Mesoscopic Structural Research Articles

Structure and dynamics of amphiphilic patchy cubes in a nanoslit under shear.

Patchy nanocubes are intriguing materials with simple shapes and space-filling and multidirectional bonding properties. Previous studies have revealed various mesoscopic structures such as colloidal crystals in the solid regime and rod-like or fractal-like aggregates in the liquid regime of the phase diagram. Recent studies have also shown that mesoscopic structural properties, such as an average cluster size M and orientational order, in amphiphilic nanocube suspensions are associated with macroscopic viscosity changes, mainly owing to differences in cluster shape among patch arrangements. Although many studies have been conducted on the self-assembled structures of nanocubes in bulk, little is known about their self-assembly in nanoscale spaces or structural changes under shear. In this study, we investigated mixtures of one- and two-patch amphiphilic nanocubes confined in two flat parallel plates at rest and under shear using molecular dynamics simulations coupled with multiparticle collision dynamics. We considered two different patch arrangements for the two-patch particles and two different slit widths H to determine the degree of confinement in constant volume fractions in the liquid regime of the phase diagram. We revealed two unique cluster morphologies that have not been previously observed under bulk conditions. At rest, the size of the rod-like aggregates increased with decreasing H, whereas that of the fractal-like aggregates remained constant. Under weak shear with strong confinement, the rod-like aggregates maintained a larger M than the fractal-like aggregates, which were more rigid and maintained a larger M than the rod-like aggregates under bulk conditions.

Damage behavior of high particle volume fraction composites with initial damage by finite element meso-modeling

Analyzing the deformation and damage processes of high particle volume fraction composites (HPVFCs) based on the meso-structure modeling plays an important role in their service lifetime and functional reliability. The reported mesoscopic structural models are usually ideal models without initial damage and apply to composites with the volume fraction of particles less than 80%. This work provides a new mesoscopic structure model called aggregate settlement-merging model to successfully simulate the mechanical behavior and damage mechanism of the polymer-bonded explosive (PBX) under quasi-static loads, and the macroscopic response is accurately predicted by the cohesive finite element method. This simulation commendably predicts the failure modes including interface debonding, binder filaments, and grain failure, which is consistent with the observation in micromechanical experiments. The multi-scale relationship between macroscopic mechanical properties and microscopic initial interface damage is quantitatively established in a form of an exponential function. This study shows a relationship between meso-structures and performance and provides strategy for designing next generation high-performance HPVFCs.

Atomic and mesoscopic structure of Dy-based surface alloys on noble metals

Surface alloys are a highly flexible class of low dimensional materials with the opportunity to tune and control the spin and charge carrier functionalities on the nanoscale. Here, we focus on the atomic and mesoscopic structural details of three distinct binary rare-earth-noble metals (NM) surface alloys by employing scanning tunneling microscopy and low energy electron diffraction. Using Dysprosium as the guest element on fcc(111) NM substrates, we identify the formation of non-commensurate surface alloy superstructures, which lead to homogeneous moiré patterns for DyCu2/Cu(111) and DyAu2/Au(111), while an inhomogeneous one is found for DyAg2/Ag(111). The local structure was analyzed for these samples and the observed differences are discussed in the light of the lattice mismatches of the alloy layer with respect to the underlying substrate. For the particularly intriguing case of a DyAg2 surface alloy, the surface alloy layer does not show a uniform long-range periodic structure, but consists of local hexagonal tiles separated by extended domain walls, which occur likely to relieve the in-plane strain within the DyAg2 surface alloy layer. Our findings clearly demonstrate that surface alloying is an intriguing tool to tailor the local atomic structure as well as the mesoscopic moiré structures of metallic heterostructures.

Numerical Study of Concrete Dynamic Splitting Based on 3D Realistic Aggregate Mesoscopic Model.

