Nucleation Precursors Research Articles

Effects of the inter- and intra-molecular hydrogen bonding interactions in forming atmospheric malonic acid-containing clusters

The possible involvement of chemical compounds in atmospheric new particle formation has received increased attention in recent years. Extensive density functional theory calculations were performed to characterize the effects of the inter- and intra-molecular hydrogen bonding interactions in forming atmospheric malonic acid (MOA)-containing clusters. Methanol, water, formaldehyde, and acetone are common atmospheric nucleation precursors, and they were chosen as the counterpart to interact with MOA. There are two types of hydrogen bonds: addition and insertion. When MOA behaves as the donor of a hydrogen bond, the formation of an inter-molecular hydrogen bond lightly increases the strength of the intra-molecular hydrogen bonds. Strong hydrogen bonds were formed when MOA executes as a hydrogen bond donor. The strength of the insertion conformer depends on the inserting molecule. The IR frequencies of the hydrogen bonded system were investigated. Furthermore, geometrically identified inter- and intra-molecular hydrogen bonds were provided by atoms in molecules approach.

Atmospheric Initial Nucleation Containing Carboxylic Acids.

The possible involvement of chemical components in atmospheric new particle formation has received increased attention in recent years. However, the deep understanding of the clusters formed between atmospheric gas-phase organic acids is incomplete. In this work, the chemical and physical properties of the cluster formed between three organic acids [glyoxylic acid (GA), oxalic acid (OA), and pyruvic acid (PA)] with common atmospheric nucleation precursors [methyl hydrogen sulfate (MHS), methanesulfonic acid (MSA), and hydroxymethanesulfonic acid (HMSA)] have been investigated with density functional theory and ab initio coupled-cluster singles and doubles with perturbative triples (CCSD(T)) theory. Six- to nine-membered cyclic ring structures are mainly arranged via two classes of intermolecular hydrogen bonds: SO-H···O and CO-H···O. The GA/OA/PA-MHS/MSA/HMSA complexes with the nine- and eight-membered cyclic ring structures are thermodynamically more stable than the others. Large red shifts of the OH-stretching vibrational frequencies of both SO-H···O (354-794 cm-1) and CO-H···O (320-481 cm-1) are obtained with regard to the isolated gas monomers. Atoms in molecules topological analysis reveals that the Laplacian of the charge density of the bimolecular interactions in the GA/OA/PA-MHS/MSA/HMSA complexes is higher than the upper value of the hydrogen bond criteria. The thermodynamic data, dipole moments, and atmospheric mixing ratios indicate that the MHS- and MSA-containing complexes possibly take part in atmospheric new particle formation. Additionally, the environmental factors, such as temperature and pressure, are also important in atmospheric particle nucleation, and the gas-mixing ratios of the clusters at 12 km are much enhanced by 18-44 times with respect to the ones at the ground level. This study suggests that small cluster calculations may be helpful in simulating atmospheric new particle formation.

Existence of multiple critical cooling rates which generate different types of monolithic metallic glass

Via fast differential scanning calorimetry using an Au-based glass as an example, we show that metallic glasses should be classified into two types of amorphous/monolithic glass. The first type, termed self-doped glass (SDG), forms quenched-in nuclei or nucleation precursors upon cooling, whereas in the so-called chemically homogeneous glass (CHG) no quenched-in structures are found. For the Au-based glass investigated, the critical cooling and heating rates for the SDG are 500 K s−1 and 20,000 K s−1, respectively; for the CHG they are 4000 K s−1 and 6000 K s−1. The similarity in the critical rates for CHG, so far not reported in literature, and CHG’s tendency towards stochastic nucleation underline the novelty of this glass state. Identifying different types of metallic glass, as is possible by advanced chip calorimetry, and comparing them with molecular and polymeric systems may help to elaborate a more generalized glass theory and improve metallic glass processing.

Impact of neutral and acidic species on cycloalkenes nucleation

Non-covalent hydrogen bond interactions between the π cloud of cycloalkenes and three atmospheric common nucleation precursors (H2S, H2O, and MeOH) have been investigated using DFT and CCSD(T). The structures and the energies of the 1:1 and 1:2 adducts were computed with the B3LYP-D3 method. The analysis of the investigated electronic properties and geometric parameters shows that cyclohexene is a stronger hydrogen bond acceptor than cyclopentene, then followed by 1,4-cyclohexadiene and 1,3-cyclohexadiene. Comparable red shifts of the OH-/SH-stretching vibrational frequencies were noticed for the studied clusters. Increasing the ring size enhances the hydrogen bond interaction, and increasing the π delocalization decreases the hydrogen bond interactions. This is further confirmed by Bader’s quantum theory of atoms in molecules. The nonadditivity effects were observed in the trimolecular complexes. All the complexes were analyzed by energy decomposition analysis to divide the interaction energy into individual components. Furthermore, the dipole moments and atmospheric implications were also investigated.

