Pores Of Different Shapes Research Articles

Numerical investigation on the discharge formation in micrometer pores in structured catalyst irradiated by a helium atmospheric pressure plasma jet

Abstract Non-thermal plasma catalysis is a promising way to achieve high efficiency in applications such as energy conversion and chemical engineering. Although synergistic effects between plasmas and catalysts have been preliminarily considered as an underlying mechanism of this type of catalysis, the formation of discharges in small-size catalyst pores, which is possibly a crucial factor in plasma-activated catalysis, is still not well understood. In this paper, investigations on the interactions between a helium atmospheric pressure plasma jet (APPJ) and catalysts with micrometer-sized pores of different shapes and sizes are conducted with a 2D fluid model. Simulation results show that the existence of pores makes a subtle difference to the APPJ by changing the equivalent capacitance, indicating the potential to achieve moderate and stable APPJ-catalyst interactions. Traces of air impurities in helium can promote discharges in catalyst pores, and thus allow discharges to form in smaller pores. In the case when the catalyst channel is too small for direct APPJ penetration, we propose a method by producing a prior discharge in a relatively large cavity to supply seed electrons to ignite discharges inside the channel. The effects of channel and cavity sizes are discussed from the perspectives of discharge behavior and plasma-surface interactions. This work will contribute to the preparation of structured catalysts to potentially achieve higher efficient plasma catalysis, and better understanding of the physical processes in plasma-surface interactions inside micrometer pores.

MICROMECHANICAL ESTIMATES FOR THE EFFECTIVE PERMEABILITY OF 2D POROUS MATERIALS WITH ARBITRARILY SHAPED PORES

The present work aims to determine the effective permeability of two-dimensional (2D) porous materials consisting of an isotropic permeable solid matrix in which arbitrarily shaped pores are embedded. The interfaces between the solid phase and pores are characterized by the Beavers-Joseph-Saffman conditions. To achieve the objective, by combining the complex variable method with the conformal mapping technique, we first solve the fundamental coupled Darcy-Stokes problem concerning the fluid flow in an infinite permeable solid containing a pore of arbitrary shape and undergoing a remote uniform pressure gradient. Next, with the help of this solution, each fluid-filled pore is replaced with an equivalent permeable inclusion whose permeability is determined. Finally, the dilute distribution, Mori-Tanaka, and differential schemes of micromechanics are applied to obtain estimates for the effective permeability of 2D composites with pores of different shapes. These estimates are compared with the relevant numerical results provided by the finite element method (FEM) and the boundary element method (BEM). In particular, the dependence of the effective permeability on the pore shapes is discussed.

Activated Carbon from Various Agricultural Wastes by H3PO4 Activation: Preparation and Characterization

AbstractActivated carbons (ACs) were prepared from various agricultural wastes, namely, lapsi seed stones, beleric seeds, peach stones, walnut shells, bamboo, and sugarcane bagasse by pyrolysis with chemical activation by phosphoric acid. As‐synthesized ACs were characterized by iodine number, methylene blue number, surface area, scanning electron microscopy, Fourier transform infrared spectroscopy, and X‐ray diffraction. The highest carbon yield was recorded for the AC derived from peach stones. The highest surface areas were achieved in ACs prepared from peach stones and walnut shells. All the ACs have an external surface covered with regular pores of different shapes and sizes, an amorphous nature, and similar oxygen‐containing surface functional groups.

Pore shapes effects on polymer translocation.

We translocated polymers through pores of different shapes and interaction patterns in three dimensions by Langevin molecular dynamics. There were four simple cylindrical pores of the same length but with different diameters. The results showed that even though decreasing the pore diameter would always decrease the translocation velocity, it was strongly dependent on the shape of the increased pore diameter. Although increasing the pore diameter made the translocation faster in simple cylindrical pores, it was complicated in different pore shapes, e.g. increasing the diameter in the middle decreased the translocation velocity. Investigating polymer shapes through the translocation process and comparing the shapes by the cumulative waiting time for different pore structures reveals the non-equilibrium properties of translocation. Moreover, polymer shape parameters such as gyration radius, polymer center of mass, and average aspect ratio help us to distinguish different pore shapes and/or different polymers.