In mesoscopic scale, concrete is regarded as a heterogeneous three-phase material composed of mortar, aggregate and interfacial transition zone (ITZ). The effect of mesoscopic structure on the mechanical behaviors of concrete should be paid more attention. The fractal characteristics of aggregate were calculated, then the geometric models of aggregate were reconstructed by using fractal Brownian motion. Based on the random distribution of aggregates, the concrete mesoscopic structure model was established. And the numerical model was generated by using grid mapping technology. The dynamic compression experiments of concrete under Split Hopkinson Pressure Bar (SHPB) loading verify the reliability and validity of the mesoscopic structural model and the parameters of the constitutive model. Based on these, a numerical study of concrete under dynamic splitting is carried out. By changing the parameters of the constitutive model, the effects of tensile strengths of aggregate, mortar and ITZ on the dynamic tensile strength of concrete are discussed. The results show that the dynamic failure of specimen usually occurs at the interfacial transition zone, then extends to the mortar, and the aggregates rarely fail. However, the increase of strain rate intensifies this process. When the strain rate increases from 72.93 s−1 to 186.51 s−1, a large number of aggregate elements are deleted due to reaching the failure threshold. The variation of tensile strengths of each phase component have the same effect on the dynamic tensile strength and energy of concrete. The dynamic tensile strength and energy of concrete are most affected by the tensile strength of mortar, following by the ITZ, but the tensile strength of aggregate has almost no effect.

Particle size distribution of aggregates effects on mesoscopic structural evolution of cemented waste rock backfill.

The most economical, environmental, and friendly method for recycling gangue is filling mining with cemented waste rock backfill (CWRB), which solves the environmental problems caused by gangue discharge and reduces the mining damages. Evaluating the mesoscopic structure of CWRB is of great significance for maximizing the utilization of gangue recycling and improving the economic benefits of filling mining. This paper constructed the particle flow model of cemented waste rock backfill (CWRB) considering particle size distribution (PSD) of aggregates and hydration of cementing material to investigate the effect of the PSD of aggregates on its mesoscopic structural evolution. The strain energy, crack, force chain, and particle fragment of CWRB during the whole loading were discussed. The binary processing and calculation on the crack image were performed to analyze the fractal dimension of crack distribution by compiling program. The influencing mechanism of the PSD of aggregates on the strength of CWRB is revealed from the mesoscopic levels of crack evolution, force chain structure, and particle fragment. The results show that the strain energy increases firstly and then decreases with the PSD fractal dimension, while the crack number decreases firstly and then increases with that. The cracks with less number and more uniform distribution present the smaller fractal dimension, CWRB with a low fractal dimension of crack distribution has higher strength, the fractal dimension of crack distribution exhibits a correlation with the PSD of aggregates. CWRBs with the PSD fractal dimensions of 2.4-2.6 have the largest strain energy and the smallest crack number, performing the superior structural evolution during loading. This study presents the huge potential of optimizing PSD in CWRB application from a new perspective, it is of great significance for strengthening the internal structure of CWRB and reducing engineering cost.

Mesoscopic study of the effective thermal conductivity of dry and moist soil

The ground heat exchanger (GHE) is a key component in the ground coupled heat pump (GCHP) system. The design of GHE significantly depends on the effective thermal conductivity of the ambient soil, which is derived from thermal response tests in the field. However, the test results normally indicate large differences in value, even at the same location. Such variations might be caused by the mesoscopic structure of soil. In order to clarify the impacts of the mesoscopic structural parameters on the effective thermal conductivity of soil-like materials, this paper presents and analyzes the findings from thermal probe tests and a modified random fractal model. It is found that porosity and degree of saturation are still the dominant factors, while the impacts of fractal dimension and size ratio are minor. The randomness of the porous structure in the soil-like materials does have considerable impact on the effective thermal conductivity.

XRCT characterization of mesoscopic structure in poured and tapped cohesive powders and prediction by DEM

Predicting structural and flow characteristics of cohesive powder systems is a technical challenge in many powder technology applications. The discrete element method (DEM) is a promising approach to model the mesoscale structure and flow behavior of cohesive granular systems. Advances in experimental techniques such as X-ray computed tomography (XRCT) have enabled characterization of powder systems with sub-micron resolution. Combined with standard bulk powder characterization methods, this technique allows for mesoscopic structural characteristics of cohesive powder systems to be compared directly with DEM simulation. In this study, DEM and XRCT are used to characterize the structural characteristics of two moderately cohesive roughly spherical “model” powders – glass and stearic acid – under loosely poured and tapped consolidated conditions. The DEM simulations are based on a constitutive elasto-plastic model that makes use of the JKR theory to account for cohesive interparticle surface forces. Powder density and mesoscopic void structure obtained from DEM simulation are compared to characterization results obtained from XRCT image analysis. The results show that DEM can accurately predict the extent of tapping induced densification and mesostructure characteristics in cohesive powders, with superior agreement for stearic acid.

