intracellular thermometry measurements using fluorescent polymeric thermometers to show that Aβ42 aggregation in live cells leads to an increase in cell-averaged temperatures. This rise in temperature is mitigated upon treatment with an aggregation inhibitor of Aβ42 and is independent of mitochondrial damage that can otherwise lead to thermogenesis. With this, we present a diagnostic assay which could be used to screen small-molecule inhibitors to amyloid proteins in physiologically relevant settings. To interpret our experimental observations and motivate the development of future models, we perform classical molecular dynamics of model Aβ peptides to examine the factors that hinder thermal dissipation. We observe that this is controlled by the presence of ions in its surrounding environment, the morphology of the amyloid peptides, and the extent of its hydrogen-bonding interactions with water. We show that aggregation and heat retention by Aβ peptides are favored under intracellular-mimicking ionic conditions, which could potentially promote thermogenesis. The latter will, in turn, trigger further nucleation events that accelerate disease progression.

Disrupt the upper or the lower conduit? The dual role of gas exsolution in the conduits of persistently active volcanoes

Many volcanoes emit a significant portion of the gas they transport to the atmosphere during continual passive degassing rather than during eruptions. To maintain a high gas and thermal flux without erupting magma, the flow field in the volcanic conduit must be approximately balanced with gas-rich, buoyant magma ascending and degassed, heavy magma descending. In vertical conduits, this exchange flow takes the form of core–annular flow, where the gas-rich magma forms a core enclosed by an annulus of degassed magma. The flow dynamics of core–annular flow have been studied extensively in fluid dynamics, but mostly for constant material properties. Our study aims to advance our understanding of how core–annular flow responds to volatile exsolution – a simple, yet ubiquitous disruption in volcanic conduits, which alters both the density and the viscosity of the core fluid. By deriving an evolution equation for the core–annular interface based on a generalized exchange-flow condition using a lubrication approximation, we find that the response of the system to volatile exsolution depends on the conduit flow regime. The same nucleation event can lead to a flow adjustment only in the upper, only in the lower or in both portions of the volcanic conduit. Our results emphasize that the thermodynamic evolution of magma properties and volcanic conduit flow are intricately linked, which may help understand the observed variability of eruptive behaviour at persistently degassing volcanoes.

Circumventing Thermodynamic Constraints in Nucleation-Controlled Crystallization of Al2TiO5-Based Chemical Vapor Deposition Coatings

Nucleation is a fundamental part in most syntheses of ceramic materials. Yet, few techniques enable control of this step, which would offer possibilities to attain full-scale kinetic selectivity of the syntheses to reach novel compounds with unique properties. Herein, we present a nucleation-controlled crystallization pathway to synthesize coatings of aluminum titanate (Al2TiO5)─renowned for its low-to-negative thermal expansion─at significantly reduced temperatures than conventional solid-state techniques. Based on a kinetic study using in situ X-ray diffraction, detailed mechanistic insights into the crystallization process and phase evolutions within the Al–Ti–O system are obtained. The lowest activation energies for crystallization are given when the Al–Ti ratio is close-to-stoichiometric or Ti-enriched. Along with these compositions’ similar kinetics at the earliest stages of the transformation, a joint nucleation behavior is discovered, revealing the elemental role of titanium in nucleating the main Al2TiO5 phase. Based on classical nucleation theory, we deduce the significant influence of the configurational entropy (Sconfig) when crystallization occurs in the nucleation-controlled domain. Finally, peculiar transition features are observed in the Al-enriched regime during annealing at intermediate temperatures, whose causes are ascribed to the presence of secondary nucleation events and possibilities of structural relaxations in the amorphous matrixes when crystallizing.

Simulation of Diffusion-Controlled Growth of Interdependent Nuclei under Potentiostatic Conditions.

