Random Pore Structure Research Articles

Effects of Pore-Scale Geometry and Wettability on Two-Phase Relative Permeabilities within Elementary Cells

We study the relative role of the complex pore space geometry and wettability of the solid matrix on the quantification of relative permeabilities of elementary cells of porous media. These constitute a key element upon which upscaling frameworks are typically grounded. In our study we focus on state immiscible two-phase flow taking place at the scale of elementary cells. Pressure-driven two-phase flow following simultaneous co-current injection of water and oil is numerically solved for a suite of regular and stochastically generated two-dimensional explicit elementary cells with fixed porosity and sharing main topological/morphological features. We show that the relative permeabilities of the randomly generated elementary cells are significantly influenced by the formation of preferential percolation paths, called principal pathways, giving rise to a strongly nonuniform distribution of fluid fluxes. These pathways are a result of the spatially variable resistance that the random pore structures exert on the fluid. The overall effect on relative permeabilities of the diverse organization of principal pathways, as driven by a given random realization at the scale of the elementary cell, is significantly larger than that of the wettability of the host rock. In contrast to what can be observed for the random cells analyzed, the relative permeabilities of regular cells display a clear trend with contact angle at the investigated scale.

Lattice Boltzmann simulation of flow and heat transfer in random porous media constructed by simulated annealing algorithm

In this article, the lattice Boltzmann (LB) method for transport phenomena is combined with the simulated annealing (SA) algorithm for digitized porous-medium construction to study flow and heat transfer in random porous media. Importantly, in contrast to previous studies which simplify porous media as arrays of regularly shaped objects or effective pore networks, the LB+SA method in this article can model statistically meaningful random porous structures in irregular morphology, and simulate pore-scale transport processes inside them. Pore-scale isothermal flow and heat conduction in a set of constructed random porous media characterized by statistical descriptors were then simulated through use of the LB+SA method. The corresponding averages over the computational volumes and the related effective transport properties were also computed based on these pore scale numerical results. Good agreement between the numerical results and theoretical predictions or experimental data on the representative elementary volume scale was found. The numerical simulations in this article demonstrate combination of the LB method with the SA algorithm is a viable and powerful numerical strategy for simulating transport phenomena in random porous media in complex geometries.

Incomparable hardness and modulus of biomimetic porous polyurethane films prepared by directional melt crystallization of a solvent

Porous materials with surprisingly diverse structures have been utilized in nature for many functional purposes. However, the structures and applications of porous man-made polymer materials have been limited by the use of processing techniques involving foaming agents. Herein, we demonstrate for the first time the outstanding hardness and modulus properties of an elastomer that originate from the novel processing approach applied. Polyurethane films of 100-μm thickness with biomimetic ordered porous structures were prepared using directional melt crystallization of a solvent and exhibited hardness and modulus values that were 6.8 and 4.3 times higher than those of the random pore structure, respectively. These values surpass the theoretical prediction of the typical model for porous materials, which works reasonably well for random pores but not for directional pores. Both the ordered and random pore structures exhibited similar porosities and pore sizes, which decreased with increasing solution concentration. This unexpectedly significant improvement of the hardness and modulus could open up new application areas for porous polymeric materials using this relatively novel processing technique.

Ceramic Filters and their Application for Cadmium Removal from Pulp Industry Effluent

The purpose of this paper was to investigate the performance of ceramic filters made from a mixture of natural clay, rice bran, and iron powder in removing cadmium from pulp industry effluent. Some parameters were examined such as acidity, total dissolved solid (TDS), total suspended solid (TSS), electrical conductivity (EC), and cadmium concentration. Results showed that the composition percentage of the ceramic filter, which in this case amounted to 87.5% natural clay, 10% rice bran, and 2.5% iron powder, may decrease cadmium concentration in pulp industry effluent by up to 99.0%. Furthermore, the permeate flux decreased after 30 minutes of filtration time, and subsequently became constant at one hour of contact time. In addition, Scanning Electron Microscope (SEM) micrographs of the ceramic filter surfaces indicate that ceramic filters have a random pore structure and can be categorized as microfiltration filters.