Random aggregate model for mesoscopic structures and mechanical analysis of fully-graded concrete

The realization of approximate practical aggregate models is a crucial link to the mesoscopic mechanical analysis for fully-graded concrete. This study systematically presents the establishment of mesostructure models of three-phase fully-graded concrete composed of anisotropic high-contented aggregates, cement paste, and interfaces bonding between them. A convex extension method is demonstrated in details to construct convex polygons and polyhedrons represented as crushed coarse aggregates in two- and three-dimensional spaces. The Walraven formula combined with the definition of an equivalent diameter is adopted to ensure the equivalence between the gradation and content of circle and spherical aggregates and that of the actual anisotropic geometric aggregates. We also propose a so-called “occupation and removal method (ORM)” to improve the efficiency of generating mesostructure models with a high aggregate content. Combining the ORM, the inter-particle overlap detection, and the random sequential packing scheme, random aggregate models (RAMs) of polygonal and polyhedral particles are realized to characterize the mesostructure models of concrete. Moreover, by the present RAMs applied, we implement a mesoscopic finite element simulation to investigate the flexural failure process of fully-graded concrete. The numerical results for dynamical flexural strength and dynamic increasing factors show a good in agreement with the experimental data reported in the literature. The proposed RAMs can simulate and explain qualitatively the effects of the mesoscopic structural properties on the macroscopic dynamic characteristics of fully-graded concrete.

The influence of society's mesoscopic structure on the rate of epidemic spreading.

In this study, we investigate the role of the mesoscopic structural properties of a scale-free social network on the contagion spreading. We focus on both the exponent of power-law community size distribution function (β) and the mixing parameter (μ). Findings show that increasing β reduces the rate of epidemic spreading. On the other hand, increasing μ increases the rate of epidemic spreading. Two innovating parameters, Temperature and cos θ, are introduced here to analyze these effects.

A Constitutive Model of Metal Rubber for Hysteresis Characteristics Based on a Meso-Mechanical Method

Based on the typical mesoscopic structural characteristics of metal rubber, the mesoscopic physical mechanism was revealed through analyzing the spatial configuration and the contact mode of wire helixes for its compression deformation process. The mesoscopic structure unit based on the curved beam of variable length and the model of the contact interaction between curved beams were proposed. Combined with the distribution law of frictional contact points, a new constitutive model of metal rubber for hysteresis characteristics was established, which included its basic structure parameters such as the diameter and the elastic modulus of wire, the diameter of wire helix, and the relative density of metal rubber. The model could describe the restoring force curves of metal rubber in initial loading, unloading and repeated loading phases, and theoretically explained its elastic characteristics and dry friction damping characteristics of multipoint contact. To verify the theory model, a comparison was made between the theoretical results and the experimental results for metal rubber specimens with different relative densities. The results show that theoretic calculations are consistent with the experimental data, which will provides a theoretical basis for predicting the stiffness and damping of metal rubber and guiding its design.

Microscopic and mesoscopic structural features of an activated carbon sample, prepared from sorghum via activation by phosphoric acid

An acidic chemical activation procedure has been used for preparing activated carbon with a surface area exceeding 1000m2/g from sorghum. In order to reveal structural features, synchrotron X-ray diffraction measurements have been performed. The structure of the material has been characterized by the total scattering structure factor and the radial distribution function describing short-range arrangement of atoms at distances of the order of a few atomic diameters as well as correlations at a longer scale, of the order of nanometers. The atomic arrangement has been found to be consistent with that of amorphous graphite-like carbon. As far as the mesoscopic structure is concerned, the presence of a characteristic distance is suggested on the basis of the clear nanometer scale oscillations of the radial distribution function, which distance may be assigned as the mesopore size in the material. It is suggested that the approach devized here may later be applied routinely for other activated carbon samples, too, for characterizing atomic and nanoscale order simultaneously.

Small angle neutron scattering as fingerprinting of ancient potteries from Sicily (Southern Italy)

Abstract The mesoscopic structure of numerous samples of Archaic Ionian Cups found in different archaeological sites of Sicily was non-invasively investigated by means of small angle neutron scattering (SANS). Similar values of the mesoscopic structural parameters, radius of gyration and fractal dimension of the interfaces of the investigated potteries, testify a similar manufacturing technique. Many samples showed mesoscopic planar anisotropy probably due to some external mechanical stress during forging. Similar porosity was found for all the studied ceramics and appeared in general agreement with the Hg porosimetry values. Finally, the firing temperatures were estimated by comparison with raw materials fired under controlled conditions.

