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Natural gas hydrates (NGHs) hold immense potential as a future energy resource and for sustainable applications such as gas capture and storage. Due to the challenging formation conditions, however, their mechanical properties remain poorly understood. Herein, the mechanical characteristics of tetrahydrofuran (THF) hydrates, a proxy for methane hydrates, were investigated at different ice contents, strain rates, and temperatures using uniaxial compressive experiments. The results unveil a distinct behavior in the peak strength of THF hydrates with a varying ice content, strain rate and temperature, exhibiting an increase as the strain rate and temperature decrease, in contrast to the peak strength-strain rate relationship observed in polycrystalline ice. Based on the experimental data, four machine learning (ML) models including extreme gradient boosting (XGboost), multilayer perceptron (MLP), gradient boosting decision tree (GBDT) and decision tree (DT) were developed to predict the peak strength. The XGboost model demonstrates superior predictive performance, emphasizing the significant influence of ice content and temperature on the peak strength of hydrates. Furthermore, molecular dynamics (MD) simulations were employed to gain insights into the dissociation and formation processes of clathrate cages, as well as phase transitions and amorphization occurring at grain boundaries (GBs) involving diverse unconventional clathrate cages, including 51265, 4151062, 4151064, 425861 and 425862, with 425861 and 425862 cages being predominant. This study enhances our understanding of the mechanical properties and deformation mechanisms of hydrates and provides a ML-based predictive framework for estimating the compressive strength of hydrates under diverse coupling conditions. The findings have significant implications for stability assessments of NGHs and the exploitation of NGH resources.

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Understanding the mechanical properties of CO2hydrate is crucial for its diverse sustainable applications such as CO2geostorage and natural gas hydrate mining. In this work, classic molecular dynamics (MD) simulations are employed to explore the mechanical characteristics of CO2hydrate with varying occupancy rates and occupancy distributions of guest molecules. It is revealed that the mechanical properties, including maximum stress, critical strain, and Young's modulus, are not only affected by the cage occupancy rate in both large 51262and small 512cages, but also by the distribution of guest molecules within the cages. Specifically, the presence of vacancies in the 51262large cages significantly impacts the overall mechanical stability compared to 512small cages. Furthermore, four distinct machine learning (ML) models trained using MD results are developed to predict the mechanical properties of CO2hydrate with different cage occupancy rates and cage occupancy distributions. Through analyzing ML results, as-developed ML models highlight the importance of the distribution of guest molecules within the cages, as crucial contributor to the overall mechanical stability of CO2hydrate. This study contributes new knowledge to the field by providing insights into the mechanical properties of CO2hydrates and their dependence on cage occupancy rates and cage occupancy distributions. The findings have implications for the sustainable applications of CO2hydrate, and as-developed ML models offer a practical framework for predicting the mechanical properties of CO2hydrate in different scenarios.

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The thermal transport properties of five-fold twinned (5FT) germanium-silicon (Ge-Si) heteronanowires (h-NWs) with varying cross-sectional areas, germanium (Ge) domain ratios and heterostructural patterns are investigated using homogeneous nonequilibrium molecular dynamics (HNEMD) simulations. The results demonstrate a distinctive behavior in the thermal conductivity (κ) of 5FT-NWs, characterized by a "flipped" trend at a critical cross-sectional area. This behavior is attributed to the hydrodynamic phonon flow, arising from the normal three-phonon scattering process in the low-frequency region. In addition, the composition ratio of 5FT-NWs has a significant impact on reducing the κ of 5FT-NWs and suppressing the hydrodynamic effect. Intriguingly, as the homogeneous element domains are separated, stronger phonon hydrodynamic flows are observed in comparison to the adjacent homogeneous element domains. By analyzing various phonon properties, including phonon dispersion, three-phonon scattering rate, and phonon mean free path, critical insights into the origin of the differential κ in different 5FT-NW structures are provided. The findings deepen the understanding of the thermal transport properties of nanomaterials and hold implications for the design and development of nanoelectronics and thermoelectric devices.

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Using molecular dynamics simulations, we investigated the effect of external electric field on ice formation with the present of a substrate surface. It turns out that the electric field can affect the ice formation on substrate surface by altering the dipole orientation of interfacial water molecules (IWs): a crossover from inhibiting to promoting ice formation with the increase of electric field strength. According to the influence of the electric field on ice formation, the electric field strength range of 0.0 V nm-1-7.0 V nm-1can be divided into three regions. In the region I and region III, there are both ice formation on the substrate surface. While, the behavior of IWs in the region I and region III are distinguished, including the arrangements of oxygen atoms and the dipole orientation distribution. In region II, ice formation does not occur in the system within 5 × 200 ns simulations. The IWs show a disorder structure, preventing the ice formation process on substrate. The interfacial water molecular orientation distribution and two-dimensional free energy landscape reveals that the electric field can alter the dipole orientation of the interfacial water and lead a free energy barrier, making the ice formation process harder. Our result demonstrates the external electric field can regulate the behavior of IWs, and further affect the ice formation process. The external electric field act as a crystallization switch of ice formation on substrate, shedding light into the studies on the control of ice crystallization.