Enhancement of Pool Boiling Heat Transfer in Water Using Sintered Copper Microporous Coatings

Abstract Pool boiling heat transfer of water saturated at atmospheric pressure was investigated experimentally on Cu surfaces with high-temperature, thermally-conductive, microporous coatings (HTCMC). The coatings were created by sintering Cu powders on Cu surfaces in a nitrogen gas environment. A parametric study of the effects of particle size and coating thickness was conducted using three average particle sizes (APSs) of 10 μm, 25 μm, and 67 μm and various coating thicknesses. It was found that nucleate boiling heat transfer (NBHT) and critical heat flux (CHF) were enhanced significantly for sintered microporous coatings. This is believed to have resulted from the random porous structures that appear to include reentrant type cavities. The maximum NBHT coefficient was measured to be approximately 400 kW/m2k with APS 67 μm and 296 μm coating thicknesses. This value is approximately eight times higher than that of a plain Cu surface. The maximum CHF observed was 2.1 MW/m2 at APS 67 μm and 428 μm coating thicknesses, which is approximately double the CHF of a plain Cu surface. The enhancement of NBHT and CHF appeared to increase as the particle size increased in the tested range. However, two larger particle sizes (25 μm and 67 μm) showed a similar level of enhancement.

Broadband and wide-angle antireflective characteristics of nanoporous anodic alumina films for silicon-based optoelectronic applications

Nanoporous anodic alumina (NPAA) films are fabricated on silicon (Si) surfaces by the anodization of aluminum (Al). Effect of the thickness of initial Al films on the antireflective characteristics of NPAA films consisting of random pore structures is investigated in the wavelength range of 300–1,100 nm at incident light angles (θ inc) of 3°–70°, together with theoretical analysis using a rigorous coupled-wave analysis method. The reflectance strongly depends on the thickness and porosity of NPAA films, resulting from anodization parameters such as applied voltage, anodization time, and pore widening time. For the obtained NPAA film at the initial Al thickness of 100 nm and the applied voltage of 30 V, the surface reflectance is reduced over a wide wavelength region of 300–1,100 nm at normal incidence of ~3°, exhibiting the relatively lower average reflectance (R avg) of ~19 % (i.e., R avg ~ 39 % for the bare Si substrate). The lower angle-dependent reflective properties are observed compared to the bare Si substrate at θ inc = 20°–70° for unpolarized light. The calculated reflectance results of NPAA films show similar trends to the measured data.

THE RANDOM POROUS STRUCTURE AND MECHANICAL RESPONSE OF LIGHTWEIGHT ALUMINUM FOAMS

ABSTRACTLightweight porous foams have been of particular interest in recent years, since they have a very unique set of properties which can be significantly different from their solid parent materials. These properties arise from their random porous structure which is generated through specialized processing techniques. Their unique structure gives these materials interesting properties which allow them to be used in diverse applications. In particular, highly porous Al foams have been used in aircraft components and sound insulation; however due to the difficulty in processing and the random nature of the foams, they are not well understood and thus have not yet been utilized to their full potential. The objective of this study was to integrate experiments and simulations to determine whether a relationship exists between the relative density (porous density/bulk density) and the mechanical properties of open-cell Al foams. Compression experiments were performed using an Instron Universal Testing Machine (IUTM) on ERG Duocel open-cell Al foams with 5.8% relative density, with compressive loads ranging from 0-6 MPa. Foam models were generated using a combination of an open source code, Voro++, and MATLAB. A Finite Element Method (FEM)-based software, COMSOL Multiphysics 4.3, was used to simulate the mechanical behavior of Al foam structures under compressive loads ranging from 0-2 MPa. From these simulated structures, the maximum von Mises stress, volumetric strain, and other properties were calculated. These simulation results were compared against data from compression experiments. CES EduPack software, a materials design program, was also used to estimate the mechanical properties of open-cell foams for values not available experimentally, and for comparison purposes. This program allowed for accurate prediction of the mechanical properties for a given percent density foam, and also provided a baseline for the Al foam samples tested via the IUTM method. Predicted results from CES EduPack indicate that a 5.8% relative density foam will have a Young’s Modulus of 0.02-0.92 GPa while its compressive strength will be 0.34-3.37 MPa. Overall results revealed a relationship between pores per inch and selected mechanical properties of Al foams. The methods developed in this study can be used to efficiently generate open-cell foam models, and to combine experiments and simulations to calculate structure-property relationships and predict yielding and failure, which may help in the pursuit of simulation-based design of metallic foams. This study can help to improve the current methods of characterizing foams and porous materials, and enhance knowledge about theirproperties for novel applications.

