Microscopic Inhomogeneities Research Articles

Easy-to-use formulations based on the homogenization theory for vascular stent design and mechanical characterization

Background and objectivesVascular stents are scaffolding structures implanted in the vessels of patients with obstructive disease. Stents are typically designed as cylindrical lattice structures characterized by the periodic repetition of unit cells. Their design, including geometry and material characteristics, influences their mechanical performance and, consequently, the clinical outcomes. Computational optimization frameworks have proven to be effective in assisting the design phase of vascular stents, facilitating the achievement of enhanced mechanical performances. However, the reliance on time-consuming simulations and the challenge of automating the design process limit the number of design evaluations and reduce optimization efficiency. In this context, a rapid and automated method for the mechanical characterization of vascular stents is presented, taking the stent geometry, conceived as the periodic repetition of a unit cell, and material as input and providing the mechanical response of the stent as output. MethodsVascular stents were assumed to be thin-walled hollow cylinders sharing the same macroscopic geometrical characteristics as the cylindrical lattice structure but composed of an anisotropic homogenized material. Homogenization theory was applied to average the microscopic inhomogeneities at the stent unit cell level into a homogenized material at the macro-scale, enabling the calculation of the associated homogenized material tensor. Analytical formulations were derived to relate the stent mechanical behavior to the homogenized stiffness tensor, considering linear elastic theory for thin-walled hollow cylinders and three loading scenarios of relevance for vascular stents: radial crimping; axial traction; torsion. Validation was conducted by comparing the derived analytical formulations with results obtained from finite element analyses on typical stent designs. ResultsHomogenized stiffness tensors were computed for the unit cells of three stent designs, revealing insights into their mechanical performance, including whether they exhibit auxetic behavior. The derived analytical formulations were successfully validated with finite element analyses, yielding low relative differences in the computed values of foreshortening, radial, axial and torsional stiffnesses for all three stents. ConclusionsThe proposed method offers a rapid, fully automated procedure that facilitates the assessment of the mechanical behavior of vascular stents and is suitable for effective integration into computational optimization frameworks.

Nanoscale lipid domains determine the dynamic molecular portraits of mixed DOPC/DOPS bilayers in a fluid phase: A computational insight

Lateral heterogeneity, or mosaicity, is a fundamental property inherent to cell membranes that is crucial for their functioning. While microscopic inhomogeneities (e.g. rafts) are easily detected experimentally, lipid domains with nanoscale dimensions (nanoclusters of nanodomains, NDs) resist reliable characterization by instrumental methods. In such a case, important insight can be gained via computer modeling. Here, NDs composed of lipid's head groups in the mixed zwitterionic dioleoylphosphatidylcholine (DOPC) and negatively charged dioleoylphosphatidylserine (DOPS) bilayers were studied by molecular dynamics. A new algorithm has been developed to identify NDs. Unlike most similar methods, it implicitly considers the heterogeneous distribution of lipid head atomic density and does not require subjectively chosen parameters. In DOPS-rich membranes, lipids form more compact and stable NDs due to strong interlipid interactions. In DOPC-rich systems, NDs arise due to the “packing” effect of weakly bound lipid heads. The clustering picture is related to the physical properties of the bilayer surface: DOPS-rich systems show more pronounced surface heterogeneity of hydrophilic/hydrophobic regions compared to DOPC-rich ones. The results obtained are important for the effective quantitative characterization of the “dynamic molecular portrait” of a membrane surface – its “fingerprint” characterizing dynamical distribution of its physicochemical properties.

Evidence of contact-induced variability in industrially-fabricated highly-scaled MoS2 FETs

Evidence of microscopic inhomogeneities of the side source/drain contacts in 300 mm wafer integrated MoS2 field-effect transistors is presented. In particular, the presence of a limited number of low Schottky barrier spots through which channel carriers are predominantly injected is demonstrated by the dramatic current changes induced by individual charge traps located near the source contact. Two distinct types of “contact-impacting traps” are identified. Type-1 trap is adjacent to the contact interface and exchanges carriers with the metal. Its impact is only observable when the adjacent contact is the reverse-biased FET source and limits the channel current. Type-2 trap is located in the AlOx gate oxide interlayer, near the source contact, and exchanges carriers with the channel. Its capture/emission time constants exhibit both a gate and drain bias dependence due to the high sensitivity of the contact regions to the applied lateral and vertical fields. Unlike typical channel-impacting oxide traps, both types of reported defects affect the Schottky barrier height and width rather than the threshold voltage and result in giant random telegraph noise (RTN). These observations indicate that the contact quality and geometry play a fundamental role in the ultimate scaling of 2D FETs.

