Mixed Hydrate Nucleation: Molecular Mechanisms and Cage Structures.

The molecular-level details of the formation of mixed gas hydrates remain elusive despite their significance for a variety of scientific and industrial applications. In this study, extensive molecular simulations have been performed to examine the behavior of CH4/H2S mixed hydrate nucleation utilizing two different simulation setups varying in compositions and temperatures. The observed behavior exhibits similar phenomenology across the various systems once differences in nucleation rates and guest uptake are accounted for. We find that CH4 is always enriched in the hydrate phase while the aqueous phase is enriched in H2S. Even with H2S as a minor component (i.e., 10% mole fraction), the system can mirror the overall nucleation kinetics of pure H2S hydrate systems with CH4-dominant nuclei. Through analyses of cages and their transitions, nonstandard cages, particularly those with 12 faces (e.g., 51062), have been found to be key intermediate cage types in the early stage of nucleation. Additionally, we present previously unreported cage types comprising heptagonal faces (e.g., 596271) as having a significant role in the early-stage gas hydrate structural transitions.

Aeolian sediment transport rates in the middle reaches of the Yarlung Zangbo River, Tibet Plateau.

Aeolian sediment emission from surfaces and subsequent transport are important geological processes. The Tibet Plateau experiences strong aeolian activity in areas such as the Yarlung Zangbo River basin. The dust storms have caused grounding of aircraft, highway closures, and other consequences for the region's residents. However, few researchers have studied this activity, which means that little knowledge is available on aeolian activity to support efforts to mitigate or prevent aeolian disasters. We measured aeolian sediment transport in the middle reaches of the Yarlung Zangbo River from 2020 to 2021. Field observations showed spatial and temporal variation of the sediment transport rate, with the greatest aeolian sediment transport in spring and winter. The largest total aeolian sediment transport rate occurred over sandy desert, with the smallest emission by a floodplain grassland. The change in sediment transport rate with height followed an exponential function, but the coefficients differed among landscapes. The mean sediment transport rate was greatest above shifting sand near riverbanks (0.21 kg m-1 d-1), where the sand is exposed in the winter and spring, followed by shifting floodplain sands (0.13 kg m-1 d-1), and was lowest above a floodplain grassland (0.03 kg m-1 d-1). Mean grain size also decreased with increasing height above 0.25 m, with a minimum mean grain size (about 52.6 µm) at 3.0 m above a floodplain grassland, and maximum mean grain size (about 100.2 µm) at 3.0 m above a floodplain shifting sand surface. The spatial variation in sediment transport rates and grain size related to the proportion of fine particles in the surface material. By comparing the aeolian sediment transport over different landscapes, we found that river banks and floodplains, which had rich deposits of very fine sand, silt, and clay, were the major sources of dust in this region. Our results indicate that efforts to mitigate or prevent aeolian disasters require a focus on riverbank and floodplain deposits.

Windbreak and airflow performance of different synthetic shrub designs based on wind tunnel experiments.

Wind erosion has gained increasing attention as one of the most serious global ecological and environmental threats. Windbreaks are effective at decreasing wind erosion by reducing wind speed to protect crops, livestock, and farmsteads, while providing wildlife habitats. Synthetic shrubs can act as novel windbreaks; however, there is limited knowledge on how their design affects wind speed. This study determined the protective effects (airflow field and sheltering efficiency) based on the design of synthetic shrubs in a wind tunnel. Broom-shaped synthetic shrubs weakened the wind speeds mainly at the middle and upper parts of the shrubs (5-14 cm), while for hemisphere-shaped shrubs this effect was greatest near their bases (below 4 cm) and least in the middle and upper parts (7-14 cm). Spindle-shaped synthetic shrubs provided the best reduction effect in wind range and strength. Moreover, the wind speed reduction ratio decreased with improved wind speeds and ranged from 26.25 cm (between the second and third rows) to 52.5 cm (after the third row). These results provide strong evidence that synthetic shrubs should be considered to decrease wind speed and prevent wind erosion.

Characterizing key features in the formation of ice and gas hydrate systems.