Compartmentalized and Internally Structured Particles for Drug Delivery - A Review

Recent advances in the fabrication, characterization and application of micro- and nano-particles that possess a non-uniform internal structure are reviewed. The particle structures include core-shell particles, particles with multiple cores or a multi-layered structure, porous particles with both regular and random pore structure, as well as complex composite particles possessing several of the above features. Particles based on "hard" inorganic materials such as silica, "soft" organic materials such as polymers and their composites are considered. The fabrication approaches include bottom-up self-assembly techniques, templating methods, microfluidics, and various emulsion-based routes to structured micro- and nano-particle formation, combined with both physical (e.g. Pickering emulsions) and chemical (e.g. polymerization, precipitation) processes for the material deposition. The applications of the structured particles for the encapsulation and controlled delivery of active substances are then reviewed with emphasis on those systems where the complex particle structure can provide specific benefits such as in-situ formation of the active substance or precise control over the release profile. Likely future research directions and prospects are discussed.

Construction and in vivo evaluation of a dual layered collagenous scaffold with a radial pore structure for repair of the diaphragm

In each organ the extracellular matrix has a specific architecture and composition, adapted to the functional needs of that organ. As cells are known to respond to matrix organization, biomaterials that take into account the specific architecture of the tissues to be regenerated may have an advantage in regenerative medicine. In this study we focussed on the diaphragm, an organ essential for breathing, and consisting of radial oriented skeletal muscle fibres diverging from a central tendon plate. To mimic this structure dual layered collagenous scaffolds were constructed with a radial pore orientation, prepared by inward out freezing, and reinforced by a layer of compressed collagen. Similar scaffolds with a random round pore structure were taken as controls. Scaffolds were first mildly crosslinked by formaldehyde vapour fixation for initial stabilization (13% and 17% crosslinking for the radial and control scaffolds, respectively), and further crosslinked using aqueous 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide/N-hydroxysuccinimide (38% and 37% crosslinking, respectively). Scaffolds were implanted into a surgically created diaphragm defect in rats and explanted after 12weeks. Macroscopically, integration of the radial scaffolds with the surrounding diaphragm was better compared with the controls. Cells had infiltrated further into the centre of the scaffolds (P=0.029) and there was a tendency of blood vessels to migrate deeper into the radial scaffolds (P=0.057, compared with controls). Elongated cells (SMA-positive) were aligned with the radial structures. In conclusion, collagenous scaffolds with a stable radial pore structure can be constructed which facilitate cellular in-growth and alignment in vivo.

Nematic Liquid Crystals Embedded in Cubic Microlattices: Memory Effects and Bistable Pixels

AbstractThe confinement of liquid crystals in geometries with frustrating boundary conditions gives rise to nontrivial effects such as bistability and memory. It is shown that large memory effects arise when nematic liquid crystals are embedded in cubic micrometer‐sized scaffolds made by two‐photon polymerization. The electric field alignment of the liquid crystals inside the porous medium is maintained when the applied field is above a threshold (approximately 2 V per micrometer of cell thickness). The onset of the memory is an on/off type process for each individual pore of the scaffold, and the memory typically starts emerging in one region of the structure and then propagates. The global memory effects in porous structures with controlled geometry are enhanced with respect to the case of random porous structures. This work is a proof of the “memory from topology” principle, which was previously suggested by computer simulations. These new materials can pave the way to new types of bistable displays.

A geometrical model of random porous structures to adsorption calculations

This paper describes a new approach to identification of random porous structures (e.g. present in cheap natural adsorbents, active carbons). It comes from a clustering based description of adsorption process assuming an exponential distribution of adsorbate stack size (the LBET model), combined with the new consistent mathematical relationships between the pore geometry, adsorption isotherm parameters and physical properties of adsorptive. The newly derived formulae are discussed, and results of their application to analysis of an active carbon structure are shown.