Modeling the Effect of Pressure on the Moduli Dispersion in Fluid‐Saturated Rocks

AbstractLaboratory experiments of ultrasonic velocities of fluid‐saturated rocks are often much higher than the predictions of the Gassmann theory. This difference is usually attributed to the velocity dispersion caused by fluid pressure relaxation between pores of different shapes and orientation. This paper proposes a simple model to characterize pressure and frequency effects on the elastic moduli of fluid‐saturated rocks in a broad frequency range. The proposed model incorporates micromechanics of wave‐induced fluid pressure relaxation at grain contacts (between crack‐like contacts and stiff pores) into pressure dependency of elastic moduli. Previously, the pressure dependency of the velocities or elastic moduli was ascribed to the progressive closure of cracks with the increasing effective pressure and expressed as an integral of crack compliance over the range of aspect ratios. For isolated cracks, this compliance is a function of crack geometry only. For cracks hydraulically connected to stiff pores, this crack compliance can be replaced by a frequency‐dependent solution of the micromechanical problem of fluid pressure relaxation between a single crack and surrounding pores. The resulting equation expresses the bulk and shear moduli of the fluid‐saturated rock as functions of both pressure and frequency. Furthermore, if pressure‐dependent moduli of both dry and fluid‐saturated moduli are known, the aspect ratio distribution can be obtained from the pressure dependency of the dry moduli, and then the saturated moduli can be computed with no adjustable parameters. The model predictions show reasonable agreement with laboratory data measured using ultrasonic and forced oscillation techniques.

Stratifying Polymer Solutions in Microsized Pores: Phase Transitions Induced by Deformation of a Porous Material

Based on the thermodynamic approach, the influence of pore morphology on the phase equilibria in stratifying liquid solutions entrapped in a porous matrix is analyzed. As illustrated by an example of a solution of oligomeric fractions of polybutadiene and polystyrene, the matrix deformation determining the geometric characteristics of pores makes it possible to control the mutual solubilities of components and the volumes and thermodynamic stability of phases coexisting at equilibrium. The change in the geometric characteristics of pores in the general case is simulated by introducing a parameter that corresponds to the degree of deviation of the interface shape from the spherical one. Moreover, the behavior of the component solubilities as a function of pore volume is different in pores of different shapes, which is explained by various mechanisms of reducing the free energy of the system. With significant matrix deformations, the phase separation in microsized pores is suppressed and any solution compositions up to equimolar ones become.

Расслаивающиеся полимерные растворы в микроразмерных порах: фазовые переходы, индуцированные деформацией пористого материала

The influence of pore morphology on phase equilibria in stratifying polymer solutions within porous matrices has been simulated by means of equilibrium chemical thermodynamics. Using the example of liquid stratifying polybutadiene-polystyrene oligomer mixtures, we have shown that deformation of a matrix which influences on pore shapes, determines mutual solubilities of components within a pore, equilibrium volume ratios of co-existing phases and their thermodynamic stability. The change in the geometric characteristics of a pore is simulated in general by introducing a parameter corresponding to the degree of deviation of the shape of the pore walls from the spherical one. The dependences of mutual solubilities of components on pore volume within pores of different shapes are different which has been explained by the existence of several mechanisms of lowering the free energy of the system. Severe deformations of a matrix lead to suppressing the stratification in microsized pores while every composition up to the equimolar one becomes thermodynamically stable.