A molecular understanding of the interaction of typical aromatic acids with common aerosol nucleation precursors and their atmospheric implications.

Aromatic acids, which are generated from numerous anthropogenic emissions and secondary transformations, have been considered to play a crucial role in new particle formation. In this study, we performed theoretical calculations at the PW91PW91/6-311++G(3df,3pd) level to investigate the interaction between typical aromatic acids namely benzoic acid (BA), phenylacetic acid (PAA), phthalic acid (PA), isophthalic acid (mPA), and terephthalic acid (PTA) and common atmospheric nucleation precursors namely sulfuric acid (SA), water (H2O), ammonia (NH3), methylamine (MA), dimethylamine (DMA), and trimethylamine (TMA). The geometric analysis, Gibbs free energy analysis, OH/NH-stretching vibrational frequency calculation, and atoms in molecules (AIM) analysis were conducted to determine the interactions in the complexes. The heterodimers formed a six to eight membered ring through four types of hydrogen bond, and the bond strength could be ranked in descending order: SO–H⋯O > O–H⋯O/N > N–H⋯O. The BA/PAA/mPA/PTA–SA complexes had the lowest Gibbs free energy values. PA was more likely to interact with NH3 or amines rather than SA due to an intra-molecular hydrogen bond. Additionally, the aromatic acids have similar ability to interact with SA and NH3 as monocarboxylic/dicarboxylic acid. The formation potential of the heterodimers from aromatic acids with common nucleation precursors in ambient atmosphere was investigated.

Defects induced multiscale kinetics of damage-failure transition under very high cycle fatigue

Multiscale mechanisms of fatigue damage-failure transitions in metals are studied for High Cycle and Very High Cycle Fatigue and analyzed as duality of inherently linked two types of singularities related to the singularity of multiscale damage kinetics under crack nucleation and singularity of stress field at the crack tip as the classical framework of fracture mechanics. The singularity of multiscale damage kinetics was established as natural precursor of crack nucleation and specific nonlinearity (metastability) of free energy release in term of damage parameter (defect induced strain). Experimentally estimated scales and power exponents were used in the constitutive laws of crack advance as the structure sensitive parameters for characteristic damage-failure transition stages.

On the dynamics of metal filled CNOs fusion: the case of sp3-rich Cu filled carbon foam

In the last decade, important research efforts have been posed towards the fabrication of novel carbon polymorphs with controlled sp3/sp2 properties. Nucleation of novel carbon polymorphs has been reported with the use of (1) cold compression experiments and (2) high temperature annealing. In the latter, the formation of a novel foam-like carbon (CFM) material has been reported as the result of an unusual spontaneous process of high temperature fusion of Fe3C-filled carbon nano-onions (Fe3C@CNOs). Being interested in investigating further such CNOs-fusion mechanism we have considered the use of Cu crystals as catalyst for the formation of the filled CNOs to be used as precursor for the CFM nucleation. The choice of these CNOs as precursors relies on their characteristic mass transport properties. This kind of effect is of important interest also for electrochemical battery-related processes. Specifically, we find that a complete conversion of the filled CNOs-precursors into CFM can be achieved within very short timescales of 6 h of annealing. This implies the presence of fast diffusion and mass transport dynamics. In addition, through the use of multiple characterization techniques, such as TEM, HRTEM, XRD, Raman Spectroscopy and XPS we investigate the variation of structural and surface characteristics of the Cu@CNOs and we demonstrate the formation of a sp3-rich mm-scale CFM material completely filled with crystalline copper after the annealing stage. Further investigations are then performed at high pressures of 5 GPa. These measurements reveal a substantial change in the CFM network-morphology and a comprehensive increase in the sp3-content.

Hydration of Atmospheric Molecular Clusters II: Organic Acid-Water Clusters.

