Uniform Packing Research Articles

Optimizing the structure and yield of vanadium oxide nanotubes by periodic 2D layer scrolling

Structural uniformity of vanadium oxide nanotubes is possible by ensuring that 2D crystalline lamina maintain a defined thickness and interlayer spacing during scrolling. These parameters are sensitive to uniform packing of amine molecular templates.

Liquid spread mechanisms in packed beds and heaps. The separation of length and time scales due to particle porosity

The distribution of liquid within a heap is a key factor in the system performance as it has a strong effect on the transport of both reagents and leached species and thus the leaching rate. How liquid spreads from drippers and the subsequent development of flow paths and any associated channelling is thus important. In this paper a pseudo 2-D column was used to investigate the horizontal spread of liquid in the vicinity of dripper in columns packed with both narrowly sized particles and more realistic particle size distributions. Both systems had distinct separation of the time scales at which different saturation features developed. There was an initial rapid formation of flow paths in the inter-particle spaces with only local wetting of the intra-particle spaces, though this was associated with little spread. Over a much longer time period there was extensive horizontal spread of the liquid within the ore particles, though this was associated with virtually no vertical flow. The externally held liquid (liquid content between the particles) showed strong channelling behaviour, especially in the realistically sized particles, despite the care that was taken to ensure uniform packing. This effect can be reduced by changing initial bed conditions and employing dense drip emitter locations, but it cannot be completely eliminated as particle level heterogeneities in heap leaching systems affect external flow paths creation. Hysteresis in the amount of liquid spread was also demonstrated, with the total spread depending not only on the current flow rate, but also on the flow history.

Evaluation of Artificial Intelligence Architectures for Optimization of Recombinant Glucoamylase Production in a Microbioreactor

Microbioreactors with immobilized cells or enzymes are normally packed uniformly. However, uniform packing is good only when the flow is uniform throughout the microtube. In real situations this is not true. For such cases it was shown recently that, although radial variations may be ignored, a topologically optimized longitudinal distribution of immobilized pellets generated higher outputs of glucoamylase by a recombinant strain of Saccharomyces cerevisiae. However, topology optimization (or any equation-based method) requires a good (and preferably simple) mathematical model. Such models are difficult to formulate for real nonideal conditions. Hence, a recent article by this author employed an expert system that selected between two artificial intelligence (AI) methods to determine the better method over successive intervals of time. However, such a switching strategy is complex and practically difficult. The present work, therefore, investigates a corresponding set of single (standalone) AI methods, each incorporating more biological information than earlier methods. For the same S. cerevisiae strain, it was seen that expressing the two input variables—feed concentration of glucose and the lengthwise distribution of immobilized cells—by genetic algorithms allowed greater flexibility and resulted in higher production rates of recombinant glucoamylase than all the methods used previously. This method is also easier to implement than topology optimization and the variable AI method.

Au@Ag core/shell cuboids and dumbbells: Optical properties and SERS response

Recent studies have conclusively shown that the plasmonic properties of Au nanorods can be finely controlled by Ag coating. Here, we investigate the effect of asymmetric silver overgrowth of Au nanorods on their extinction and surface-enhanced Raman scattering (SERS) properties for colloids and self-assembled monolayers. Au@Ag core/shell cuboids and dumbbells were fabricated through a seed-mediated anisotropic growth process, in which AgCl was reduced by use of Au nanorods with narrow size and shape distribution as seeds. Upon tailoring the reaction rate, monodisperse cuboids and dumbbells were synthesized and further transformed into water-soluble powders of PEGylated nanoparticles. The extinction spectra of AuNRs were in excellent agreement with T-matrix simulations based on size and shape distributions of randomly oriented particles. The multimodal plasmonic properties of the Au@Ag cuboids and dumbbells were investigated by comparing the experimental extinction spectra with finite-difference time-domain (FDTD) simulations. The SERS efficiencies of the Au@Ag cuboids and dumbbells were compared in two options: (1) individual SERS enhancers in colloids and (2) self-assembled monolayers formed on a silicon wafer by drop casting of nanopowder solutions mixed with a drop of Raman reporters. By using 1,4-aminothiophenol Raman reporter molecules, the analytical SERS enhancement factor (AEF) of the colloidal dumbbells was determined to be 5.1×106, which is an order of magnitude higher than the AEF=4.0×105 for the cuboids. This difference can be explained by better fitting of the dumbbell plasmon resonance to the excitation laser wavelength. In contrast to the colloidal measurements, the AEF=5×107 of self-assembled cuboid monolayers was almost twofold higher than that for dumbbell monolayers, as determined with rhodamine 6G Raman reporters. According to TEM data and electromagnetic simulations, the better SERS response of the self-assembled cuboids is due to uniform packing and more efficient generation of electromagnetic hot spots, as compared to the dumbbell monolayers.