Finite Element Modeling of Porous Microstructures With Random Holes of Different-Shapes and -Sizes to Predict Their Effective Elastic Behavior

Porous materials are promising media for designing medical instruments, drug carriers, and bioimplants because of their excellent biocompatibility, ease of design, and large variation of elastic moduli. In this study, a computational strategy using the finite element method is developed to model the porous microstructures and to predict the relevant elastic moduli considering the actual characteristics of the micropores and their distributions. First, an element-based approach is presented to generate pores of different shapes and sizes according to the experimental observations. Then, a computational scheme to evaluate the effective moduli of macroscopically isotropic porous materials based on their micro-mechanics is introduced. Next, the accuracy of our approach is verified with the analytical solutions of the extreme bounds of the elastic isotropic moduli of a simplified model and with the experimental data available in the literature. Finally, the influence of the shape of pores and their distribution modes are assessed.

Pore deformation and grain boundary migration during sintering in porous materials: a phase-field approach

Grain growth in porous ceramics is a complex process due to the variety of interactions between pores and grains. In this study, the pore deformation and grain boundary migration during porous ceramic sintering are simulated by the phase-field method, and the variety of diffusions during sintering is considered. Pores of different shapes and sizes are induced into the simulations to investigate the grain boundary migration and pore deformations during grain growth. Simulation results indicate that the porous microstructure is determined by the contacting mode between pore surface and grain boundaries, which is in good agreement with experimental observations. The efficiency of the grain boundary migration is analyzed via calculating the forces applied on the interfaces between grains and pores, and the mechanism of the pore deformation during grain boundary migration is discussed. Therefore, controlling the grain–pore microstructure by adjusting the synthesis process is essential to reach the desired mechanical and physical properties of sintered materials.

Frequency dependence of anisotropy in fluid saturated rocks – Part I: aligned cracks case

ABSTRACTA major cause of attenuation in fluid‐saturated media is the local fluid flow (or squirt flow) induced by a passing wave between pores of different shapes and sizes. Several squirt flow models have been derived for isotropic media. For anisotropic media however, most of the existing squirt flow models only provide the low‐ and high‐frequency limits of the saturated elastic properties. We develop a new squirt flow model to account for the frequency dependence of elastic properties and thus gain some insight into velocity dispersion and attenuation in anisotropic media. In this paper, we focus on media containing aligned compliant pores embedded in an isotropic background matrix. The low‐ and high‐frequency limits of the predicted fluid‐saturated elastic properties are respectively consistent with Gassmann theory and Mukerji–Mavko squirt flow model. Results are also expressed in terms of Thomsen anisotropy parameters. It turns out that the P‐wave anisotropy parameter ε tends to zero in the high‐frequency limit, whereas the δ parameter remains the only indicator of P‐S⊥ anisotropy. The S‐wave anisotropy parameter γ is not affected by the presence of fluid and remains the same for all frequency ranges. A new definition for attenuation anisotropy parameters is also proposed to quantify the attenuation anisotropy. In the most important case of liquid saturation, analytical expressions are derived for elastic properties, velocity anisotropy parameters, quality factors, and attenuation anisotropy parameters. A companion paper considers the case of cracks with an ellipsoidal distribution of orientations resulting from the application of anisotropic stress.

Frequency dependence of anisotropy in fluid saturated rocks – Part II: Stress‐induced anisotropy case

ABSTRACTA major cause of attenuation in fluid‐saturated media is the local fluid flow (or squirt flow) induced by a passing wave between pores of different shapes and sizes. Several squirt flow models have been derived for isotropic media. For anisotropic media, however, most of the existing squirt flow models only provide the low‐ and high‐frequency limits of the saturated elastic properties. We develop a new squirt flow model to account for the frequency dependence of elastic properties and thus gain some insight into velocity dispersion and attenuation in anisotropic media. In a companion paper, we focused on media containing aligned compliant pores embedded in an isotropic background matrix. In this paper, we investigate the case for which anisotropy results from the presence of cracks with an ellipsoidal distribution of orientations due to the application of anisotropic stress. The low‐ and high‐frequency limits of the predicted fluid‐saturated elastic properties are respectively consistent with Gassmann theory and Mukerji–Mavko squirt flow model. In the most important case of liquid saturation, analytical expressions are derived for elastic properties and Thomsen anisotropy parameters. The main observations drawn from this model are as follows. Crack closure perpendicular to the applied stress leads to an increase in seismic velocities as a function of stress in the direction of applied stress and a decrease in squirt‐flow‐induced dispersion and attenuation in this direction. The anisotropy of squirt flow dispersion engenders a decrease in the degree of anisotropy with frequency. The stress‐induced anisotropy remains elliptical, even in saturated media, for all frequency ranges.