Using computational methods, we study the gas phase hydration of three different atmospherically relevant organic acids with up to 10 water molecules. We study a dicarboxylic acid (pinic acid) and a tricarboxylic acid (3-methyl-1,2,3-butanetricarboxylic acid (mbtca)) that are both identified as products from α-pinene oxidation reactions. We also study a 2-oxohexanediperoxy acid (ohdpa) that has been identified as a product from cyclohexene autoxidation. To sample the cluster structures, we employ our recently developed systematic hydrate sampling technique and identify a total of 551 hydrate clusters. The cluster structures and thermochemical parameters (at 298.15 K and 1 atm) are obtained at the ωB97X-D/6-31++G(d,p) level of theory, and the single point energy of the clusters have been refined using a high level DLPNO-CCSD(T)/aug-cc-pVTZ calculation. We find that all three tested organic acids interact significantly more weakly with water compared to the primary nucleation precursor sulfuric acid. Even at 100% relative humidity (298.15 K and 1 atm), we find that ohdpa remains unhydrated and only the monohydrate of pinic acid and mbtca are slightly populated (4% and 2%, respectively). From the obtained molecular structures, potential implications for the ice nucleating ability of aerosol particles is discussed.

Study of carbon suboxide-containing clusters: A potential sink for cumulene

Carbon suboxide (C3O2) has a sizable fraction in the atmosphere. The environmental fate of C3O2 was studied by density functional theory. Methanol (MeOH), dimethylamine (DMA), and methane sulfonic acid (MSA) are typical atmospheric nucleation precursors. This study investigated how the acidic, neutral and basic precursors impact atmospheric new particle formation. The formation of 1:1 and 1:2 complexes were studied. The binding energies and geometric parameters clearly show that MSA is the strongest hydrogen bond donor than MeOH and DMA. This is consistent with the comparable red shifts and QTAIM analysis of the MSA-containing complexes. The neutral precursor (MeOH) and the basic one (DMA) were found to weakly interact with C3O2. Thus, the acidic species would promote the nucleation with the neutral C3O2.

Optimal Design of High-Strength Ti‒Al‒V‒Zr Alloys through a Combinatorial Approach

The influence of various Zr contents (0–45 wt.%) on the microstructure and mechanical properties of Ti6Al4V alloy was investigated through a combinatorial approach. The diffusion multiples of Ti6Al4V–Ti6Al4V20Fe–Ti6Al4V20Cr–Ti6Al4V20Mo–Ti6Al4V45Zr were manufactured and diffusion-annealed to obtain a large composition space. Scanning electron microscopy, electron probe micro-analysis, and a microhardness system were combined to determine the relationships among the composition, microstructure, and hardness of these alloys. The Ti–6Al–4V–30Zr alloy was found to contain the thinnest α lath and showed peak hardness. X-ray diffraction and transmission electron microscope results indicated that after quenching from the β-field, the metastable α″-phase formed; moreover, at the secondary aging stage, the metastable α″-phase acted as precursor nucleation sites for the stable α-phase. The bulk Ti6Al4V30Zr alloy was manufactured. After aging at 550 °C, the alloy showed excellent balance of strength and ductility, and the tensile strength was 1464 MPa with a moderate elongation (8.3%). As the aging temperature increased, the tensile strength and yield strength of the alloys rose, but the total elongation decreased. The lamella thickness and volume fraction of the α-phase were the major factors that had great impacts on the mechanical properties.

Gas phase transformation from organic acid to organic sulfuric anhydride: Possibility and atmospheric fate in the initial new particle formation

New particle formation (NPF) process has been observed frequently in various environments and produces a large fraction of atmospheric aerosols. However, the chemical species participating in the nucleation as well as the corresponding nucleation mechanism in the atmosphere still remain ambiguous. Recent research by Leopold et al. shows that cycloaddition reaction of SO3 to carboxylic acids could contribute to the formation of organic sulfuric anhydride which would have lower vapor pressure compared with the corresponding carboxylic acid and hence kick-start new particle formation in the gas phase. In the present study, energy profile for the formation of 3-methyl-1,2,3-butanetricarboxylic sulfuric anhydride (MBTCSA) through the cycloaddition of SO3 to 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) has been investigated using computational methods. As a result, such a process would be effectively barrierless for one of the terminal carboxy group and has very low energy barriers for the other two carboxy groups (0.6 and 2.8 kcal/mol, respectively), indicating the whole process is a plausible gas phase pathway to MBTCSA formation. Furthermore, by evaluating the stability of the generated atmospheric clusters through topological and kinetic analysis, interaction between atmospheric nucleation precursor with MBTCSA is found to be more thermodynamically favourable and stronger than those with sulfuric acid and MBTCA which is identified from further-generation oxidation of a-pinene. Hence MBTCSA is speculated to be a potential participator in the initial new particle formation and the further particles growth.