Pore structure characterization and permeability prediction of uranium-bearing sandstone based on digital core

The permeability of the ore-bearing layer is an important indicator affecting the in-situ leaching (ISL) of uranium-bearing sandstone, which is related to various factors such as pore shape, distribution, and size. In order to study the effect of pore structure on seepage in low-permeability uranium-bearing sandstone, CT scanning tests were conducted to create a 3D digital core based on scanning images and to calculate the fractal dimension using the box counting dimension method, which integrated fractal theory to define the core samples' pore structure. The permeability prediction was realized based on the porosity-permeability model and the fractal theory model. Results indicated that this type of sandstone is obviously characterized by pore connectivity, large differences in distribution, and strong microscopic inhomogeneity. The pores are dominated by micro- and nano-pores, as well as small pores, accounting for 90 %; macropores are few in number, but the diameters of their single pores are large. The distribution of pore structure in this type of sandstone exhibits a good fractal characteristic; the three-dimensional fractal dimensionality is 2.044–2.310. The porosity-permeability model was established, and permeability prediction was realized by combining the fractal theory to provide theoretical support for determining the values of well field parameters in ISL.

Towards more homogeneous character in 3D printed photopolymers by the addition of nanofillers

The performance of 3D printed materials differs from that of fully cured polymer materials because of the presence of interfacial areas between consecutively joined layers. These interfaces result in an inhomogeneous character of the printed objects and is frequently reported as their main cause of failures. We noted that the presence of nanosilica particles strengthens the 3D printed layers of the polymer matrix by inducing its additional crosslinking. A model resin composed of poly (ethylene glycol) diacrylate (PEGDA) and nanosilica (Aerosil R972) is used for vat photopolymer 3D printing. Evolution of the interface properties at different nanosilica loadings is tracked by mapping its surface stiffness (Young's modulus mapping) using quantitative Atomic Force Microscopy (AFM). Our research demonstrates that incorporating 6% w/v nanosilica in the polyPEGDA matrix unifies its mechanical properties within the layer, leading to a substantial reduction of microscopic inhomogeneity in the final 3D printed materials.

Identification the Role of Grain Boundaries in Polycrystalline Photovoltaics via Advanced Atomic Force Microscope.

Atomicforce microscopy (AFM)-based scanning probing techniques, including Kelvinprobe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM), have been widely applied to investigate thelocal electromagnetic, physical, or molecular characteristics of functional materials on a microscopic scale. The microscopic inhomogeneities of the electronic properties of polycrystalline photovoltaic materials can be examined by these advanced AFM techniques, which bridge the local properties of materials to overall device performance and guide the optimization of the photovoltaic devices. In this review, the critical roles of local optoelectronic heterogeneities, especially at grain interiors (GIs) and grain boundaries (GBs) of polycrystalline photovoltaic materials, including versatile polycrystalline silicon, inorganic compound materials, and emerging halide perovskites, studied by KPFM and C-AFM, are systematically identified. How the band alignment and electrical properties of GIs and GBs affect the carrier transport behavior are discussed from the respective of photovoltaic research. Further exploiting the potential of such AFM-based techniques upon a summary of their up-to-date applications in polycrystalline photovoltaic materials is beneficial to acomprehensive understanding of the design and manipulation principles of thenovel solar cells and facilitating the development of the next-generation photovoltaics and optoelectronics.

A new method for the characterization of microcracks based on seepage characteristics

Microcracks are the main seepage channels and reservoir space for oil and gas in dense sandstone reservoirs, and the degree of development dominates the reservoir’s high and stable production capacity. A new method has been devised to address the lack of quantitative identification and characterization methods for microcrack networks. The method is based on core stress sensitivity, permeability anisotropy, and two-phase seepage rule testing. By improving upon the traditional black oil model, this method can accurately calculate the impact that microcracks of varying degrees of development have on the capacity of tight oil reservoirs. The study shows that 1) the higher the degree of microcrack development, the stronger the reservoir stress sensitivity and the greater the permeability anisotropy. As the degree of microcrack development increases, the irreducible water saturation decreases, the residual oil saturation gradually increases, and the oil–water two-phase co-infiltration zone becomes more extensive and smaller. The degree of microcrack development in tight reservoirs can be characterized based on the seepage characteristic parameters; 2) a microcrack characterization method and classification criteria have been established. It is based on stress sensitivity coefficients, permeability anisotropy parameters, and phase seepage characteristics in cores with different microcrack development degrees. For the first time, the method enables a macroscopic-level description of microcrack seepage; 3) numerical calculations show that the degree of microcrack development significantly affects the reservoir’s oil production and water production. The higher the degree of microcrack development, the higher the reservoir’s initial oil production and cumulative oil production. However, when the degree of microcrack development is too high, the microcracks are connected, thus exhibiting the nature of large fractures. This strengthens the bypassing communication effect and causes the microscopic inhomogeneity to strengthen, the oil production decreases rapidly, and water production increases quickly at the later stage. This research result enriches the reservoir microcrack characterization and evaluation system, which has essential theoretical guidance and practical significance for the rational and effective development of tight oil and tight sandstone gas.