Numerical simulation of mesomechanical behavior of porous brittle materials

The influence of the types of porous structure on the features of deformation, damage accumulation and fracture of mesovolumes of brittle materials is studied. At the mesoscale, the pores of different shape are taken into account explicitly. The digital models were made using random values of coordinates and radii of spherical voids or solid spheres. For numerical modeling of the mechanical behavior up to failure, the evolutionary approach is applied with considering the nonlinear constitutive equations to describe damage accumulation and its influence on the degradation of the strength properties of the frame of porous ceramics. The calculated averaged stress-strain diagrams were shown to be sensitive not only to the value of porosity but also to the shape of pores. The simulation results are validated with experimental data for zirconia and alumina ceramics. Good qualitative and quantitative agreement of modeling results with experimental data suggests that taking into account of two-scale porosity in the form of explicit consideration of large pores at the mesoscale and implicit integrated consideration of tiny pores and cracks from the microscale in the form of accumulated damage is quite sufficient in the framework of the hierarchical modeling.

Determination of electrical conductivity of double-porosity formations by using generalized differential effective medium approximation

Abstract In this paper, we propose an approach for simulating the effective electrical conductivity of porous rocks. This approach is based on the Generalized Differential Effective Medium (GDEM) method that allows us to calculate the effective conductivity of a rock up to the percolation threshold. We consider a porous medium composed of a porous matrix with small scale pores (primary pores) and large-scale pores placed in the matrix (secondary pores). For double porosity formations we applied two-step homogenization scheme starting from the smallest scale of inclusions. At the first step, we apply a model composed by a conductive host and two types of inclusions placed in it: nonconductive grains and pores with a conductive fluid. The electrical conductivities of the host and saturating fluid are equal. At the second homogenization step we consider the conductive host with effective electrical conductivity determined at the first step and large-scale inclusions with conductivity corresponded to the pore saturating fluid. To take into account the percolation for the secondary pore system we introduce two types of inclusions. For porosities less than the critical porosity (secondary pores are disconnected), we consider spheroids that describe secondary pores of different shapes (vugs and cracks). For porosities more than the critical value, we additionally introduce infinite ellipsoidal cylinders that ensure the pore connectivity. Application of the proposed model allows us to take into account the void percolation for a porous matrix and the percolation threshold for a secondary pore system that we consider as an advantage of the proposed method with comparison of Archie law or classical micromechanical methods.

Selective laser sintered poly-ε-caprolactone scaffold hybridized with collagen hydrogel for cartilage tissue engineering

Selective laser sintering (SLS), an additive manufacturing (AM) technology, can be used to produce tissue engineering scaffolds with pre-designed macro and micro features based on computer-aided design models. An in-house SLS machine was built and 3D poly-ε-caprolactone (PCL) scaffolds were manufactured using a layer-by-layer design of scaffold struts with varying orientations (0°/45°/0°/45°, 0°/90°/0°/90°, 0°/45°/90°/135°), producing scaffolds with pores of different shapes and distribution. To better enhance the scaffold properties, chondrocytes were seeded in collagen gel and loaded in scaffolds for cartilage tissue engineering. Gel uptake and dynamic mechanical analysis demonstrated the better suitability of the 0°/90°/0°/90° scaffolds for reconstructive cartilage tissue engineering purposes. Chondrocytes were then seeded onto the 0°/90°/0°/90° scaffolds in collagen I hydrogel (PCL/COL1) and compared to medium-suspended cells in terms of their cartilage-like tissue engineering parameters. PCL/COL1 allowed better cell proliferation when compared to PCL or two-dimensional tissue culture polystyrene. Scanning electron microscopy and confocal microscopy observations demonstrated a similar trend for extracellular matrix production and cell survival. Glycosaminoglycan and collagen II quantification also demonstrated the superior matrix secretion properties of PCL/COL1 hybrid scaffolds. Collagen-gel-suspended chondrocytes loaded in SLS-manufactured PCL scaffolds may provide a means of producing tissue-engineered cartilage with customized shapes and designs via AM technology.

