Fluid Pores Research Articles

Research on influence laws of aggregate sizes on pore structures and mechanical characteristics of cement mortar

Aggregate plays an important role in cement-based materials. Evaluating the impact of aggregate gradation on pore structure and mechanical properties will help to optimize the mortar ratio and improve its durability. In this study, artificial sand with 5 different particle sizes was used as aggregate, and the synthetic gradation formed by their coupling was compared. The pore structure distribution of mortar was quantitatively characterized by nuclear magnetic resonance (NMR) technology. Surface-to-volume ratio (SVR) and ρd (aggregate packing density) were used to characterize the aggregate grading characteristics. The correlation between aggregate gradation characteristics and mortar parameters, such as porosity, pore structure fractal dimension, permeability coefficient, and mechanical strength, was analyzed, and a compressive modulus of mortar was established. The results indicate that the pore structure characteristics, K (permeability), and mechanical characteristics of mortar are all influenced by both SVR and ρd. The predominant pores in the mortar are micropores and movable fluid pores, and the addition of 0.1–0.5 mm aggregates reduces the formation of micropores, mesopores, macropores, and movable fluid pores in the mortar. As the number of pores increases, the complexity of the pore structure decreases, leading to higher homogeneity. Moreover, the K shows a strong power function relationship with the total porosity and the total porosity fractal. The compressive strength and Ec (compression modulus) of GM1–6 are significantly higher than those of GM7–9, correlating with the minimum aggregate size and the inherent strength of the aggregates. The regression results of the strength correlation model are significant, and the strength model established based on the porosity of micropores and Ec can accurately predict the compressive strength of mortar.

Multifractal characteristics on pore structure of Longmaxi shale using nuclear magnetic resonance (NMR)

The efficient exploration and extraction of shale gas is essential for China's energy structure as a replacement for natural gas. In this research, shale samples from the Longmaxi Formation in Changning, Sichuan Province were investigated for their pore structure. To achieve this, X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) experiments were performed on the samples. Fractal theory was employed to characterize the pore structure and fractal features of the shale reservoirs. The fractal characteristics of reservoir rocks were correlated with physical parameters and mineral components to identify the controlling factors for the multifractal dimension. The results indicate the following: ① Quartz predominates in the mineral composition of shale in the study area, while clay minerals are the least abundant. ② The primary pore types observed in the samples are bound fluid pores, primarily controlled by mesopores (2 nm ≤ r ≤ 50 nm). ③ The calculated multifractal parameters show that the heterogeneous pore structure exhibits multifractal characteristics. ④ The low probability multifractal parameters Dmin-D0 and αmax-α0 were found to be negatively correlated with reservoir porosity, permeability, and clay mineral content, while the high probability multifractal parameters D0-Dmax and α0-αmin were found to be positively correlated with them. The comprehensive fractal characterization of reservoir rocks demonstrates the applicability of multifractal dimensions in characterizing the heterogeneity of shale reservoir pore structures, which holds potential promise for coal methane exploration.

Pore structure characteristics of artificial sand aggregate mortar

With the progressive depletion of natural sand, the substitution of natural sand with artificial sand as a construction aggregate has received significant attention. To analyzed the pore structure characteristics of artificial sand aggregate mortars, nuclear magnetic resonance (NMR) technology was utilized to analyze the distribution characteristics and dynamic changes of five types of aggregate gradation mortars, and fractal theory was employed for characterization. The study examined on the correlation between aggregate gradation and pore structure, fractal dimension, and assessed the connectivity of the pore structure. The results showed that: water-saturated samples were mainly composed of micropores and movable fluid pores after 28 days of hydration, while the micropores and clay-bound fluid pores dominated after 365 days of hydration. The formation of the pore structure in multi-aggregate-graded mortar is influenced by both the surface-to-volume ratio and aggregate packing density. At the same hydration time, lower porosity in water-saturated mortar results in increased non-homogeneity of the pore structure and a reduced permeability coefficient. Well-graded aggregate mortar has stronger resistance to hydraulic erosion, and the stronger the erosion effect of water will be with more developed pore structure.