Signal Processing in Optical Frequency Domain Reflectometry Systems Based on Self-Sweeping Fiber Laser with Continuous-Wave Intensity Dynamics

We report on the development of an optical frequency domain reflectometry (OFDR) system based on a continuous-wave Er-doped self-sweeping fiber laser. In this work, we investigate the influence of the input data processing procedure in an OFDR system on the resulting reflectograms and noise level. In particular, several types of signal averaging (in time and frequency domain) and Fourier analysis are applied. We demonstrate that the averaging in the frequency domain can be applied to evaluate absolute values of the local scattering amplitudes related to the Rayleigh light scattering (RLS), which is associated with the interference of scattering signals on microscopic inhomogeneities in optical fibers. We found that the RLS signal remains unchanged in the case of signal averaging in time domain, while the noise floor level decreases by 30 dB with an increasing number of points from 1 to ~450. At the same time, it becomes possible to detect the spectral composition of the scattering at each point of the fiber using windowed Fourier transform. As a result, the sensitivity of the developed system allows us to measure the RLS signal at a level of about 20 dB above the noise floor. The described analysis methods can be useful in the development of distributed sensors based on Rayleigh OFDR systems.

Quantitative study of the dynamics and energetics of peptide-lipid membrane interaction using coarse-grained molecular dynamics simulations.

Peptide-lipid membrane interactions are fundamental in molecular biology. A quantitative description, at single amino acid level, of the interaction of peptides with lipid membranes is essential in understanding the subtle microscopic mechanisms of membrane protein partitioning and folding. Here we demonstrate that long time-scale coarse-grained molecular dynamics (CG-MD) simulations can be used effectively to study both the conformational dynamics and the energetics of peptides and peripheral membrane proteins interacting with lipid bilayers. First, the penetration depth distribution of individual amino acids, obtained from tens of µs long CG-MD trajectories, provide a detailed description of the binding of the peptide to the lipid membrane. Second, the free energy profile (potential of mean force or PMF), as a function of the peptide-lipid membrane separation, can be calculated using the CG umbrella sampling method. The features of the PMF appear to be consistent with existing experimental results. Furthermore, we find that repeated CG-MD simulations that take into account the microscopic inhomogeneities of the system lead to a stochastic ensemble of PMFs that can be used to identify the dominant dissociation pathways of the peptide from the lipid bilayer. Third, the reconstructed PMF can be used to (i) interpret the peptide-lipid membrane dissociation force distribution measured in AFM force spectroscopy experiments, and (ii) infer the corresponding dissociation rate. We apply the above approach to investigate two exemplary systems, namely: (1) three Wimley-White pentapeptides (Ac-WLXLL, with guest residues X = R,I and L) interacting with POPC and POPG lipid bilayers, and (2) the extreme N-terminus (SecA2-11) of the SecA peripheral membrane protein from E. coli, interacting with a POPC bilayer.

Synthesis of nano (Ti,W)C powder with preferred orientation and twin boundary structure

(Ti,W)C is a novel additive for high performance cermets. In this study, (Ti0.88W0.12)C with the lowest formation energy is synthesized by carbothermal reduction-carbonization in Ar. The starting materials included WO2.72 with one-dimensional nanostructure, TiO2 and carbon black. The phase transition temperatures were established by thermal analysis. XRD analysis results disclose that once TiC is formed at a temperature over 1220 °C, W atoms begin to diffuse into the TiC lattices, which is independent of the existing form of tungsten. At a condition of 1500 °C for 180 min, W and C atoms from the decomposed W2C and WC are fully dissolved in the TiC lattices. Under such a TiC-centered atomic reconfiguration environment, the as-synthesized powder is featured with a BET particle size of 76 nm and texture coefficients TC(111) of 1.53 and TC(200) of 1.33. Results from SEM and HRTEM reveal that the roughly equiaxed powder particles have characteristics of readily identified twin boundary structure and stacking faults. Microscopic inhomogeneity of W solution atoms is discussed. The revelation of the easily identified twin boundary structure and stacking faults is of great significance to the hard phase regulation for high performance Ti(C,N)-based cermets.