General framework for nonclassical nucleation

A great deal of experimental evidence suggests that a wide spectrum of phase transitions occur in a multistage manner via the appearance and subsequent transformation of intermediate metastable states. Such multistage mechanisms cannot be explained within the realm of the classical nucleation framework. Hence, there is a strong need to develop new theoretical tools to explain the occurrence and nature of these ubiquitous intermediate phases. Here we outline a unified and self-consistent theoretical framework to describe both classical and nonclassical nucleation. Our framework provides a detailed explanation of the whole multistage nucleation pathway showing in particular that the pathway involves a single energy barrier and it passes through a dense phase, starting from a low-density initial phase, before reaching the final stable state. Moreover, we demonstrate that the kinetics of matter inside subcritical clusters favors the formation of nucleation clusters with an intermediate density, i.e. nucleation precursors. Remarkably, these nucleation precursors are not associated with a local minimum of the thermodynamic potential, as commonly assumed in previous phenomenological approaches. On the contrary, we find that they emerge due to the competition between thermodynamics and kinetics of cluster formation. Thus, the mechanism uncovered for the formation of intermediate phases can be used to explain recently reported experimental findings in crystallization: up to now such phases were assumed a consequence of some complex energy landscape with multiple energy minima. Using fundamental concepts from kinetics and thermodynamics, we provide a satisfactory explanation for the so-called nonclassical nucleation pathways observed in experiments.

A study on the microscopic mechanism of methanesulfonic acid-promoted binary nucleation of sulfuric acid and water

Methanesulfonic acid (MSA) is believed to play an important role in the formation and growth of atmospheric organic aerosols and could facilitate the binary nucleation of sulfuric acid (SA)–water (W). However, understanding of larger clusters formed by gas-phase MSA with atmospheric nucleation precursors from microscopic aspect is lacking. In this work, to study the microscopic mechanism of the ternary nucleation, the structural characteristics and thermodynamics of MSA clusters with SA in the presence of up to six W molecules have been investigated. It was found that MSA forms relatively stable clusters with SA and W molecules and that acid dissociation plays an important role. The analysis of the atmospheric relevance indicates that the heterodimer MSA–SA and monohydrated cluster MSA–SA–W1 show an obvious relative concentration in the atmosphere, and thus, these species likely participate in new particle formation (NPF). However, with an increasing number of W molecules, the concentration of clusters gradually decreases. Additionally, the minimum energy isomer of MSA–SA–W4 is predicted to possess a relatively stable configuration under the employed temperature dependence analysis, and evaporation rate analysis. The detailed non-covalent interactions of MSA-SA-Wn, n = 3–6 cluster have been thoroughly studied for the first time.

On the Binding Free Energy and Molecular Origin of Sickle Cell Hemoglobin Aggregation.

Protein aggregation is associated with various diseases, including Alzheimer and Parkinson as well as sickle cell disease (SCD). From a molecular point of view, protein aggregation depends on a complex balance of electrostatic and hydrophobic interactions mediated by water. An impressive manifestation of the importance of this balance concerns the human hemoglobin (HbA) mutant, HbS (sickle cell Hb), where a single substitution at the 6th position of HbA β-chains, from glutamic acid to valine, causes the polymerization of deoxygenated HbS (deoxy-HbS), responsible for SCD. HbS polymerization is believed to occur via a double nucleation mechanism initiated by the formation of HbS fibers (homogeneous nucleation), followed by fiber growth. Furthermore, it was proposed that homogeneous nucleation proceeds through a two-step mechanism, where metastable dense clusters play the role of nucleation precursors. Thus, hindering or delaying the formation of such precursors could represent a potential SCD therapeutic route. Here, we study, through molecular dynamics, the binding free energy and protein-protein contacts involved in the deoxy-HbS dimer aggregation and stabilization process. A binding free energy of ∼-14.0 ± 1 kcal/mol is estimated from a one-dimensional potential of mean force. Analysis of protein-protein interactions shows that both electrostatic and van der Waals interactions play an important role on the aggregation of HbS. With respect to the former, our results indicate that aggregation is largely favored by the formation of salt bridges (SB), mostly, Lys-Glu, Lys-Asp, and Heme-Lys SB, which outweigh electrostatic repulsions involving similar residues. Thus, our results suggest that a potential antisickling drug could be one with the ability to weaken or hinder the formation of a few SB between carboxylate and ammonium groups.