Transferable Coarse-Grained Models for Ionic Liquids

The effective force coarse-graining (EF-CG) method was applied to the imidazolium-based nitrate ionic liquids with various alkyl side-chain lengths. The nonbonded EF-CG forces for the ionic liquid with a short side chain were extended to generate the nonbonded forces for the ionic liquids with longer side chains. The EF-CG force fields for the ionic liquids exhibit very good transferability between different systems at various temperatures and are suitable for investigating the mesoscopic structural properties of this class of ionic liquids. The good additivity and ease of manipulation of the EF-CG force fields can allow for an inverse design methodology of ionic liquids at the coarse-grained level. With the EF-CG force field, the molecular dynamics (MD) simulation at a very large scale has been performed to check the significance of finite size effects on the structural properties. From these MD simulation results, it can be concluded that the finite size effect on the phenomenon of ionic liquid spatial heterogeneity (Wang, Y.; Voth, G. A. J. Am. Chem. Soc. 2005, 127, 12192) is small and that this phenomenon is indeed a nanostructural behavior which leads to the experimentally observed mesoscopic heterogeneous structure of ionic liquids.

Structure and tectonics of the Achankovil Shear Zone, southern India

An integrated approach to resolve the kinematics of the controversial Achankovil Shear Zone (AKSZ) has been attempted involving remote sensing data, shaded relief topo-maps, ground details of lithology and mesoscopic structures. An excellent correlation of structural trends exists on all scales of observation. The AKSZ is distinctly defined by NW–SE trending foliation fabrics with steep dips to southwest. The adjacent Madurai block and Trivandrum block show contrasting lithological and structural characteristics as shown in structural cross-sections. The mesoscopic structural studies reveal the presence of sub-horizontal stretching lineations, asymmetric structures like S–C′ fabrics, porphyroclasts, ‘S’ shaped folds and shear bands confirming the strike-slip component of shear along AKSZ. The deformation undergone by the AKSZ could be described in terms of an initial dextral deformation — D 1, reactivated and superimposed by sinistral kinematics — D 2, which is also supported by megascopic structural interpretation of remote sensing data. The megascopic structural interpretation of AKSZ displays en-echelon pattern of lineaments with right overstepping arrangement, which can be interpreted as an evidence of the latest sinistral transpressional deformation.

Probing structural relaxation in complex fluids by critical fluctuations

Complex fluids, such as polymer solutions and blends, colloids and gels, are of growing interest in fundamental and applied soft-condensed-matter science. A common feature of all such systems is the presence of a mesoscopic structural length scale intermediate between atomic and macroscopic scales. This mesoscopic structure of complex fluids is often fragile and sensitive to external perturbations. Complex fluids are frequently viscoelastic (showing a combination of viscous and elastic behaviour) with their dynamic response depending on the time and length scales. Recently, non-invasive methods to infer the rheological response of complex fluids have gained popularity through the technique of microrheology, where the diffusion of probe spheres in a viscoelastic fluid is monitored with the aid of light scattering or microscopy. Here we propose an alternative to traditional microrheology that does not require doping of probe particles in the fluid (which can sometimes drastically alter the molecular environment). Instead, our proposed method makes use of the phenomenon of "avoided crossing" between modes associated with the structural relaxation and critical fluctuations that are spontaneously generated in the system.

Small-Angle Scattering Study of Mesoscopic Structures in Charged Gel and Their Evolution on Dehydration

Mesoscopic structures, with length scales ∼102 Å, were investigated by small-angle X-ray and neutron scattering (SAXS and SANS) in several N-isopropylacrylamide−sodium acrylate (NIPA−SA) copolymeric hydrogels with varying [NIPA]/[SA] ratios and water contents. The SAXS experiments reveal that, depending upon the [NIPA]/[SA] ratio, the dehydrated NIPA−SA gel shows two mesoscopic structures: one consists of randomly distributed SA-rich islands in NIPA matrix, while the other is a microphase-separated structure, composed of NIPA-rich and SA-rich domains. In addition, the SANS experiments reveal the mesoscopic structural features during the dehydration process. As the concentration of the network polymers increases, NIPA-rich and water-rich domains segregate in the gel. Then, an electrostatic interaction between the segregated domains induces a microphase-separated structure in the limit of the dehydrated NIPA−SA gel.

