Search

Particle morphology plays a significant role in influencing engineering behavior of granular materials. Surface texture, roundness, and sphericity represent distinct multiscale measures needed to fully describe particle morphology. Most studies reported in the literature rely on two-dimensional (2D) projected images of particles with a few three-dimensional (3D) images that mostly focused on relatively large-size aggregate samples. In this paper, 3D synchrotron microcomputed tomography (SMT) was used to acquire high-resolution images of glass beads, F-35 Ottawa sand, #1 dry glass sand, GS#40 Columbia sand, Toyoura sand, and Hostun RF sand. New roundness and sphericity indexes are proposed and calculated for the samples based on 3D measurements of surface area, volume, and three orthogonal diameters of particles. In addition, the surface texture of particles were measured using optical interferometry technique. The measurements reported in this paper can serve as a good source for other researchers working on sands to build on these intrinsic particle properties to link engineering behavior of sands to their morphology. 2014 American Society of Civil Engineers. National Science Foundation Scopus

We present in this paper a methodology to calculate the distribution of local void ratio in porous media systems from high-resolution three-dimensional images. We introduce an algorithm to calculate the distribution of local void ratio from 3D images based on distance and watershed transforms. The watershed transform is used to segment touched or overlapped particles in an efficient way and the distance transform is used to calculate the boundaries of local void regions. The algorithm is validated using computer simulated 3D images of regular packing, irregular (non-spherical particles) packing, and random uniform spherical packing. Results show that the algorithm is robust, accurate and can be used to calculate local void ratio distribution of 3D systems regardless of irregularity in shapes, sizes, or arrangement of particles. X-ray microtomography images of different glass bead systems are used to calculate distributions of local void ratio. Parameters of distributions are function of porosity and particle-size distribution. The maximum local void ratio in each system is less than 3.0 and the minimum is greater than 0.2.

Pore-scale processes govern fundamental behavior in multiphase porous media systems. A high-resolution, three-dimensional image of the interior of a multiphase porous media system was obtained using synchrotron X-ray tomography. The system was imaged at a resolution of 12.46 mum following entrapment of the nonwetting phase at residual saturation. First, the physically representative network structure of the porous media system is extracted from the void space. This provides a direct mapping of the pore bodies and throats and enables pore-level calculations of coordination numbers, aspect ratios, and pore body and throat correlations. Next, algorithms developed to calculate properties of the entrapped nonwetting phase, such as volume, sphericity, interfacial area, and orientation, are applied to the residual nonwetting phase blobs. Finally, correlations between the pore network structure and nonwetting phase characteristics are examined. As expected, it was found that the nonwetting phase was trapped primarily in the largest pore spaces, the pore bodies with the highest aspect ratios, and the pore bodies with the highest coordination numbers. This work shows that, while there may be limitations related to the ability to capture REV-sized domains for some of the multiphase flow properties and phenomena, high-resolution X-ray tomography is able to provide the high quality datasets needed to observe and quantify the pore-scale phenomena and processes that govern multiphase flow in unconsolidated porous media systems.

This paper presents application of a series of algorithms used to extract pore network structure from high-resolution three-dimensional synchrotron microtomography images of unconsolidated porous media systems. These algorithms are based on the three-dimensional skeletonization that simplifies the pore space to networks in the form of nodes connected to paths. Dilation algorithms were developed to generate inscribed spheres on the nodes and paths of the medial axis to represent pore-bodies and pore-throats of the network, respectively. The end result is a physically representative pore network structure, i.e. three-dimensional spatial distribution (i.e. x-, y-, and z-coordinates) of pore-bodies and pore-throats, pore-body size distribution, pore-throat size distribution, and the connectivity. Systems analyzed in this study include different glass bead systems and natural marine sand. The media ranged in size from 0.123 to 1.0 mm, while the image volumes ranged between 7.7 and 108.9 mm3. In addition to extracting the pore network structure, the porosity, specific surface area, and representative elementary volume analysis on the porosity were calculated. Spatial correlation between pore-body sizes in the network was investigated using semivariograms and integral scale concepts. The impact of resolution on the calculated property was also investigated. In this work, we show that microtomography is an effective tool to non-destructively extract the structure of many systems. The quality of the datasets depends on photon energy, photon flux, size of the sample, type of the sample, and size of the sample ‘features’. Results show that the developed method of extracting pore network structure is applicable to ideal and natural porous media systems. The impact of resolution on the quantification of the network structure properties varies in its significance based on feature size of the system and the properties being calculated. Therefore, a thorough resolution sensitivity analysis should be carried out to determine the degree of error associated with a system imaged at a given resolution.