Enhanced dispersoid precipitation and dispersion strengthening in an Al alloy by microalloying with Cd

The dispersion hardening effect of Mn(Fe)-containing dispersoids in aluminium alloys has long been ignored since it is difficult to achieve a high number density of fine dispersoids with conventional alloying compositions. This work demonstrates a minor addition of Cd (0.05 at.%) can dramatically enhance the precipitation of α-Al(Mn,Fe)Si dispersoids and therefore the dispersion strengthening of AA3003 alloy. Similar to the 3003 base alloy, a peak hardness in the Cd-containing alloy was obtained after continuous heating to 450 °C. However, an improvement in yield strength by 25% was achieved by the Cd addition. Detailed transmission electron microscopy (TEM) and atom probe tomography (APT) investigations show that the Cd addition has changed the nucleation behaviour of α-Al(Mn,Fe)Si dispersoids from the conventional heterogeneous nucleation on dislocations to a more homogeneous manner. It is found that a high number density of Al-Cd nanoprecipitates formed during heating between 150 and 250 °C. These Al-Cd precipitates attracted Mn and Si atoms to form Mn,Si-rich clusters in/around them, which acted as the precursors for the later nucleation of α-Al(Mn,Fe)Si dispersoids at ∼300 °C. As a result, the number density of dispersoids formed in the Cd-containing alloy after heating to 350–450 °C is about twice as that in the base alloy subjected to the same heat treatment. This work proposes a new approach to enhance the nucleation of α-Al(Mn,Fe)Si dispersoids, which can help to further develop cheap Mn(Fe)-containing dispersoid-strengthened aluminium alloys for high-temperature applications.

An investigation about the structures, thermodynamics and kinetics of the formic acid involved molecular clusters

Despite the very important role of atmospheric aerosol nucleation in climate change and air quality, the detailed aerosol nucleation mechanism is still unclear. Here we investigated the formic acid (FA) involved multicomponent nucleation molecular clusters including sulfuric acid (SA), dimethylamine (DMA) and water (W) through a quantum chemical method. The thermodynamics and kinetics analysis was based on the global minima given by Basin-Hopping (BH) algorithm coupled with Density Functional Theory (DFT) and subsequent benchmarked calculations. Then the interaction analysis based on ElectroStatic Potential (ESP), Topological and Atomic Charges analysis was made to characterize the binding features of the clusters. The results show that FA binds weakly with the other molecules in the cluster while W binds more weakly. Further kinetic analysis about the time evolution of the clusters show that even though the formic acid’s weak interaction with other nucleation precursors, its effect on sulfuric acid dimer steady state concentration cannot be neglected due to its high concentration in the atmosphere.

Constraints on Metal Oxide and Metal Hydroxide Abundances in the Winds of AGB Stars: Potential Detection of FeO in R Dor

Abstract Using the Atacama Large Millimeter/submillimeter Array (ALMA), we observed the stellar wind of two oxygen-rich asymptotic giant branch stars, IK Tau and R Dor, between 335 and 362 GHz. One aim was to detect metal oxides and metal hydroxides (AlO, AlOH, FeO, MgO, and MgOH), some of which are thought to be direct precursors of dust nucleation and growth. We report on the potential first detection of FeO (v = 0, Ω = 4, J = 11–10) in R Dor (mass-loss rate ∼ 1 × 10−7 M ⊙ yr−1). The presence of FeO in IK Tau ( ∼ 5 × 10−6 M ⊙ yr−1) cannot be confirmed, due to a blend with 29SiS, a molecule that is absent in R Dor. The detection of AlO in R Dor and of AlOH in IK Tau was reported earlier by Decin et al. All other metal oxides and hydroxides, as well as MgS, remain undetected. We derive a column density N(FeO) of 1.1 ± 0.9 × 1015 cm−2 in R Dor, or a fractional abundance [FeO/H] ∼ 1.5 × 10−8 accounting for non-local thermodynamic equilibrium effects. The derived fractional abundance [FeO/H] is a factor ∼20 larger than conventional gas-phase chemical-kinetic predictions. This discrepancy may be partly accounted for by the role of vibrationally excited OH in oxidizing Fe, or it may be evidence for other currently unrecognized chemical pathways producing FeO. Assuming a constant fractional abundance w.r.t. H2, the upper limits for the other metals are [MgO/H2] < 5.5 × 10−10 (R Dor) and <7 × 10−11 (IK Tau), [MgOH/H2] < 9 × 10−9 (R Dor) and <1 × 10−9 (IK Tau), [CaO/H2] < 2.5 × 10−9 (R Dor) and <1 × 10−10 (IK Tau), [CaOH/H2] < 6.5 × 10−9 (R Dor) and <9 × 10−10 (IK Tau), and [MgS/H2] < 4.5 × 10−10 (R Dor) and <6 × 10−11 (IK Tau). The retrieved upper-limit abundances for these latter molecules are in accord with the chemical model predictions.