Crystallization in liquids is critical to a range of important processes occurring in physics, chemistry and life sciences. In this article, we review our efforts towards understanding the crystallization mechanisms, where we focus on theoretical modelling and molecular simulations applied to ice and gas hydrate systems. We discuss the order parameters used to characterize molecular ordering processes and how different order parameters offer different perspectives of the underlying mechanisms of crystallization. With extensive simulations of water and gas hydrate systems, we have revealed unexpected defective structures and demonstrated their important roles in crystallization processes. Nucleation of gas hydrates can in most cases be characterized to take place in a two-step mechanism where the nucleation occurs via intermediate metastable precursors, which gradually reorganizes to a stable crystalline phase. We have examined the potential energy landscapes explored by systems during nucleation, and have shown that these landscapes are rugged and funnel-shaped. These insights provide a new framework for understanding nucleation phenomena that has not been addressed in classical nucleation theory. This article is part of the theme issue 'The physics and chemistry of ice: scaffolding across scales, from the viability of life to the formation of planets'.

Does Local Structure Bias How a Crystal Nucleus Evolves?

The broad scientific and technological importance of crystallization has led to significant research probing and rationalizing crystal nucleation processes. Previous work has generally neglected the possibility of the molecular-level dynamics of individual crystal nuclei coupling to local structures. However, recent experimental work has conjectured that this can occur. Therefore, to address a deficiency in scientific understanding of crystallization, we have probed the nucleation of prototypical single and multicomponent crystals (specifically, ice and mixed gas hydrates). We establish that local structures can bias the evolution of nascent crystal phases on a nanosecond time scale by, for example, promoting the appearance or disappearance of specific crystal motifs and thus reveal a new facet of crystallization behavior. Moreover, we demonstrate structural biases are likely present during crystallization processes beyond ice and gas hydrate formation. Structurally biased dynamics are a lens for understanding existing computational and experimental results while pointing to future opportunities.

Bridging solution properties to gas hydrate nucleation through guest dynamics.

By investigating the aqueous solution properties of several hydrate guests with molecular simulations, we find that with increasing guest concentration, the guest's hydration shell becomes more ordered and the system entropy decreases. A common critical value of the self-diffusion coefficient of different guest molecules is identified, below which hydrates will nucleate very readily.

Effects of gas reservoir configuration and pore radius on shale gas nanoflow: A molecular dynamics study.

We calculated methane transport through cylindrical graphite nanopores in cyclical steady-state flows using non-equilibrium molecular dynamics simulations. First, two typical gas reservoir configurations were evaluated: open (OS) and closed (CS) systems in which pores connect to the gas reservoir without/with a graphite wall parallel to the gas flow. We found that the OS configuration, which is commonly used to study nanoflows, exhibited obvious size effects. Smaller gas reservoir cross-sectional areas were associated with faster gas flows. Because Knudsen diffusion and slip flow in pores are interrupted in a gas reservoir that does not have walls as constraints, OSs cannot be relied upon in cyclical nanoflow simulations. Although CSs eliminated size effects, they introduced surface roughness effects that stem from the junction surface between the gas reservoir and the pore. To obtain a convergent nanoflow, the length of a side of the gas reservoir cross-section should be at least 2 nm larger than the pore diameter. Second, we obtained methane flux data for various pore radii (0.5-2.5 nm) in CSs and found that they could be described accurately using the Javadpour formula. This is the first direct molecular simulation evidence to validate this formula. Finally, the radial density and flow-velocity distributions of methane in CS pores were analyzed in detail. We tested pores with a radius between 0.5 nm and 2.5 nm and determined that the maximum ratio (∼34%) of slip flow to overall flow occurred in the pore with a radius of 1.25 nm. This study will aid in the design of gas reservoir configurations for nanoflow simulations and is helpful in understanding shale gas nanoflows.

The effects of ice on methane hydrate nucleation: a microcanonical molecular dynamics study.

Although ice powders are widely used in gas hydrate formation experiments, the effects of ice on hydrate nucleation and what happens in the quasi-liquid layer of ice are still not well understood. Here, we used high-precision constant energy molecular dynamics simulations to study methane hydrate nucleation from vapor-liquid mixtures exposed to the basal, prismatic, and secondary prismatic planes of hexagonal ice (ice Ih). Although no significant difference is observed in hydrate nucleation processes for these different crystal planes, it is found, more interestingly, that methane hydrate can nucleate either on the ice surface heterogeneously or in the bulk solution phase homogeneously. Several factors are mentioned to be able to promote the heterogeneous nucleation of hydrates, including the adsorption of methane molecules at the solid-liquid interface, hydrogen bonding between hydrate cages and the ice structure, the stronger ability of ice to transfer heat than that of the aqueous solution, and the higher occurrence probability of hydrate cages in the vicinity of the ice surface than in the bulk solution. Meanwhile, however, the other factors including the hydrophilicity of ice and the ice lattice mismatch with clathrate hydrates can inhibit heterogeneous nucleation on the ice surface and virtually promote homogeneous nucleation in the bulk solution. Certainly, the efficiency of ice as a promoter and as an inhibitor for heterogeneous nucleation is different. We estimate that the former is larger than the latter under the working conditions. Additionally, utilizing the benefit of ice to absorb heat, the NVE simulation of hydrate formation with ice can mimic the phenomenon of ice shrinking during the heterogeneous nucleation of hydrates and lower the overly large temperature increase during homogeneous nucleation. These results are helpful in understanding the nucleation mechanism of methane hydrate in the presence of ice.