Characterization, Classification, and Evaluation of the Reservoir Pore Structure Features of Lacustrine Fine-Grained Sedimentary Rocks. A Case Study of the Fourth Member of the Shahejie Formation in the Chenguanzhuang Area of the Southern Gently Sloping Zone of the Dongying Depression, Bohai Bay Basin

With the development of unconventional oil and gas exploration “from sea to land,” lacustrine fine-grained sedimentary rocks (FSR) have gradually attracted the attention of scholars and become an important topic in the field of unconventional oil and gas, but the research is still in its initial stage. In this study, lacustrine FSR in the Dongying Depression of the Bohai Bay Basin are used as the research object, and nuclear magnetic resonance (NMR) and quantitative image characterization are used to characterize the pore structure of the reservoir in the study area on multiple scales, analyze the reservoir characteristics control factors, and classify and evaluate the reservoir. The results show that: 1) the favorable petrographic phases of the FSR reservoir can be classified into six types of organic-rich lime mudstone, organic-rich laminoid lime clay rock, organic-rich laminoid clay micritic limestone, organic-bearing banding clay micritic limestone, organic-rich banding lime clay rock, and organic-bearing lumpy clay micritic limestone. With an average porosity of 12.3% and an average permeability of 10.58 mD, the overall reservoir is a typical low-porosity-low-permeability type; 2) the reservoir space types are diverse, with strong microscopic inhomogeneity; pores with a pore size of less than 2 nm almost have no contribution to the reservoir space; the pore volume and pore area are mainly provided by organic matter pores at the 100 nm level, mineral intergranular pores, and clay mineral shrinkage pores/slits. The FSR reservoirs in the study area are classified into three categories, and the pore structure of the reservoirs from categories I to III deteriorates in turn. This study provides a basis for the microscopic characterization, classification, and evaluation of lacustrine FSR reservoirs and their exploration.

Determination of the magnetic signature due to the presence of a nonlinear material property gradient in a nickel-based superalloy by thermoelectric means

It was showed that the thermoelectric coupling in a metallic medium can be applied as a viable means of inspecting inhomogeneities and defects in materials. This nondestructive sensing is carried out by using magnetometers to monitor the thermoelectric currents around the inspected area when a temperature gradient is induced inside the sample. In the NDE thermoelectric technique, the detection probability of defects in metals depends on the thermoelectric background signature produced by the intrinsic texture and inhomogeneity of the material. The measured magnetic signature is due to a combination of both effects. This paper investigate s the magnetic signature produced by a microscopic inhomogeneity such as precipitation and/or segregation when the material properties exhibit a non linear spatial variation in the cross section of a slender bar. An analytical method has been developed for calculating the magnetic field produced by the quadratic spatial dependence of the material properties. Experimental data from an inhomogeneous nickel-based superalloy 825 sample aged at 650 °C for 312 h were correlated with the theoretical predictions and fully verified the analytical model. Finally, the relationship between the strength of the magnetic signature and the complex microstructural and chemical features of this popular high-strength superalloy was studied.

Studying on the Internal Quality of Steel at Various Stages of Production: Slab-Rolled Metal

In this work, the characteristics of the cast structure formed under various conditions of slab casting and the micro-structure of hot-rolled sheets made from them are studied by a complex of chemical and metal-graphic analysis methods. This article presents the results on cast structure's dispersion studying of continuously cast slab that depend on the casting speed as well as hot rolled steel's the microstructure and mechanical properties produced from experimental slabs. A comprehensive study shows that continuously cast slab's solidification rate determines the dispersion of the dendritic structure and the microscopic inhomogeneity degree. It is established that the dendritic structure density is the degree indicator of cast steel’s microchemical homogeneity. In all structural zones and in the whole thickness of a continuous slab with a thickness of 105 mm, the dendritic structure density and the micro-chemical homogeneity increases with increasing crystallization rate. Using the methods of quantitative metallography, it is shown that with an increase in the crystallization rate, the resulting cast structure becomes significantly dispersed. The study revealed a pattern between the initial austenitic structure of a continuously cast slab obtained at different crystallization rates and the structure of hot-rolled rolled products obtained by direct rolling, when hot slabs are placed in a heating furnace before phase transformations γ→α in the steel structure.