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Here, using homogeneous nonequilibrium molecular dynamics simulations, we report the thermal transport characteristics of thin Si nanowires (NWs) with varying size and isotope doping ratio. It is identified that crossover in the thermal conductivity (κ) of both isotope doping-free and isotope doped Si-NWs appears at critical sizes, below whichκis enlarged with decreasing size because the hydrodynamic phonon flow predominates, above which, due to the dominant phonon boundary scattering, opposite behavior is observed. With increasing isotope doping, however, the critical size in minimizing theκis moved to small values because the phonon impurity scattering caused by isotope doping is critically involved. Moreover, there is a critical isotope doping (<50%) in the critical size motion, originating from that, above which, the critical size no longer moves due to the persistence of hydrodynamic phonon flow. This study provides new insights into the thermal transport behaviors of quasi-1D structures.

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Design and fabrication of functional materials for anti-icing and deicing attract great attention from both the academic research and industry. Among them, the study of fish-scale-like materials has proved that enabling sequential rupture is an effective approach for weakening the intrinsic interface adhesion. Here, graphene platelets were utilized to construct fish-scale-like surfaces for easy ice detachment. Using a biomimicking arrangement of the graphene platelets, the surfaces were able to alter their structural morphology for the sequential rupture in response to external forces. With different packing densities of graphene platelets, all the surfaces showed universally at least 50% reduction in atomistic tensile ice adhesion strength. Because of the effect of sequential rupture, stronger ice-surface interactions did not lead to an obvious increase in ice adhesion. Interestingly, the high packing density of graphene platelets resulted in stable and reversible surface morphology in cyclic tensile and shearing tests, and subsequently high reproducibility of the sequential rupture mode. The fish-scale-like surfaces built and tested, together with the nanoscale deicing results, provided a close view of ice adhesion mechanics, which can promote future bioinspired, stress-responsive, anti-icing surface designs.

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Microemulsions have been attracting great attention for their importance in various fields, including nanomaterial fabrication, food industry, drug delivery, and enhanced oil recovery. Atomistic insights into the self-microemulsifying process and the underlying mechanisms are crucial for the design and tuning of the size of microemulsion droplets toward applications. In this work, coarse-grained models were used to investigate the role that droplet sizes played in the preliminary self-microemulsifying process. Time evolution of liquid mixtures consisting of several hundreds of water/surfactant/oil droplets was resolved in large-scale simulations. By monitoring the size variation of the microemulsion droplets in the self-microemulsifying process, the dynamics of diameter distribution of water/surfactant/oil droplets were studied. The underlying mass transport mechanisms responsible for droplet size evolution and stability were elucidated. Specifically, temperature effects on the droplet size were clarified. This work provides the knowledge of the self-microemulsification of water-in-oil microemulsions at the nanoscale. The results are expected to serve as guidelines for practical strategies for preparing a microemulsion system with desirable droplet sizes and properties.

Asunto(s)

RESUMEN

Microemulsions exist widely in nature, daily life and industrial manufacturing processes, including petroleum production, food processing, drug delivery, new material fabrication, sewage treatment, etc. The mechanical properties of microemulsion droplets and a correlation to their molecular structures are of vital importance to those applications. Despite studies on their physicochemical determinants, there are lots of challenges of exploring the mechanical properties of microemulsions by experimental studies. Herein, atomistic modelling was utilized to study the stability, deformation, and rupture of Janus oligomer enabled water-in-oil microemulsion droplets, aiming at revealing their intrinsic relationship with Janus oligomer based surfactants and oil structures. The self-emulsifying process from a water, oil and surfactant mixture to a single microemulsion droplet was modulated by the amphiphilicity and structure of the surfactants. Four microemulsion systems with an interfacial thickness in the range of 7.4-17.3 Å were self-assembled to explore the effect of the surfactant on the droplet morphology. By applying counter forces on the water core and the surfactant shell, the mechanical stability of the microemulsion droplets was probed at different ambient temperatures. A strengthening response and a softening regime before and after a temperature-dependent peak force were identified followed by the final rupture. This work demonstrates a practical strategy to precisely tune the mechanical properties of a single microemulsion droplet, which can be applied in the formation, de-emulsification, and design of microemulsions in oil recovery and production, drug delivery and many other applications.

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Very recently synthesized carbon nitride nanothreads (CNNTs) by compressing crystalline pyridine show outperform diamond nanothreads in chemical and physical properties. Here, using first-principles-based ReaxFF molecular dynamics (MD) simulations, a comprehensive investigation on the mechanical characteristics of seven experimentally synthesized CNNTs has been performed. All CNNTs exhibit unique tensile properties that change with molecular morphology, atomic arrangement and the distribution of nitrogen in the skeleton. The CNNTs with more effective covalent bonds at cross-sections are more mechanically robust. Surprisingly, a tiny CNNT with periodic unit structures of 5462-cage shows extreme ductility because of the formation of a linear polymer via 4-step dissociation-and-reformation of bonds at extremely low temperatures in the range of 1-15 K; however, it shows brittle failure at one cross-section with low ductility at higher temperatures similar to other CNNTs at different temperatures; this offers a feasible way to design a kind of lightweight material that can be used in ultra-low temperature conditions, for example, the harsh deep space environment. The results also show that temperature significantly affects the fracture stress and rupture strain but not the effective stiffness. The analysis of atomic bond orders and bond lengthening reveals that the unique nonlinear elasticity of CNNTs is attributed to the occurrence of local bond transformations. This study provides physical insights into the tensile characteristics of CNNTs for the design and application of CNNT-based nanostructures as multifunctional materials.