Study on spontaneous imbibition and displacement characteristics of mixed-wet tight sandstone reservoir based on high-precision balance and NMR method

Wettability is the key to understanding the complex seepage characteristics of tight sandstone reservoirs, which will affect the distribution of the residual hydrocarbon resources during the reservoir exploitation process. Complex wettability, strong heterogeneity and tiny pore space make it more challenging to study the imbibition characteristics of mixed-wet tight sandstone reservoirs. In this study, the spontaneous imbibition (SI) and displacement characteristics of mixed-wet tight sandstone reservoirs are studied by high precision balance and nuclear magnetic resonance (NMR) T2 experiment. 3 rock plungers (A1, B1, C1) were put into the CaCl2 solution for SI, while their parallel samples (A2, B2, C2) were carried out SI experiments in the experimental oil. Then all the plungers are respectively displaced by the imbibition liquid after SI experiments. The weight and NMR T2 spectra of the plungers in the above different conditions were measured. According to the experiment results, the pores of tight sandstone can be divided into 3 types with different wettability. The adsorption pores with T2 distribution of 0.01–2.5 ms are water-wet, the capillary bound fluid pores with T2 between 2.5 and 50 ms and the movable fluid pore with T2 greater than 50 ms are mainly oil-wet. The SI and displacement behaviors of different pores were studied, the results show the oil absorbed per cc rock is more than 4 times that of brine. The weight gain after SI in the first 24 h can indicate the total SI result. The study results can accurately characterize the wettability of different pores and contribute to a novel wettability evaluation method in mixed wettable tight sandstone.

Pore-fracture network alteration during forced and spontaneous imbibition processes in shale formation

Imbibition controls mass transfer in the complex pore-fracture network in shale, which may change the pore-fracture network and lead to a low efficient flowback of fracturing fluid. Thus, it is necessary to accurately characterize the pore-fracture network alteration during the imbibition process. In this study, forced and spontaneous imbibition tests were conducted under the confining pressure on selected shale core samples with induced fractures, collected from Longmaxi Formation, Sichuan Basin, China. The low-field nuclear magnetic resonance (NMR) spectrometer was employed to monitor the variation of pores and micro-fractures in the shale core samples during the dynamic imbibition process. In addition, optimal surface relaxivities, ranging from 0.019 to 0.033 μm/ms, were determined by comparing the NMR T2 distributions with the pore size distributions (PSDs) measured via high-pressure mercury intrusion (HPMI) tests. Then, the measured dynamic T2 spectra with three distinct peaks were converted into the corresponding PSDs to quantitatively analyze the number, size, and connectivity changes of small pores, large pores, and micro-fractures in shale. Results show that the total porosities of the four shale core samples are increased by 3.5%, 10.2%, 32.9%, and 36.3% after the imbibition tests. The forced imbibition leads to more remarkable improvements in the pore volume of large pores with radius between 0.2 and 3.6 μm. In contrast, the spontaneous imbibition results in more significant increases in the number and size of small pores with radius between 0.0004 and 0.36 μm. It is also found that the total porosity increment is primarily an outcome of small pore alteration during the imbibition. Moreover, the enlarged pores and micro-fractures are mainly categorized as the capillary bounded fluid pores and movable fluid pores, which significantly affect the efficiency of oil and gas transfer in shale. The findings of our study demonstrate the comprehensive effects of capillary force, clay hydration, osmotic potential, confining and pore pressures, and creep deformation and failure on the pore-fracture network alteration in shale and advance the understanding of the mechanisms behind the forced and spontaneous imbibition processes.

The Effect of Textile structure on the Comfort and Protection Properties of Clothes (Cloth masks)

The purpose of the present research is to study the effect of textile structure (Coverage factor and the type of fabrics) on the comfort properties (breathability) and protection properties (resistance to fluid permeability and pores size). Where the problem with this research in lies in the lack or scarcity of research that dealt with the properties of comfort and protection, especially in fabric masks, and the experimental approach was used to reach the best results . Whereas, woven and knitted fabrics were used in the research samples. The Results of air permeability (breathability) showed that the textile structure has a great impact on the breathability property of both woven and knitted fabrics, as the higher the density of the threads at a certain limit in the warp and weft directions in woven fabrics and the number of rows and columns in knitted fabrics, the air permeability and breathability decreased. The results showed that the cloth mask made of knitted fabrics is better than the cloth mask made of woven fabrics in terms of comfort and protection properties. The reason of respect are knitted fabrics better than woven fabrics when used to make cloth masks Based on the results given in the research, the two-layer cloth masks made of knitted fabrics are better in the comfort properties of breathable, expressed in air permeability, according to the technical requirements issued by the Egyptian Ministry of Industry and Trade, as well as according to the standard specification. Also, masks made of knitted fabrics were better than masks made from woven fabrics in terms of protection properties represented in the size of pores as well as resistance to spray permeability, expressed by the fabrics' resistance to fluid permeability under hydrostatic pressure according to the standard specification.