The problem of diffusion-controlled growth following an instantaneous nucleation event was studied within the framework of a new numerical model, considering the spatial distribution of hemispherical nuclei on the electrode surface and the mutual influence of growing nuclei via the collision of 3D diffusion fields. The simulation of the diffusion-controlled growth of hexagonal and random ensembles was performed at the overpotential-dependent number density of nuclei. The diffusion flow to each nucleus within a random ensemble was simulated by the finite difference method using the derived analytical expressions for the surface areas and the volumes formed at the intersection of 3D diffusion fields with the side faces of a virtual right prism with a Voronoi polygon base. The implementation of this approach provides an accurate calculation of concentration profiles, time dependences of the size of nuclei, and current transients. The results, including total current density transients, growth exponents, and nucleus size distribution, were compared with models developed within the concept of planar diffusion zones, the mean-field approximation and the Brownian dynamics simulation method, as well as with experimental data from the literature. The prospects of the model for studying the initial stages of electrocrystallization were discussed.

Structure of a pathologic amyloid nucleus determined by rational genetic deconstruction of an intracellular nucleation barrier

A long‐standing goal of the study of amyloids has been to characterize the structural basis of the rate‐determining nucleating event. However, the ephemeral nature of that event has made it inaccessible to classical biochemistry, structural biology, and computational approaches. Here, we addressed that limitation by using Distributed Amphifluoric FRET to measure the dependence of amyloid formation on concentration and conformational templates in living cells, whose volumes are sufficiently small to resolve the outcomes of independent nucleation events. We characterized numerous rationally designed sequence variants of polyglutamine (polyQ), a polypeptide that precipitates Huntington’s and other amyloid‐associated neurodegenerative diseases when its length exceeds a characteristic threshold. This effort uncovered a pattern of approximately twelve Qs, only for polypeptides exceeding the clinical length threshold, that allow for amyloid nucleation to occur spontaneously within single polypeptides. Nucleation was inhibited by intermolecular phase separation. Using atomistic molecular dynamics simulations, we found that the pattern encodes a minimal steric zipper of interdigitated side chains. Lateral growth of the steric zipper competed with axial growth to produce “pre‐amyloid” oligomers. By illuminating the structural mechanism of polyQ amyloid formation in cells, our findings reveal a potential molecular etiology for polyQ diseases, and may provide a roadmap for the design of new therapies.

Evaluate the effect of melt pool convection on grain structure of IN625 in laser melting process using experimentally validated process-structure modeling

During the laser based manufacturing process such as metal additive manufacturing (AM) and laser cladding, enhancing or inhibiting liquid metal flow within the laser-induced melt pool provides a promising approach to tune material microstructure and the resulting mechanical properties. However, the effect of convection flow, specifically the dominant Marangoni convection flow in the melt pool, on the as-solidified material microstructure is still vague. This study aims to reveal convection-modified grain evolution in the laser melting process, e.g., the selective laser melting AM. We use a process-microstructure model and systematically design comparative simulation cases (with and without convection flow) to identify the effects of convection flow on melt pool geometry, solidification conditions, and as-solidified grain structure formation. The model is validated by secondary electron images and electron backscatter diffraction of the laser melted IN625 alloy provided by NIST Additive Manufacturing Benchmark Test Series. It is found that the strong Marangoni convection flow can widen the melt pool and notably affect the solidified microstructure in terms of grain growth directions and bulk nucleation events. This study provides a quantitative basis for controlling the as-solidified microstructure by manipulating the convection flow in the laser-induced melt pool.

The effect of bubble nucleation on the performance of a wickless heat pipe in microgravity

Bubble nucleation was investigated in a 20-mm-long, wickless heat pipe on the International Space Station. Over 20 h of running experiments using pentane as the working fluid, more than 100 nucleation events were observed. Bubble nucleation at the heater end temporarily boosted peak pressures and vapor temperatures in the device. At the moment of nucleation, the heater wall temperature significantly decreased due to increased evaporation and the original vapor bubble collapsed due to increased pressure. A thermal model was developed and using the measured temperatures and pressures, heat transfer coefficients near the heater end of the system were extracted. Peak heat transfer coefficients during the nucleation event were over a factor of three higher than at steady-state. The heat transfer coefficient data were all collapsed in the form of a single, linear correlation relating the Nusselt number to the Ohnesorge number.