One‐step Fabrication of Nanoporous Black Silicon Surfaces for Solar Cells using Modified Etching Solution

Currently, a conventional two‐step method has been used to generate black silicon (BS) surfaces on silicon substrates for solar cell manufacturing. However, the performances of the solar cell made with such surface generation method are poor, because of the high surface recombination caused by deep etching in the conventional surface generation method for BS. In this work, a modified wet chemical etching solution with additives was developed. A homogeneous BS layer with random porous structure was obtained from the modified solution in only one step at room temperature. The BS layer had low reflectivity and shallow etching depth. The additive in the etch solution performs the function of pH‐modulation. After 16‐min etching, the etching depth in the samples was approximately 200 nm, and the spectrum‐weighted‐reflectivity in the range from 300 nm to 1200 nm was below 5%. BS solar cells were fabricated in the production line. The decreased etching depth can improve the electrical performance of solar cells because of the decrease in surface recombination. An efficiency of 15.63% for the modified etching BS solar cells was achieved on a large area, p‐type single crystalline silicon substrate with a 624.32‐mV open circuit voltage and a 77.88% fill factor.

Synthesis of mesoporous spherical silica via spray pyrolysis: Pore size control and evaluation of performance in paclitaxel pre-purification

Spherical silica particles with various pore sizes were synthesized by spray pyrolysis using organic templates (CTAB, P123) and PS nanoparticles. The performance of the prepared silica was evaluated as an absorbent for the removal of impurities in the pre-purification processes of paclitaxel from plant-derived cell cultures. From the investigation of the synthetic temperature effect, it was found that the drying-zone temperature rather than the pyrolysis temperature greatly affects the change in the surface area of silica. At the optimal temperatures, the CTAB templates formed hexagonal-structured pores like MCM-41, whereas the silica synthesized using other templates (P123, CTAB/P123, PS nanoparticles) showed a random pore structure. By changing the template types, the pore size and the surface area of silica were controlled from 2.4nm to 24.4nm and from 131m2/g to 1670m2/g (CTAB), respectively. According to the evaluation of the adsorbent properties, the pore size rather than the surface area of silica was found to play a more important role in the pre-purification of paclitaxel. In terms of achieving the high purity and yield of paclitaxel, it was concluded that silica should have 8–15nm in pore size, with as high a surface area as possible. Finally, we confirmed that the mesoporous silica synthesized by spray pyrolysis has better performance than the commercialized sylopute.

Effect of Pore Morphology on the Adsorption of Methane/Hydrogen Mixtures on Carbon Micropores

The effect of pore structure and geometry on the selectivity of carbon materials for the adsorption of methane from a mixture of methane and hydrogen at 50 bar and 298 K was studied using a grand canonical Monte Carlo (GCMC) simulation technique. Hydrogen and methane were modeled by classical (nonquantum) Lennard-Jones intermolecular potentials. The pore morphologies studied included slits, nanotubes, a random porous structure, and a foamlike structure. The GCMC results show that both the pore size distribution and pore structure have significant effects on the selective adsorption of methane molecules from CH4/H2 mixtures. The selectivity depends on both energetic and packing effects. Pores that can accommodate either one or two layers of methane molecules give the optimum pore width for the separation of methane molecules. In the pressure range studied, of the four pore structures considered, nanotubes were found to have the highest selectivity (in the range of 15–51) toward the separation of methane molecules, followed by open structured foams (6–14), slits (6.6–16), and dense random structures (4.2–5.3). Prediction of the adsorption behavior of these mixtures by means of pure-component information seems to be inadequate for the very confined geometries.