Pore structure characteristics and evaluation of lacustrine mixed fine-grained sedimentary rocks: A case study of the Lucaogou Formation in the Malang Sag, Santanghu Basin, Western China

The lithology of the mixed fine-grained sedimentary rock reservoirs in the Permian Lucaogou Formation of the Santanghu Basin is very complex, and the reservoirs are dominated by light- and dark-colored laminated structures with extremely heterogeneous pore structure. In this study, the mineral compositions and pore types of the reservoirs were analyzed using thin sections, whole rock X-ray diffraction (XRD), scanning electron microscopy (SEM), and Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN). This analysis was then combined with high-pressure mercury injection (HPMI), water-saturated nuclear magnetic resonance (NMR), and nano-CT scanning to quantitatively characterize the pore structure. In addition, the fractal dimensions obtained by NMR experimentation were used to comprehensively evaluate the pore structure and analyze the effect of volcanic ash on that structure. The results revealed that the reservoirs are characterized by fine-grained (<0.1 mm) minerals of tuffaceous material (felsic minerals) and carbonate, with light-colored lamination containing carbonate minerals and dark-colored lamination containing tuffaceous material. The petrophysical properties of the reservoir were found to be poor, with porosity ranging from 2% to 8% and permeability generally < 0.1 mD. Nanometer scale intergranular pores, intercrystalline pores, vugs, and microfractures are the main reservoir pore types. The NMR T2 spectra of the dolomite reservoir are mainly characterized by the right unimodal type (1–100 ms), which was mainly caused by the dolomite intercrystal pores and intercrystal dissolution pores, with the larger T2 components (>100 ms) mostly representing microfractures or vugs. In addition, the HPMI curves display a relatively low entry pressure (3–5 MPa), with medium sorting. The NMR T2 spectra of the tuff and transitional lithology reservoirs are diverse, however, with the low T2 components (0.01–1 ms) representing the intracrystal pores, intergranular pores that are mostly blocked by organic matter (OM), and some remaining intergranular pores during compaction, while the right peaks (1–100 ms) are similar to the dolomite reservoir. Also, the HPMI curves exhibit a higher entry pressure (>6 MPa) with poor sorting. Based on the T2cutoff values, the fractal dimensions obtained by the NMR experiment could be divided into two distinct segments, representing different pore structure characteristics. The fractal dimension of the movable fluid pores (D2) ranged from 2.493 to 2.973 (average 2.765); nevertheless, the fractal dimension of bound fluid pores (D1) was determined to be unsuitable for fractal theory. Due to the influence of volcanic ash, there is a positive correlation among D2, felsic mineral content, and TOC, while the D2 displays a negative correlation with the movable fluid porosity, permeability, and dolomite content. Hence, D2 increases with increasing felsic mineral content and decreasing dolomite content, indicating that the tuffaceous material can make the pore structure more complex. The relationship between the calcite content and D2 is not obvious, however, and the clay mineral contents are so low that their effects on the pore structure are negligible. The results of this study indicate that the fractal dimension can comprehensively reflect the pore structure complexity of a mixed fine-grained sedimentary rock reservoir that affected by volcanic ash.

Analysis of ZTE MRI application to sandstone and carbonate

AbstractMicrotomography (μCT) and nuclear magnetic resonance (NMR) have been used to characterize porous media for decades. Magnetic resonance imaging (MRI) enables direct visualization of pore architecture and many pulse sequences exist. In this work, we tested the MRI pulse sequence Zero Echo Time (ZTE) to study sandstone and carbonate for its ability to address short relaxation times. We aimed at resolving two fluid conduit scales, that is, pores and fractures. In this research, we study tighter porous systems than those previously reported using ZTE. Additionally, pore cluster analysis (PCA), combined with ZTE, can be used to analyze pore‐fracture connectivity of relatively large core plugs. We show that ZTE can resolve two‐scale pore systems simultaneously, that is, fractures and pores. By combining time‐domain NMR pore‐size analysis and PCA, we show that careful selection of resolution is necessary to understand transport in porous media.