Synchrotron X-ray tomography is used to image, at 3.34 μm resolution, a mixture containing high-impact polystyrene (HIPS) and a two-component flame retardant, a brominated phthalimide dimer (Saytex BT-93), and a synergist, antimony oxide (Sb 2 O 3 ). Complete tomography data sets were acquired at seven X-ray energies in the range of 12-40 keV, closely spanning Br and Sb Is electron binding energies at 13.474 and 30.491 keV, respectively. Data acquisition required 8 h. Quantification is done for a representative subvolume of 0.582 mm 3 containing 250 3 cubic volume elements (voxels) which is fitted to a model for X-ray voxel linear attenuation coefficient, yielding the three-dimensional concentration distribution of the flame retardant and the synergist. With successive thresholding, a particle size distribution algorithm yields regions of low flame retardant concentration-a potential safety concern-and regions of excessively high flame retardant concentration-a waste of an expensive product.

While network models of porous materials have traditionally been constructed using regular or disordered lattices, recent developments allow the direct modeling of more realistic structures such as sphere packings, microtomographic images, or computer-simulated materials. One of the obstacles in these newer approaches is the generation of network structures that are physically representative of the real systems. In this paper, we present and compare two different algorithms to extract pore network parameters from three-dimensional images of unconsolidated porous media systems. The first approach, which utilizes a pixelized image of the pore space, is an extension to unconsolidated systems of a medial-axis based approach (MA). The second approach uses a modified Delaunay tessellation (MDT) of the grain locations. The two algorithms are validated using theoretical packings with known properties and then the networks generated from random packing are compared. For the regular packings, both methods are able to provide the correct pore network structure, including the number, size, and location of inscribed pore bodies, the number, size, and location of inscribed pore throats, and the connectivity. Despite the good agreement for the regular packings, there were differences in both the spatial mapping and statistical distributions in network properties for the random packings. The discrepancies are attributed to the pixelization at low resolution, non-uniqueness of the inscribed pore-body locations, and differences in merging processes used in the algorithms, and serve to highlight the difficulty in creating a unique network from a complex, continuum pore space.

The objective of this work was to investigate the impact of grain geometry (size and shape) of porous media on the morphology of residual NAPL. Synchrotron microtomography was used to obtain maps of residual NAPL in multiphase systems. High-resolution, three-dimensional images of natural sand systems, comprising a range of grain sizes and shapes were imaged and analyzed. Findings indicate that residual NAPL saturation is influenced by the shapes of grains of the porous medium more than their sizes. In systems composed of grains with similar sphericity and angularity, residual saturations are independent of median grain sizes at the same operating regime (capillary-controlled regime in this work). Residual saturations tend to increase as the system comprised more angular or non-spherical grains where relatively large NAPL blobs are entrapped in such systems. While volumes of individual blobs tend to decrease as grain size decreases, grain geometry has more profound effects on the morphology of the residual NAPL blobs. Within a system composed of grains with similar shape characteristics, total NAPL-water interfacial area increases as grain sizes decrease where a large number of small blobs are trapped. Total meniscus NAPL-water interfacial area exhibits a linear relation with total interfacial area where it tends to increase as grain sizes decrease. However, while meniscus interfacial areas of individual blobs are highly influenced by pore geometry; residual blobs trapped in pores with complex geometry tend to have higher meniscus interfacial areas due to their branched nature which increases contacts with the wetting phase. U.S. Department of Energy, the National Science Foundation (Contract No. HRD-0503362) and Louisiana Board of Regents (Contract No. LINK-45). Scopus