Grain‐scale melt distribution in two contact aureole rocks: implications for controls on melt localization and deformation

AbstractThe grain‐scale spatial arrangement of melt in layer‐parallel leucosomes in two anatectic rocks from two different contact aureoles located in central Maine, USA, is documented and used to constrain the controls on grain‐scale melt localization. The spatial distribution of grain‐scale melt is inferred from microstructural criteria for recognition of mineral pseudomorphs after melt and mineral grains of the solid matrix that hosted the melt. In both rocks, feldspar mimics the grain‐scale distribution of melt, and quartz is the major constituent of the solid matrix. The feldspar pockets consist of individual feldspar grains or aggregates of feldspar grains that show cuspate outlines. They have low average width/length ratios (0.54 and 0.55, respectively), and are interstitial between more rounded and equant (width/length ratios 0.65 for both samples) quartz grains. In two dimensions, the feldspar pockets extend over distances equivalent to multiple quartz grain diameters, possibly forming a connected three‐dimensional intergranular network. Both samples show similar mesoscopic structural elements and in both samples the feldspar pockets have a shape‐preferred orientation. In one sample, feldspar inferred to replace melt is aligned subparallel to the shape‐preferred orientation of quartz, indicating that pre‐ or syn‐anatectic strain controlled the grain‐scale distribution of melt. In the other sample, the preferred orientation of feldspar inferred to replace melt is different from the orientations of all other mesoscopic or microscopic structures in the rock, indicating that differential stress controlled grain‐scale melt localization. This is probably facilitated by conditions of higher differential stress, which may have promoted microfracturing. Grain‐scale melt distribution and inferred melt localization controls give insight into possible grain‐scale deformation mechanisms in melt‐bearing rocks. Application of these results to the interpretation of deep crustal anatectic rocks suggests that grain‐scale melt distribution should be controlled primarily by pre‐ or syn‐anatectic deformation. Feedback relations between melt localization and deformation are to be expected, with important implications for deformation and tectonic evolution of melt‐bearing rocks.

Effect of salt and heating on a mesoscopic structure composed of ovalbumin globules in aqueous solution.

Mesoscopic structural changes of an ovalbumin solution by heating and adding NaCl have been investigated with a small-angle neutron scattering method. In the natural solution, a broad peak at q = 0.057 A(-1), which disappeared by adding NaCl, indicates the existence of an electrostatic long-range interaction between the ovalbumin globules. Along with the broad peak, a prominent intensity increase in a very small q region was observed on heating except for the initial and final stages, indicating the coexistence of a disordered structure of the denatured ovalbumin and a regular-interspacing structure of the natural ovalbumin globules. Though the macroscopic feature in the final stage, whether the solution forms a gel (10 wt %) or not (5 wt %), strongly depended on the concentration of the ovalbumin, the scattering profiles showed a common characteristic feature: the appearance of a new peak around q = 0.023 A(-1), which indicates the emergence of another regular structure.

Reversible mesoscopic structural transformations in vacuum evaporated amorphous Ge 30Se 70 film studied by Raman scattering

The Raman scattering properties at high temperatures in non-crystalline Ge 30Se 70 prepared by melt-quenching (MQ) and vacuum evaporation (VE) are investigated from the view point of the structural difference between them. In the VE film with increasing temperatures, the mesoscopic structure containing edge-sharing GeSe 4/2 tetrahedra changes to the structure only containing the corner-sharing tetrahedra even at temperatures well below the glass transition temperature. In the MQ-glass, the structure containing edge-sharing tetrahedra is more stable than that in the VE-film.

Mesoscopic structure formation in colloidal aggregation and gelation

Recent small-angle light scattering experiments have revealed that diffusively aggregating spherical particles develop structure on a mesoscopic length scale (∼ tens of particles). The mesoscopic structural length scale persists even when the aggregation proceeds to the formation of a space-spanning network (a gel). We review the technique of small-angle light scattering, survey the experimental evidence for mesoscopic structure formation, discuss attempts at understanding these experimental observations by computer simulation of irreversible and reversible diffusion-limited cluster aggregation (DLCA), and propose a coherent picture for the understanding of non-equilibrium aggregation in the context of phase transitions.