A novel pore size classification method of coals: Investigation based on NMR relaxation

Nuclear magnetic resonance (NMR) severs as a nondestructive and relative new technique that has been widely used in characterizing reservoir fluids and pore size distribution (PSD) of coals. Conventionally, pore fluids in coals are classified into movable fluid and irreducible fluid based on a single NMR T2 cutoff value (T2C). However, the single NMR T2 cutoffs model has some apparent defects in pore fluid/size classification, and few researches have reported the limitation of the single T2 cutoffs model. In contrast, the dual T2 cutoffs model may provide an accurate quantified model to classify different pore fluid types in coals. In this study, fifteen coal samples with different ranks were conducted in systematic NMR and centrifugal experiments to investigate the characteristics of pore fluid typing and PSD. Results show that when tried applying the single NMR T2 cutoffs model to classify the pore fluid typing, there are still some movable fluids when T2 < T2C. At the same time, when T2 > T2C, there is remaining some irreducible fluid in pores after high pressure centrifugal experiments. These results indicated the limited application of single NMR T2 cutoffs model in pore fluid classification. By introducing a novel pore fluid classification method (i.e. the dual T2 cutoffs model), a typical T2 spectrum under fully-saturated condition, the absolute irreducible fluid T2 cutoffs (T2C1) and absolute movable fluid T2 cutoff (T2C2) can re-divide the pore fluid typing of coal into three types: absolute irreducible fluid (T2 < T2C1), partial movable fluid (T2C1 < T2 < T2C2), and absolute movable fluid (T2 > T2C2). The results show that the T2C1 is in the range of 0.10–0.32 ms, while the T2C2 has a wider range from 36.12 ms to 89.07 ms. Finally, a conceptional model were proposed to clarify a full-scale PSD classification that includes the absolute irreducible fluid pores, partial movable fluid pores and absolute movable fluid pores. The model established in this study, can also be applicable for other rock types (e.g., sandstones, carbonates and shales).

Pore structure evaluation of tight reservoirs in the mixed siliciclastic-carbonate sediments using fractal analysis of NMR experiments and logs

Abstract The study on mixed depositional reservoirs is widely attended in recent years. However, the mixed depositional reservoirs are generally characterized by complex pore structure, which is caused by strong heterogeneity, complex mineral compositions, and varying pore sizes, presenting a major challenge to describe the difference in pore structure between different samples. To address this challenge, the pore structure of mixed depositional reservoirs was evaluated by the fractal dimensions, and the influences of various mineral compositions on pore structure were analyzed. The fractal dimensions derived from NMR measurements or NMR logging data can be divided into two segments at T 2cutoff values, representing different fractal values and characteristics of pore structure. For the E 3 2 Formation in Yingxi field, the fractal dimension of movable fluid pores ranges from 2.628 to 2.917 (average 2.765), whereas the bound fluid pore systems are not following the fractal theory. Meanwhile, the fractal dimensions show a negative relationship with movable fluid porosity and permeability. The effects of various mineral compositions on fractal laws of the pore networks are different. Fractal dimension increases with the increases of clay mineral contents, and the decrease of dolomite content, while the effect of calcite content on fractal laws of the pore system is uncertain, and quartz from different genetic type represents different effect on fractal dimensions. This study is contributed for petroleum exploitation in mixed depositional reservoirs and shows that the fractal dimension can comprehensively indicate the complexity of the pore structure in mixed depositional reservoirs.