Atmospheric new particle formation at the research station Melpitz, Germany: connection with gaseous precursors and meteorological parameters

Abstract. This paper revisits the atmospheric new particle formation (NPF) process in the polluted Central European troposphere, focusing on the connection with gas-phase precursors and meteorological parameters. Observations were made at the research station Melpitz (former East Germany) between 2008 and 2011 involving a neutral cluster and air ion spectrometer (NAIS). Particle formation events were classified by a new automated method based on the convolution integral of particle number concentration in the diameter interval 2–20 nm. To study the relevance of gaseous sulfuric acid as a precursor for nucleation, a proxy was derived on the basis of direct measurements during a 1-month campaign in May 2008. As a major result, the number concentration of freshly produced particles correlated significantly with the concentration of sulfur dioxide as the main precursor of sulfuric acid. The condensation sink, a factor potentially inhibiting NPF events, played a subordinate role only. The same held for experimentally determined ammonia concentrations. The analysis of meteorological parameters confirmed the absolute need for solar radiation to induce NPF events and demonstrated the presence of significant turbulence during those events. Due to its tight correlation with solar radiation, however, an independent effect of turbulence for NPF could not be established. Based on the diurnal evolution of aerosol, gas-phase, and meteorological parameters near the ground, we further conclude that the particle formation process is likely to start in elevated parts of the boundary layer rather than near ground level.

A specific form of prefibrillar aggregates that functions as a precursor of amyloid nucleation

Non-fibrillar protein aggregates that appear in the earlier stages of amyloid fibril formation are sometimes considered to play a key role in amyloid nucleation; however, the structural features of these aggregates currently remain unclear. We herein identified a characteristic pathway of fibril formation by human insulin B chain, in which two major species of prefibrillar aggregates were identified. Based on the time-resolved tracking of this pathway with far-UV circular dichroism (CD) spectroscopy, dynamic light scattering (DLS), and 1H-NMR spectroscopy, the first prefibrillar aggregate with a hydrodynamic diameter of approximately 70 nm accumulated concomitantly with the formation of a β-sheet structure, and the size further evolved to 130 nm with an additional structural development. These prefibrillar aggregates were metastable and survived at least 24 hours as long as they were maintained under quiescent conditions. The energy barrier for nucleation was overcome by shaking or even by applying a single short ultrasonic pulse. Furthermore, an investigation where nucleation efficiency was monitored by fibrillation rates with varying the timing of the ultrasonic-pulse treatment revealed that the second prefibrillar aggregate specifically produced amyloid nuclei. These results suggest that the second form of the prefibrillar aggregates acts as a direct precursor for the amyloid nucleation.

Root-growth of boron nitride nanotubes: experiments and ab initio simulations.

We have synthesized boron nitride nanotubes (BNNTs) in an arc in the presence of boron and nitrogen species. We find that BNNTs are often attached to large nanoparticles, suggesting that root-growth is a likely mechanism for their formation. Moreover, the tube-end nanoparticles are composed of boron, without transition metals, indicating that transition metals are not necessary for the arc synthesis of BNNTs. To gain further insight into this process we have studied key mechanisms for root growth of BNNTs on the surface of a liquid boron droplet by ab initio molecular dynamics simulations. We find that nitrogen atoms reside predominantly on the droplet surface where they organize to form boron nitride islands below 2400 K. To minimize contact with the liquid particle underneath, the islands assume non-planar configurations that are likely precursors for the thermal nucleation of cap structures. Once formed, the caps are stable and can easily incorporate nitrogen and boron atoms at their base, resulting in further growth. Our simulations support the root-growth mechanism of BNNTs and provide comprehensive evidence of the active role played by liquid boron.