Diffusion-bonded CNT carpets for fundamental CDI studies

ABSTRACTUncertainty about future energy and water supplies suggests a pressing need to develop efficient technologies for water desalination. Capacitive deionization (CDI), a method that captures ions in the electrical double layer (EDL) of an electrochemical capacitor, is a promising technology that can potentially fulfill those requirements. Similar to supercapacitors, ideal CDI electrodes should have a large electrolyte-accessible specific surface area available for ion adsorption with rapid charging/discharging characteristics. Unlike supercapacitors, CDI electrodes are required to operate in aqueous electrolytes with low ionic concentrations in a non-linear charging regime. To explore this practically and theoretically important regime, we developed robust, electrochemically-compatible carbon nanotube (CNT) carpet electrodes that posses a well-defined and uniform pore structure that is more readily analyzed in comparison to the random and multi-scale pore structure of typical carbon electrodes. The fabricated electrodes were characterized using cyclic voltammetry and potentiostatic charging in aqueous NaCl solutions (no = 20 - 90 mM) using a three electrode setup. Examination of the CV and potentiostatically-measured capacitances were consistent with EDL behavior dictated by the Stern layer. However, some deviations from the expected behavior were observed with increasing salt concentration during potentiostatic testing.

Effect of pore structure on the selectivity of carbon materials for the separation of CO2/H2 mixtures: new insights from molecular simulation

Molecular simulations were performed to study the effect of the nanoporous structure on the selectivity of carbon materials for the adsorption of carbon dioxide from mixtures of carbon dioxide and hydrogen at 298 K and for fluid compositions: xCO2/xH2 = 1/9 and 2/8. Both carbon dioxide and hydrogen were studied using classical Lennard-Jones intermolecular potentials. Typical pore geometries such as slit-shaped pores and nanotubes were considered, along with a hypothetical foam-like structure and a carbon model exhibiting a random porous structure with a wide pore size distribution. Simulation results show that selectivity for carbon dioxide is sensitive to pore structure and composition; the solid/fluid interactions play a decisive role in the selectivity and most of the effects can be explained by the independent analysis of the interactions of carbon dioxide with the pore walls. In the range of pressure and composition studied, nanotubes have the highest selectivity towards carbon dioxide (100–313), followed by slit (9–63), foam-like (29–35) and random porous carbon (8–30). Molecular simulations further indicate that predicting the adsorption behavior for a CO2/H2 mixture from pure component isotherms is inadequate due, to the competing effects of the molecules with the pore walls.

Modeling Microbial Dynamics in Heterogeneous Environments: Growth on Soil Carbon Sources

We have developed a new kinetic model to study how microbial dynamics are affected by the heterogeneity in the physical structure of the environment and by different strategies for hydrolysis of polymeric carbon. The hybrid model represented the dynamics of substrates and enzymes using a continuum representation and the dynamics of the cells were modeled individually. Individual-based biological model allowed us to explicitly simulate microbial diversity, and to model cell physiology as regulated via optimal allocation of cellular resources to enzyme synthesis, control of growth rate by protein synthesis capacity, and shifts to dormancy. This model was developed to study how microbial community functioning is influenced by local environmental conditions in heterogeneous media such as soil and by the functional attributes of individual microbes. Microbial community dynamics were simulated at two spatial scales: micro-pores that resemble 6-20-μm size portions of the soil physical structure and in 111-μm size soil aggregates with a random pore structure. Different strategies for acquisition of carbon from polymeric cellulose were investigated. Bacteria that express membrane-associated hydrolase had different growth and survival dynamics in soil pores than bacteria that release extracellular hydrolases. The kinetic differences suggested different functional niches for these two microbe types in cellulose utilization. Our model predicted an emergent behavior in which co-existence of membrane-associated hydrolase and extracellular hydrolases releasing organisms led to higher cellulose utilization efficiency and reduced stochasticity. Our analysis indicated that their co-existence mutually benefits these organisms, where basal cellulose degradation activity by membrane-associated hydrolase-expressing cells shortened the soluble hydrolase buildup time and, when enzyme buildup allowed for cellulose degradation to be fast enough to sustain exponential growth, all the organisms in the community shared the soluble carbon product and grew together. Although pore geometry affected the kinetics of cellulose degradation, the patterns observed for the bacterial community dynamics in the 6-20 μm-sized micro-pores were relevant to the dynamics in the more complex 111-μm-sized porous soil aggregates, implying that micro-scale studies can be useful approximations to aggregate scale studies when local effects on microbial dynamics are studied. As shown with examples in this study, various functional niches of the bacterial communities can be investigated using complex predictive mathematical models where the role of key environmental aspects such as the heterogeneous three-dimensional structure, functional niches of the community members, and environmental biochemical processes are directly connected to microbial metabolism and maintenance in an integrated model.