Shale pore size classification: An NMR fluid typing method

Abstract Shale reservoir is characterized by complex pore networks, within which there are various pore fluids including unrecoverable fluid, capillary bound fluid and movable fluid. Although considerable literature has investigated the pore structure of gas shale using different laboratory testing techniques, few papers have provided a quantified model to distinguish different types of fluids (corresponding to different pore types) in shale. In this study, seven shale core plugs from the Sichuan Basin were measured in a series of NMR experiments under full brine-saturated, centrifugal and heat-treated conditions to analyze the pore structure information and pore fluid transport during the processes of centrifuging and heating. For a typical T2 spectrum of 100% brine-saturated shale, the movable fluid T2 cutoff (T2C1) and unrecoverable fluid cutoff (T2C2) were derived from NMR centrifugal and heat-treated experiments to distinguish the unrecoverable fluid (T2 T2C1). Our results show that for the investigated shales, the T2C2 ranges from 0.09 ms to 0.36 ms, and T2C1 has a wide range from 0.45 ms to 2.98 ms. The surface relaxivities range from 0.00426 μm/ms to 0.02822 μm/ms, and the shales having high silicate mineral contents commonly have low surface relaxivities. A conceptional model based on the dual T2 cutoff method was constructed to illustrate the full-scale pore size distribution: unrecoverable fluid pores, capillary bound fluid pores and movable fluid pores. This study provides a new method of pore fluid typing and full-scale pore size distribution classification for shales.

Micromechanical processes in consolidated granular salt

Granular salt is likely to be used as backfill material and a seal system component within geologic salt formations serving as a repository for long-term isolation of nuclear waste. Pressure from closure of the surrounding salt formation will promote consolidation of granular salt, eventually resulting in properties comparable to native salt. Understanding dependence of consolidation processes on stress state, moisture availability, temperature, and time is important for demonstrating sealing functions and long-term repository performance. This study characterizes laboratory-consolidated granular salt by means of microstructural observations. Granular salt material from mining operations was obtained from the bedded Salado Formation hosting the Waste Isolation Pilot Plant and the Avery Island salt dome. Laboratory test conditions included hydrostatic consolidation of jacketed granular salt with varying conditions of confining isochoric stress to 38 MPa, temperature to 250 °C, moisture additions of 1% by weight, time duration, and vented and non-vented states. Resultant porosities ranged between 1% and 22%. Optical and scanning electron microscopic techniques were used to ascertain consolidation mechanisms. From these investigations, samples with 1% added moisture or unvented during consolidation, exhibit clear pressure solution processes with tightly cohered grain boundaries and occluded fluid pores. Samples with only natural moisture content consolidated by a combination of brittle, cataclastic, and crystal plastic deformation. Recrystallization at 250 °C irrespective of moisture conditions was also observed. The range and variability of conditions applied in this study, combined with the techniques used to display microstructural features, are unique, and provide insight into an important area of governing deformation mechanism(s) occurring within salt repository applications.

Structure Tensor Graph Searches Based Fully Automated Grading and 3D Profiling of Maculopathy From Retinal OCT Images

Maculopathy is a collective group of diseases that damages the central region of a retina known as macula. The major two forms of maculopathy are macular edema (ME) and central serous chorioretinopathy (CSCR). Different researchers have worked on the identification of these macular disorders using optical coherence tomography (OCT) images. However, to the best of our knowledge, no research is reported until now that can automatically extract retinal information for the grading of ME and CSCR as per clinically significant ME (CSME), non-clinically significant ME (non-CSME), Type-I CSCR, and Type-II CSCR clinical standards from OCT images. Therefore, this paper presents a novel structure tensor graph searches (ST-GS)-based segmentation framework that combines a structure tensor and graph theory to automatically extract retinal and choroidal layers along with fluid pores followed by automated reconstruction of 3-D retinal surfaces. The ST-GS can extract retinal information even from highly degraded OCT scans. Furthermore, the proposed system automatically grades ME and CSCR pathologies as CSME and non-CSME and Type-I CSCR and Type-II CSCR, respectively. The proposed system extracts seven distinct features, and it is trained on 30 (10 healthy, 10 ME, and 10 CSCR) labeled OCT volumes containing 3840 brightness scans (B-scans). After that, 90 (30 healthy, 30 CSCR, and 30 ME) unlabeled OCT volumes containing 11520 B-scans were used for testing the proposed system, where it correctly classified 88 out of 90 cases with the sensitivity, specificity, and accuracy ratings of 96.77%, 100%, and 97.78%, respectively. Furthermore, the proposed system has achieved a mean dice coefficient of 0.875±0.0342 and 0.92±0.0258 for extracting cyst and serous fluids, respectively.