Interfacing polymeric scaffolds with primary pancreatic ductal adenocarcinoma cells to develop 3D cancer models.

We analyzed the interactions between human primary cells from pancreatic ductal adenocarcinoma (PDAC) and polymeric scaffolds to develop 3D cancer models useful for mimicking the biology of this tumor. Three scaffold types based on two biocompatible polymeric formulations, such as poly(vinyl alcohol)/gelatin (PVA/G) mixture and poly(ethylene oxide terephthalate)/poly(butylene terephthalate) (PEOT/PBT) copolymer, were obtained via different techniques, namely, emulsion and freeze-drying, compression molding followed by salt leaching, and electrospinning. In this way, primary PDAC cells interfaced with different pore topographies, such as sponge-like pores of different shape and size or nanofiber interspaces. The aim of this study was to investigate the influence played by the scaffold architecture over cancerous cell growth and function. In all scaffolds, primary PDAC cells showed good viability and synthesized tumor-specific metalloproteinases (MMPs) such as MMP-2, and MMP-9. However, only sponge-like pores, obtained via emulsion-based and salt leaching-based techniques allowed for an organized cellular aggregation very similar to the native PDAC morphological structure. Differently, these cell clusters were not observed on PEOT/PBT electrospun scaffolds. MMP-2 and MMP-9, as active enzymes, resulted to be increased in PVA/G and PEOT/PBT sponges, respectively. These findings suggested that spongy scaffolds supported the generation of pancreatic tumor models with enhanced aggressiveness. In conclusion, primary PDAC cells showed diverse behaviors while interacting with different scaffold types that can be potentially exploited to create stage-specific pancreatic cancer models likely to provide new knowledge on the modulation and drug susceptibility of MMPs.

Geometry as a Factor for Tissue Growth: Towards Shape Optimization of Tissue Engineering Scaffolds

Scaffolds for tissue engineering are usually designed to support cell viability with large adhesion surfaces and high permeability to nutrients and oxygen. Recent experiments support the idea that, in addition to surface roughness, elasticity and chemistry, the macroscopic geometry of the substrate also contributes to control the kinetics of tissue deposition. In this study, a previously proposed model for the behavior of osteoblasts on curved surfaces is used to predict the growth of bone matrix tissue in pores of different shapes. These predictions are compared to in vitro experiments with MC3T3-E1 pre-osteoblast cells cultivated in two-millimeter thick hydroxyapatite plates containing prismatic pores with square- or cross-shaped sections. The amount and shape of the tissue formed in the pores measured by phase contrast microscopy confirms the predictions of the model. In cross-shaped pores, the initial overall tissue deposition is twice as fast as in square-shaped pores. These results suggest that the optimization of pore shapes may improve the speed of ingrowth of bone tissue into porous scaffolds.

Description and reconstruction of the soil pore space using correlation functions

In this paper a method for the description and reconstruction of the soil pore space using correlation functions has been examined. The reconstruction procedure employed here is the best way of verification of the potential descriptor of the soil pore space. Thin sections representing eight major types of pore space in zonal loamy soils and parent materials of the Russian Plain with pores of different shapes and orientations have been chosen for this study. Comparison based on the morphological analysis of the original pore space images and their correlation function reconstructions obtained using simulated annealing technique indicates that this method of reconstruction adequately describes the isometric soil pore space with isometric dissected, isometric slightly dissected, and rounded pores. The two-point correlation functions calculated with the use of the orthogonal method proved to be different for the examined types of soil pore space; they reflect the soil porosity, specific surface, and pore structure correlations at different lengths. The results of this study allow us to conclude that the description of the soil pore space with the help of correlation functions is a promising approach, but requires more development. Further directions of the development of this method for describing the soil pore space and determining the soil physical processes are outlined.