Assessment of iron nanoparticle distribution in mouse models using ultrashort-echo-time MRI.

Microscopic magnetic field inhomogeneities caused by iron deposition or tissue‐air interfaces may result in rapid decay of transverse magnetization in MRI. The aim of this study is to detect and quantify the distribution of iron‐based nanoparticles in mouse models by applying ultrashort‐echo‐time (UTE) sequences in tissues exhibiting extremely fast transverse relaxation. In 24 C57BL/6 mice (two controls), suspensions containing either non‐oxidic Fe or AuFeO x nanoparticles were injected into the tail vein at two doses (200 μg and 600 μg per mouse). Mice underwent MRI using a UTE sequence at 4.7 T field strength with five different echo times between 100 μs and 5000 μs. Transverse relaxation times T 2* were computed for the lung, liver, and spleen by mono‐exponential fitting. In UTE imaging, the MRI signal could reliably be detected even in liver parenchyma exhibiting the highest deposition of nanoparticles. In animals treated with Fe nanoparticles (600 μg per mouse), the relaxation time substantially decreased in the liver (3418 ± 1534 μs (control) versus 228 ± 67 μs), the spleen (2170 ± 728 μs versus 299 ± 97 μs), and the lungs (663 ± 101 μs versus 413 ± 99 μs). The change in transverse relaxation was dependent on the number and composition of the nanoparticles. By pixel‐wise curve fitting, T 2* maps were calculated showing nanoparticle distribution. In conclusion, UTE sequences may be used to assess and quantify nanoparticle distribution in tissues exhibiting ultrafast signal decay in MRI.

Anomalous broadening of specific heat jump at T c in high-entropy-alloy-type superconductor TrZr2

A high-entropy-alloy-type (HEA-type) superconductor is new category of highly disordered superconductors. Therefore, finding brand-new superconducting characteristics in the HEA-type superconductors would open new avenue to investigate the relationship between structural disorder and superconductivity. Here, we report on the remarkable broadening of specific heat jump near a superconducting transition tempreature (T c) in transition-metal zirconides (TrZr2) with different mixing entropy (ΔS mix) at the Tr site. With increasing ΔS mix, the superconducting transition seen in specific heat became broader, whereas those seen in magnetization were commonly sharp. Therefore the broadening of specific heat jump would be related to the microscopic inhomogeneity of the formation of Cooper pairs behind the emergence of bulk superconductivity states.

Lattice Boltzmann simulations of drying suspensions of soft particles.

The ordering of particles in the drying process of a colloidal suspension is crucial in determining the properties of the resulting film. For example, microscopic inhomogeneities can lead to the formation of cracks and defects that can deteriorate the quality of the film considerably. This type of problem is inherently multiscale and here we study it numerically, using our recently developed method for the simulation of soft polymeric capsules in multicomponent fluids. We focus on the effect of the particle softness on the film microstructure during the drying phase and how it relates to the formation of defects. We quantify the order of the particles by measuring both the Voronoi entropy and the isotropic order parameter. Surprisingly, both observables exhibit a non-monotonic behaviour when the softness of the particles is increased. We further investigate the correlation between the interparticle interaction and the change in the microstructure during the evaporation phase. We observe that the rigid particles form chain-like structures that tend to scatter into small clusters when the particle softness is increased. This article is part of the theme issue 'Progress in mesoscale methods for fluid dynamics simulation'.

Cobalt Iron Oxides Prepared by Acidic Redox-Assisted Precipitation: Characterization, Applications, and New Opportunities.

The microscopic homogeneity of mixed metals in a single-phase oxide is a critical issue in improving material performance. Aqueous alkaline precipitation is the most common approach but it has the limits of microscopic inhomogeneity because of intrinsically different precipitation rates between metal cations. Herein, we demonstrate a new preparation of uniformly structural substituted cobalt iron oxides via acidic redox-assisted precipitation (ARP) upon the interaction of CoII and K2FeO4. This low-pH synthesis features the redox process between Co and Fe, presumably through the formation of inner-sphere complexes such as [(H2O)5CoII-O-FeVIO3]. With the nucleation starting from such complexes, one obtains a product with predominantly mixed-metal Co-O-Fe moieties, which improves the electrical conductivity of the product. This work further analyzes how the properties of the product species evolve during the hydrothermal synthesis step in the ARP process. We see that the Co/Fe ratio slowly increases from about 1:1 to a final value of 2:1, but does not reach the expected redox stoichiometry of 3:1. At the same time, the magnetization also increases, reaching a value of 16.9 emu g-1 for the final superparamagnetic product, which is three times higher than the value of monometallic Co3O4 and Fe2O3. The cobalt iron oxide samples obtained from ARP also possess superior oxygen evolution activity (307 mV overpotential at 10 mA cm-2 μg-1) compared to a mixture of Co3O4 and Fe2O3 (422 mV) or pure cobalt oxide (350 mV), highlighting the structure-induced enhancement of the catalytic activity. The difficult synthesis of evenly blended trinary/quaternary metals in a single-oxide phase may become possible in the future via ARP.