A preliminary batch study of sorption kinetics of Cr(VI) ions from aqueous solutions by a magnetic ion exchange (MIEX®) resin and determination of film/pore diffusivity

A magnetic ion exchange resin was tested for the removal of Cr(VI) ions from aqueous solutions in a batch reactor at different resin dosages (0.5–1.5gL−1), pH values (2−10), temperatures (25°C–55°C) and initial concentrations of Cr(VI) (0.11–2.12mmolL−1 or 5.7–110mgL−1). The experimental data were evaluated using kinetic models based on the rate controlling steps relevant to pseudo-first order or pseudo-second order reactions or diffusion through fluid film or pores of the MIEX resin beads. Fast uptake of Cr(VI) ions by MIEX resin facilitates the attainment of equilibrium within 60min. The sorption process is strongly dependent on the initial pH of the solution with maximum occurring at pH4–6. The sorption of Cr(VI) at equilibrium increases with the increase in initial concentration according to the Langmuir isotherm, with a maximum sorption capacity of 93mgg−1 on MIEX resin. Relatively low activation energy of 43kJmol−1 at a high initial concentration of 1mmolL−1 Cr(VI) suggests a mixed chemical-diffusion controlled process, supported by the agreement of the sorption results with the homogeneous particle diffusion model with a film diffusivity of Df=1×10−9m2s−1 and a pore diffusivity of Dp=8×10−13m2s−1.

Visualization of Solution Chemistry by X-ray Crystallography Using Porous Coordination Networks

Abstract Porous coordination networks have two distinctive characteristics: robust coordination frameworks and fluid pores wherein molecules are mobile. These characteristics allow solution chemistry to be performed in the crystalline state, thus enabling single-crystal X-ray observation of various reactions and host–guest complexation usually observed in solution. From this viewpoint, porous coordination networks can be used as a tool for visualizing solution-based chemical reactions. Using this tool, we have visualized chemical reactions within porous coordination network crystals to elucidate the reaction mechanisms and to determine the molecular structures of the products and intermediates. When the nanospace of porous coordination networks is cleverly designed on the basis of the structures of hollow molecular hosts in solution, solution-like host–guest chemistry is also reproduced in crystals, therefore enabling their visualization by X-ray crystallography. Furthermore, a new method based on crystalline-state host–guest chemistry was developed for the single-crystal X-ray analysis of trace amounts of noncrystalline compounds using porous coordination networks. In this account, we review and discuss the visualization of unique solution-like phenomena by single-crystal X-ray analysis using porous coordination networks as crystalline hosts.

The configuration evolution and macroscopic elasticity of fluid-filled closed cell composites : micromechanics and multiscale homogenization modelling

For fluid-filled closed cell composites widely distributed in nature, the configuration evolution and effective elastic properties are investigated using a micromechanical model and a multiscale homogenization theory, in which the effect of initial fluid pressure is considered. Based on the configuration evolution of the composite, we present a novel micromechanics model to examine the interactions between the initial fluid pressure and the macroscopic elasticity of the material. In this model, the initial fluid pressure of the closed cells and the corresponding configuration can be produced by applying an eigenstrain at the introduced fictitious stress-free configuration, and the pressure-induced initial microscopic strain is derived. Through a configuration analysis, we find the initial fluid pressure has a prominent effect on the effective elastic properties of freestanding materials containing pressurized fluid pores, and a new explicit expression of effective moduli is then given in terms of the initial fluid pressure. Meanwhile, the classical multiscale homogenization theory for calculating the effective moduli of a periodical heterogeneous material is generalized to include the pressurized fluid inclusion effect. Considering the coupling between matrix deformation and fluid pressure in closed cells, the multiscale homogenization method is utilized to numerically determine the macroscopic elastic properties of such composites at the unit cell level with specific boundary conditions. The present micromechanical model and multiscale homogenization method are illustrated by several numerical examples for validation purposes, and good agreements are achieved. The results show that the initial pressure of the fluid phase can strengthen overall effective bulk modulus but has no contribution to the shear modulus of fluid-filled closed cell composites.