Evaluation of Microstructural Properties of Coffee Beans by Synchrotron X‐Ray Microtomography: A Methodological Approach

Synchrotron radiation microtomography was used as a nondestructive imaging technique to investigate the microstructural properties of green and roasted coffee beans. After image acquisition, 2D images have been reconstructed and 3D images of the beans have been then obtained. Qualitative and quantitative analysis of the images allow to fully characterize the morphological and structural features of the coffee beans. Roasting causes meaningful changes in the microstructure of the coffee bean tissue with the development in the entire bean of a porous structure with pores of different shape and size depending on the zone of the bean and cracks occurring mainly in the more external regions and between parenchyma and mucilage. The highly contrasted X-ray images have been analyzed to determine the pore size and its distribution in different regions of the coffee beans by selecting Volume-of-Interest (VoI). The use of phase-contrast hard X-rays imaging techniques represents an interesting tool of investigation of the internal structure, morphology, as well as the quality of whole coffee beans. Moreover, the high potentiality of 3D X-ray imaging and the approach used in this study could be applied in understanding the effects of roasting process conditions on the evolution of microstructural properties of the bean that may affect the stability as well the grinding and brewing performances. Synchrotron radiation microtomography is an elegant nondestructive imaging technique to investigate the microstructural properties of porous cellular matrices like the green and roasted coffee beans. The quantitative analysis of the resulting 2D and 3D images allows a more comprehensive and objective characterization of the sample under investigation as a whole or of extracted Volumes-of-Interest in the bean. This imaging technique could have a major role in understanding the effects of roasting process conditions on the microstructural properties of the bean.

A method for the determination of accessible surface area, pore volume, pore size and its volume distribution for homogeneous pores of different shapes

We develop a novel method to determine the accessible pore volume, the accessible pore size and its distribution for pores having homogeneous surfaces but taking an arbitrary shape. The accessible pore volume is essentially the volume space that is accessible to the centre of an adsorbate molecule, while the accessible pore size is defined by the largest sphere that can be accommodated in the accessible space. The size of this sphere depends on the point in the accessible volume that we select. The accessible pore size is therefore, a local variable and this means that even a geometrically simple pore can possess many sizes. Each local accessible pore size is associated with a local accessible pore volume and the relationship between this pore volume and pore size is called the accessible pore size distribution. In this paper, we illustrate this methodology with a number of model pores ranging from simple to complex geometry and present the analytical accessible pore size distribution.

A simple model for squirt-flow dispersion and attenuation in fluid-saturated granular rocks

A major cause of seismic attenuation in fluid-saturated rocks is the flow of the pore fluid induced by the passing wave. At sonic and ultrasonic frequencies, attenuation appears to be dominated by the local (pore-scale) flow between pores of different shapes and orientations. A simple squirt flow model is developed in which all of the parameters can be independently measured or estimated from measurements. The pore space of the rock is assumed to consist of stiff porosity and compliant (or soft) pores present at grain contacts. The effect of isotropically distributed compliant pores is modeled by considering pressure relaxation in a disk-shaped gap between adjacent grains. This derivation gives the complex and frequency-dependent effective bulk and shear moduli of a rock, in which the compliant pores are liquid saturated and stiff pores are dry. The resulting squirt model is consistent with Gassmann’s and Mavko–Jizba equations at low and high frequencies, respectively. The magnitude of attenuation and dispersion given by the model is directly related to the variation of dry bulk modulus with pressure and is relatively independent of fluid properties.