Unraveling Mixed Hydrate Formation: Microscopic Insights into Early Stage Behavior.

The molecular-level details of mixed hydrate nucleation remain unclear despite the broad implications of this process for a variety of scientific domains. Through analysis of mixed hydrate nucleation in a prototypical CH4/H2S/H2O system, we demonstrate that high-level kinetic similarities between mixed hydrate systems and corresponding pure hydrate systems are not a reliable basis for estimating the composition of early stage mixed hydrate nuclei. Moreover, we show that solution compositions prior to and during nucleation are not necessarily effective proxies for the composition of early stage mixed hydrate nuclei. Rather, microscopic details, (e.g., guest-host interactions and previously neglected cage types) apparently play key roles in determining early stage behavior of mixed hydrates. This work thus provides key foundational concepts and insights for understanding mixed hydrate nucleation.

Effects of ensembles on methane hydrate nucleation kinetics.

By performing molecular dynamics simulations to form a hydrate with a methane nano-bubble in liquid water at 250 K and 50 MPa, we report how different ensembles, such as the NPT, NVT, and NVE ensembles, affect the nucleation kinetics of the methane hydrate. The nucleation trajectories are monitored using the face-saturated incomplete cage analysis (FSICA) and the mutually coordinated guest (MCG) order parameter (OP). The nucleation rate and the critical nucleus are obtained using the mean first-passage time (MFPT) method based on the FS cages and the MCG-1 OPs, respectively. The fitting results of MFPT show that hydrate nucleation and growth are coupled together, consistent with the cage adsorption hypothesis which emphasizes that the cage adsorption of methane is a mechanism for both hydrate nucleation and growth. For the three different ensembles, the hydrate nucleation rate is quantitatively ordered as follows: NPT > NVT > NVE, while the sequence of hydrate crystallinity is exactly reversed. However, the largest size of the critical nucleus appears in the NVT ensemble, rather than in the NVE ensemble. These results are helpful for choosing a suitable ensemble when to study hydrate formation via computer simulations, and emphasize the importance of the order degree of the critical nucleus.

Microcanonical molecular simulations of methane hydrate nucleation and growth: evidence that direct nucleation to sI hydrate is among the multiple nucleation pathways.

The results of six high-precision constant energy molecular dynamics (MD) simulations initiated from methane-water systems equilibrated at 80 MPa and 250 K indicate that methane hydrates can nucleate via multiple pathways. Five trajectories nucleate to an amorphous solid. One trajectory nucleates to a structure-I hydrate template with long-range order which spans the simulation box across periodic boundaries despite the presence of several defects. While experimental and simulation data for hydrate nucleation with different time- and length-scales suggest that there may exist multiple pathways for nucleation, including metastable intermediates and the direct formation of the globally-stable phase, this work provides the most compelling evidence that direct formation to the globally stable crystalline phase is one of the multiple pathways available for hydrate nucleation.

[Effect of dust deposition collection methods on collection efficiency].

There are lots of dust deposition collection methods on dust deposition, but there is no standard field observation method. At present, researchers have studied dust deposition using different methods in this issue, but due to the different observation method and collection efficiency, the research results are incomparable. The efficiency of the standard dust deposition gauge including dry, wet, net, net and glass ball and slowing speed methods was studied in the Tengger Desert. The amount of the dry method collected material was only about 5% to 62% of the wet method, the net method was only about 43% to 89% of the wet method, the net and glass ball method was only about 40% to 80% of the wet method. Wind speed obviously affected the dust deposition efficiency in all observation methods, the dust deposition efficiency decreased with increasing wind speed when the wind speed was smaller than 2.5 m.s-1, but the dust deposition efficiency had no clear trend with wind speed when the wind speed was larger than 2.5 m.s-1. The particle size of collected material by wet method was the smallest, followed by the slowing speed method, the net and glass ball method, the wet method and the dry method. There are relationships between the averaged wind speed, the averaged larger than 5 m.s-1 wind speed and the amount of dust deposition collected by the wet and the slowing speed methods, which can be expressed as exponential function. There are also linear relationships between the amount of collected material by the wet and dry, net, net and glass ball and slowing speed methods. The wet method is the best dust deposition collecting method, followed by the slow wind speed method. Therefore, in the arid and semi-arid regions, the slowing wind speed method can be used to replace the wet method to collect dust deposition.