Robust Pore Size Analysis of Filamentous Networks from 3D Confocal Microscopy

We describe a robust method for determining morphological properties of filamentous biopolymer networks, such as collagen or other connective tissue matrices, from confocal microscopy image stacks. Morphological properties including pore size distributions and percolation thresholds are important for transport processes, e.g. particle diffusion or cell migration through the extracellular matrix. The method is applied to fluorescently labeled fiber networks prepared from rat tail tendon and calf skin collagen, at concentrations of 1.2, 1.6 and 2.4 mg/ml. The collagen fibers form an entangled and branched network. The medial axes, or skeletons, representing the collagen fibers are extracted from the image stack by threshold intensity segmentation and distance-ordered homotopic thinning. The size of the fluid pores as defined by the radii of largest spheres that fit into the cavities between the collagen fibers is derived from Euclidean distance daps and maximal covering radius transforms of the fluid phase. The size of the largest sphere that can traverse the fluid phase between the collagen fibers across the entire probe, called the percolation threshold, was computed for both horizontal and vertical directions. We demonstrate that by representing the fibers as the medial axis the derived morphological network properties are both robust against changes of the value of the segmentation threshold intensity and robust to problems associated with the point-spread function of the imaging system. We also provide empirical support for a recent claim that the percolation threshold of a fiber network equals the fiber diameter for which the Euler index of the networks becomes zero.

Robust Pore Size Analysis of Filamentous Networks from Three-Dimensional Confocal Microscopy

We describe a robust method for determining morphological properties of filamentous biopolymer networks, such as collagen or other connective tissue matrices, from confocal microscopy image stacks. Morphological properties including pore size distributions and percolation thresholds are important for transport processes, e.g., particle diffusion or cell migration through the extracellular matrix. The method is applied to fluorescently labeled fiber networks prepared from rat-tail tendon and calf-skin collagen, at concentrations of 1.2, 1.6, and 2.4mg/ml. The collagen fibers form an entangled and branched network. The medial axes, or skeletons, representing the collagen fibers are extracted from the image stack by threshold intensity segmentation and distance-ordered homotopic thinning. The size of the fluid pores as defined by the radii of largest spheres that fit into the cavities between the collagen fibers is derived from Euclidean distance maps and maximal covering radius transforms of the fluid phase. The size of the largest sphere that can traverse the fluid phase between the collagen fibers across the entire probe, called the percolation threshold, was computed for both horizontal and vertical directions. We demonstrate that by representing the fibers as the medial axis the derived morphological network properties are both robust against changes of the value of the segmentation threshold intensity and robust to problems associated with the point-spread function of the imaging system. We also provide empirical support for a recent claim that the percolation threshold of a fiber network is close to the fiber diameter for which the Euler index of the networks becomes zero.

Effective and coupled thermal conductivities of isotropic open‐cellular foams

AbstractThe effective and the coupled thermal conductivity of the solid microstructure of open‐cellular foams and an accompanying saturation fluid are defined in a conceptual representation of conductive heat transfer in a two‐phase system of which the phases are in a thermal nonequilibrium state. The effective and coupled thermal conductivities were determined from a close observation of the relationship between microscopic and macroscopic temperature distributions. Temperature distributions were obtained from the numerical solution of the three‐dimensional (3‐D) conduction equation in a representative geometrical model of the foam solid microstructure and the fluid pores. Empirical correlations are provided for the effective and coupled thermal conductivities in terms of the solid and the fluid thermal conductivity and foam porosity. Thermal radiation was not considered in the energy transfer process. © 2004 American Institute of Chemical Engineers AIChE J, 50: 547–556, 2004

A model for high-frequency acoustic backscatter from gas bubbles in sandy sediments at shallow grazing angles

A model for acoustic backscatter from trapped bubbles in sandy sediments was developed. The model combines a Biot acoustic penetration model with a resonance scattering mechanism from trapped bubbles. The bubble size distribution is assumed to mirror the size distribution of the fluid pores that exist between sand grains. An estimate of the pore size distribution is constructed from the grain size distribution, based on the known pore structure between dense random packings of hard spheres. The principle of acoustic reciprocity is employed to compute backscattered acoustic pressure in terms of the incident pressure and the scattering cross section of a bubble distribution. The model is applied to data from experiments recently taken at sea. It is concluded that trapped gas is often a likely cause of observed backscatter from sandy sediments. Very small amounts of gas appear to be sufficient to produce significant backscatter.