Micro X-ray CT를 이용한 글라스 비드의 3차원 간극 구조 정량화

무질서하고 불균질한 형상을 갖는 지반 재료 내 간극 구조는 하중에 의한 재료의 변형 및 간극 내 유체의 흐름 등 물리 역학적 거동에 중요한 영향 인자이다. 최근 들어 X-ray CT에 의한 비파괴 검사를 통해 지반 재료의 내부 구조를 마이크로미터 단위의 높은 해상도를 통해 평가하는 기법이 사용되고 있다. CT 이미지는 재료의 많은 정보를 포함하고 있음에도 그에 따른 이미지 해석 기법의 개발이 다소 미흡하여 2, 3차원 이미지의 정성적 관찰 및 간극비와 같은 거시적인 물성치 획득만이 이루어지고 있다. 본 연구에서는 연속적으로 획득된 글라스 비드의 2차원 CT 이미지에 기반하여 3차원 입자 및 간극 구조를 형성하고, 복잡한 간극구조를 간극셀과 간극채널로 정량적 분리를 실시하였다. 이를 위해 좌표 변환법, 이진화, 들로네 삼각망, 그리고 유클리디안 거리변환법과 같은 이미지 프로세싱 기법을 3차원 CT 이미지에 적용하였고 불균질한 글라스 비드의 간극구조에 대해 정량적으로 간극셀의 분포 및 간극간의 연결도 평가가 가능함을 확인하였다. The random and heterogeneous pore structure is a significant factor that dominates physical and mechanical behaviors of soils such as fluid flow and geomechanical responses driven by loading. The characterization method using non-destructive testing such as micro X-ray CT technique which has a high resolution with micrometer unit allows to observe internal structure of soils. However, the application has been limited to qualitatively observe 2D and 3D CT images and to obtain the void ratio at macro-scale although the CT images contain enormous information of materials of interests. In this study, we constructed the 3D particle and pore structures based on sequentially taken 2D images of glass beads and quantitatively defined complex pore structure with void cell and void channel. This approach was enabled by implementing image processing techniques that include coordinate transformation, binarization, Delaunay Triangulation, and Euclidean Distance Transform. It was confirmed that the suggested algorithm allows to quantitatively evaluate the distribution of void cells and their connectivity of heterogeneous pore structures for glass beads.

Evaluation of hydraulic conductivity in 3D random and heterogeneous particulate materials using network model

In this study we present a three-dimensional network model for the evaluation of hydraulic conductivity using new pore structure characterization algorithms. We define a porous medium from the sphere packing generated by discrete element method (DEM). The network model is based on connectivity of pore chambers linked along discretized pathway using a harmonic mean radius value computed from a series of radius-varying tubes that represent the irregularly shaped pore channel. Particle packing cases under investigation include regular packings, monodispersed, and polydispersed cases with varying porosity. Finite element analysis is implemented to obtain hydraulic conductivity values for regular packing cases for comparison. Laboratory experimentation results corroborate numerically obtained values with good agreement. Observations show the pore space should be tessellated once pore chambers are established in order to preserve the geometrical uniqueness of randomly shaped pore channel, avoid redundancy, and minimize pore channel overlap. Also, it is observed that limits must be defined for the total number of grains needed for representativeness and discretization density for the quantification of random heterogeneous pore structure.

Molecular simulations to study the effect of pore geometry and structure on the adsorption of CH4/H2 mixtures in carbon pores

AbstractWe used molecular simulations to study the effect of pore structure and geometry on the selectivity of carbon materials for the adsorption of methane from a mixture of methane and hydrogen at 50 bar and 298 K. We modeled hydrogen as a more realistic dumb-bell shaped fluid and methane as spherical Lennard-Jones sphere. The pore geometry studied includes the slit, nanotube, random carbon structure and carbon pores that mimic a foam structure. Our results confirm that both the pore size and pore structure have a significant effect on the selectivity for methane molecules. An array of nanotubes that have a closed structure has the highest selectivity for methane molecules, followed by carbon foam, slit and random pore structures. Energetic and packing effects at low and high pressures, respectively, influence the selectivity for methane. Reduction in entropy of hydrogen in smaller pores increases the selectivity for methane molecules.