Дифференциальный фон электрического сигнала, снимаемого с индукционной магнитной головки

Experimental dependences U(t) of the electric voltage taken from an induction magnetic head (MH) moving relative to a magnetic carrier (MH) are presented. The backgrounds of the edges of the MN, local defects of the MN, the background of the inhomogeneous magnetic field of the local source, the backgrounds of defects and structural inhomogeneities of the object, the etheric electromagnetic background, the background of the quality of the surface of the object and surface microscopic inhomogeneities of the material have been identified and investigated. The resonant backgrounds of self-excitation of the measuring system on the signals of the edges of the MN, defects of the MN, instrument and network pickups and interference, object defects, and etheric electromagnetic fields are revealed and investigated. Resonance peaks are the result of self-excitation of the measuring system, which includes the MG, and arise on the trailing edges of any signals of sufficient magnitude, the duration of the trailing edge of which is about a quarter of the period of natural oscillations of the measuring system. The amplitude and frequency spectra of the background signals of object defects, MI and noise and the analytical expressions describing them are determined. The results of the extraction of the useful signal from the complete signal recorded on the MN are shown. Investigations of the differential background of an electric signal allow, together with the previously developed methods of hysteresis interference, to control the properties of objects in an automatic mode with program control, which significantly increases the sensitivity and accuracy of control. To achieve this goal, it is recommended to set the parameters of the measurement system at the threshold of the onset of natural free oscillations in it.

A model for freeze-thaw-induced plastic volume changes in saturated clays

A new model is proposed for describing freeze-thaw-induced plastic volume changes in saturated clays based on microscopic inhomogeneities inherent in the soil structure and the pore water transfer between them. It is shown through a simple inhomogeneous mesostructure model that the equilibrium state reached after freezing serves as an important reference state for interpreting the volumetric behaviour. The soil state evolution towards this freeze equilibrium state describes many features seen in the experimentally observed freeze-thaw behaviour, such as the existence of a limiting stress level above which no plastic volume change occurs. The process of freezing/thawing is modelled as a rate process approaching the freeze equilibrium line (FEL) and representing the eventual equilibrium states at a particular sub-freezing temperature, exploiting a newly defined state parameter, ψfe. The model offers a mesoscopic three-dimensional yield surface in the specific volume – effective stress – temperature space, expressed by a simple equation. Although the model’s essence is encapsulated in only two simple equations, it can simulate well the experimentally observed volume change accumulation with freeze-thaw cycles under different stress levels and densities (or over-consolidation ratios). As the model is formulated in an incremental rate form, it can be applied to general freeze-thaw problems, even those with irregular temperature and stress history – a feature not seen in existing models.

Solvation shell dynamics of supercritical water–cyclohexane mixtures in relation to the translational and rotational dynamics as studied by molecular dynamics simulation

The translational dynamics of water and cyclohexane in supercritical binary mixtures were investigated using molecular dynamics simulations. The effects of the local composition were examined through a decomposition scheme of the conditional time-correlation functions based on the solvation numbers for water and cyclohexane. The self-diffusion of water was found to be largely controlled by the continuous and collective attractive interactions with surrounding water molecules, while interactions with cyclohexane have minimal impact on water diffusion. On the other hand, the self-diffusion of cyclohexane is dominantly determined by uncorrelated collisional interactions with neighboring cyclohexane molecules. The results demonstrate the dynamic aspect of microscopic inhomogeneity and highlight the significance of interactions between molecules of the same species. An examination of the dependence of self-diffusion on the lifetime of the solvation shell indicated that the self-diffusion of water is confined within the solvation shell. This is attributed to the hydrogen bond interactions with neighboring water molecules, which create an energy barrier to the water molecules diffusing out of the hydration cage. In contrast, diffusing cyclohexane molecules migrate beyond the solvation shell, particularly at large water contents.