Improving separation efficiency of crystallization by ultrasound-accelerated nucleation: The role of solute diffusion and solvation effect

The nucleation rate significantly affects the separation and purification efficiency of crystallization. The proper choice of solvent combined with the intensification of ultrasound can further accelerate the nucleation. However, the difference in the ability of ultrasound to promote nucleation in different solvents needs to be considered. Here, the comparative study of crystal nucleation in various solvent environments under ultrasonic and non-ultrasonic conditions was explored. The probability distribution method was employed to better describe the randomness of nucleation events. Over 1500 nucleation induction time experiments of 3,5-dimethoxybenzoic acid in three solvents (ethanol, acetonitrile, acetone) were performed. The results showed that the enhancement on nucleation rate by ultrasound in different solvents ranged from 1% to 116%, which had obvious differences in different solvents. It is revealed that the enhancement degree is mainly affected by solute diffusion and solvation effect. On the one hand, mean square displacement (MSD) analysis of solute molecule demonstrates that, the weaker the diffusion ability of solute in a certain solvent, the more significant the enhancement effect of ultrasound on the nucleation in such solvent system. On the other hand, molecular dynamics (MD) simulation, density functional theory (DFT) calculation and spectroscopy method indicate that the solvation effect is the main factor determining the order of nucleation rate in the three solvents. The stronger the solvation of solvent on the solute, the more difficult the nucleation. So that in our system, the introduction of ultrasound enhances nucleation in each solvent but does not change the order of nucleation rate in the three solvents.

Analysis of aerosol particle number size distribution and source attribution at three megacities in China

Air pollution has been a major concern in China, especially in densely populated megacities. Particulate pollutants, generally known as aerosols, have sizes that range widely and complex physical and chemical compositions with high spatiotemporal variabilities. We have conducted a series of experiments using the same comprehensive observation system, measuring various aerosol properties in the three megacities of Beijing, Shanghai, and Guangzhou. Although there are differences in the total particle number concentration and particle number size distribution among the three sites due to differences in emission sources and sampling seasons, a high proportion of ultrafine particles was observed in all cities, i.e., 86.33%, 88.34%, and 81.1%, respectively. The high particle number concentration at the Beijing and Guangzhou sites was caused by an increase in nucleation-mode particles at nighttime. The concentration of nucleation-mode particles at nighttime was significantly higher than during the daytime. We call this phenomenon a nocturnal nucleation event, and combined with analyses of factors involved, attribute this event to traffic emissions. To investigate the pollution sources at each location, we identified and quantified the pollution contributions from different sources by applying the k-means clustering analysis to observational datasets of environmental quantities. Clustering results showed that traffic emissions were the main contributor in Beijing and Guangzhou, with direct and significant impacts ranging from 37% to 45%, respectively.

A leukemia-protective germline variant mediates chromatin module formation via transcription factor nucleation

Non-coding variants coordinate transcription factor (TF) binding and chromatin mark enrichment changes over regions spanning >100 kb. These molecularly coordinated regions are named “variable chromatin modules” (VCMs), providing a conceptual framework of how regulatory variation might shape complex traits. To better understand the molecular mechanisms underlying VCM formation, here, we mechanistically dissect a VCM-modulating noncoding variant that is associated with reduced chronic lymphocytic leukemia (CLL) predisposition and disease progression. This common, germline variant constitutes a 5-bp indel that controls the activity of an AXIN2 gene-linked VCM by creating a MEF2 binding site, which, upon binding, activates a super-enhancer-like regulatory element. This triggers a large change in TF binding activity and chromatin state at an enhancer cluster spanning >150 kb, coinciding with subtle, long-range chromatin compaction and robust AXIN2 up-regulation. Our results support a model in which the indel acts as an AXIN2 VCM-activating TF nucleation event, which modulates CLL pathology.