The union of anti-CD34 antibody can improve the performance of drug-eluting stents.

OBJECTIVES: The authors investigate whether the combination of anti-CD34 antibody with DES is win-win cooperation. BACKGROUND: DES may reduce the risk of restenosis compared to bare-metal stents (BMS), but they were found to inhibit the healing process of intima. METHODS: Fifteen BMS, 17 DES, and 16 combined anti-CD34 antibody and DES were randomly implanted in the coronary arteries of 22 minipigs. Ten minipigs were followed up to 2 weeks. The stenting coronary segments were examined by histological examination and scanning electron microscopy after in vivo coronary angiography and intracoronary optical coherence tomography (OCT) examinations. The other 12 minipigs were followed up to 3 months. Coronary angiography and intracoronary OCT examination were performed in vivo and histological examination was performed on the stenting coronary segments. RESULTS: After 2 weeks, the neointimal covering level of the DES was lower than that in BMS, but the covering level of the combined stents was even better than the BMS. After 3 months, neointimal hyperplasia was significant in the BMS, but not in the other two types of stents. The in-stent late lumen loss of the combined stents even showed a decreasing tendency when compared with the DES. CONCLUSION: The combination of anti-CD34 antibody and DES can not only well offset the short-term inhibitory effect on re-endothelialization but also slightly enhance the long-term antiproliferative effect.

Mesenchymal stem cell seeding promotes reendothelialization of the endovascular stent.

This study is designed to make a novel cell seeding stent and to evaluate reendothelialization and anti-restenosis after the stent implantation. In comparison with cell seeding stents utilized in previous studies, Mesenchymal stem cells (MSCs) have advantages on promoting of issue repair. Thus it was employed to improve the reendothelialization effects of endovascular stent in present work. MSCs were isolated by density gradient centrifugation and determined as CD29(+) CD44(+) CD34(-) cells by immunofluorescence and immunocytochemistry; gluten and polylysine coated stents were prepared by ultrasonic atomization spray, and MSCs seeded stents were made through rotation culture according to the optimized conditions that were determined in previous studies. The results from animal experiments, in which male New Zealand white rabbits were used, show that the reendothelialization of MSCs coated stents can be completed within one month; in comparison with 316L stainless steel stents (316L SS stents) and gluten and polylysine coated stents, the intimal hyperplasia and in-stent restenosis are significantly inhibited by MSCs coated stents. Endovascular stent seeded with MSCs promotes reendothelialization and inhibits the intimal hyperplasia and in-stent restenosis compared with the 316L SS stents and the gluten and polylysine coated stents.

Synthesis and insecticidal activity of N-tert-butyl-N,N'-diacylhydrazines containing 1,2,3-thiadiazoles.

N-tert-Butyl-N,N'-diacylhydrazines are nonsteroidal ecdysone agonists used as environmental benign pest regulators. In this paper, two series of new N-tert-butyl-N,N'-diacylhydrazine derivatives containing 1,2,3-thiadiazole were designed and synthesized. All structures of the synthesized compounds were confirmed by proton nuclear magnetic resonance ((1)H NMR), infrared spectroscopy (IR), and high-resolution mass spectrometry (HRMS). Bioasssay results indicated that most of the synthesized compounds possessed good insecticidal activities against Plutella xylostella L. and Culex pipiens pallens as compared with the positive control, tebufenozide. The results of this study indicated that 1,2,3-thiadiazoles, as an important active substructure, could improve or maintain the activity of the dicylhydrazines and favor novel pesticide development.

[The research actualities of drug eluting-stents and the developing trend].

The development of drug eluting-stents has been a breakthrough for the prevention of PCI postoperative restenosis, and the correlative researches have not yet stopped and are still the focus of the world. We review, in this article, the history of drug eluting-stents researches, analyze the research actualities and the problems existing, and forecast the developing trend in the future.