Finite Element Modeling on the Compaction of Copper Powder Under Different Conditions

Single-action die compaction of copper powders with initial loose natural packing and vibrated random dense packing structures was carried out numerically by finite element method and physically for validation. Furthermore, the compaction under various cyclic loadings was modeled to identify its effects on the compact properties. The results were analyzed and compared between compacts formed at different initial packing structures and different forming conditions, which indicate that at the same pressure, single-action die compaction on the dense uniform initial packing can produce compacts with high relative density, uniform density and stress distributions, which implies the necessity to improve initial packing density and uniformity in forming high performance compacts. Meanwhile, by using cyclic loading on such dense initial packing structures, compacts with higher packing density and more uniform density and stress distributions can be created. The numerical and physical results are comparable and in good agreement with the proposed double logarithmic equation.

Colloidal stabilization of cerium-gadolinium oxide (CGO) suspensions via rheology

A rheological method based on the analysis of the flow index is proposed for the optimization of ceramic suspensions with respect to dispersant-ceramic affinity, dispersant concentration, and ceramic loading. The single-flow index (SFI) feature was identified as the criterion defining the optimized colloidally stable state. The method was applied to explore the ability of four commercial dispersants (acidic affine, neutral, basic affine, and polyvinylpyrrolidone (PVP)) to disperse cerium-gadolinium oxide (CGO) in ethanol. Only the acidic affine and the PVP dispersants were found to efficiently disperse the CGO powder. The acidic affine dispersant was further demonstrated to impart superior packing properties due to the formation of a thinner monolayer around the ceramic surface. CGO suspensions using the acidic affine at optimized amount were prepared and processed via tape casting. The resulting green tapes exhibited uniform and high packing density, producing a theoretical density in the sintered tapes of ca. 97–98%.

Photophysical study of P3HT/NDI based hybrid nanoparticles

Electron donor-acceptor based hybrid novel structure remains a frontier area of research to design optoelectronic, photovoltaic, and light harvesting devices. Here, we report the synthesis of interdyad and intradyad nanoparticles by using the electron donating polymer Poly-3-(hexylthiophine)) (P3HT) and the electron accepting molecule 1, 4, 5, 8 naphthalene tetracarboxylic diimide (NDI). The intradyad nanostructures are fabricated in situ by adding donor and acceptor molecules simultaneously whereas interdyad nanoparticles are fabricated by attaching the donor and acceptor nanoparticles electrostatically. The differential scanning calorimetry (DSC) confirms the segmental motion of the polymer chain and the uniform packing in intradyad nanostructures which is absent in the interdyad system. The photoluminescence quenching and the shortening of decay time of the excited state of the donor molecule were observed with increasing the concentration of acceptor molecule in the intradyad system which is attributed to the photoinduced electron transfer from donor to the acceptor molecule. However, in the case of the interdyad system, the change in photoluminescence quenching and the decay time is less significant due to different photophysical processes.

Application of one-body hybrid solid pellets to sorption-enhanced water gas shift reaction for high-purity hydrogen production

Interest in hydrogen, regarded as a new clean energy carrier, has been increasing with expectation of the approaching hydrogen economy. In the hydrogen economy, hydrogen will replace the conventional fuels that have caused pollution problems. As one of the methods for the mass production of hydrogen, water gas shift (WGS) reaction (CO + H2O ↔ H2 + CO2) has been highlighted for synthesis gas feed, which is produced by coal and biomass gasification. Recently, the performance of WGS reaction has been improved significantly through application of the sorption-enhanced WGS (SE-WGS) reaction concept, where WGS reaction and CO2 sorption are carried out simultaneously. High-purity hydrogen can be directly produced through the SE-WGS reaction, without need for further purification processes. In the SE-WGS reaction, uniform packing of the mixture of catalyst and sorbent is important; however, this is difficult to manage with conventional catalyst and sorbent pellets. In this study, novel one-body hybrid solid pellets consisting of the mixture of catalyst and sorbent were prepared to address this shortcoming and applied to SE-WGS reactions. From experiments, the effect of different ratio of catalyst/sorbent in one-body hybrid solid pellets was studied. A novel multi-section packing concept was also applied to SE-WGS reaction with one-body hybrid solid pellets. The experimental results showed that one-body hybrid solid pellets can be successfully used and that multi-section packing can increase the hydrogen productivity in SE-WGS reaction.

Formation and control of line defects caused by tectonics of water droplet arrays during self-organized honeycomb-patterned polymer film formation

This study describes the formation of macro-scale defects of the honeycomb-patterned polymer film and we discovered two types of new line defects which differ from the defects reported in the past studies. We examined the formation mechanisms of the line defects and clarified two types of formation mechanisms of the "Divergent" mode line defects and the "Convergent" mode line defects caused by the "tectonics" of water droplet arrays on polymer solutions. The regions causing the macro-scale line defects are made clear in the phase diagram represented by the radius and the density of the micro-scale water droplets. In addition, the results of our calculations made it possible to theoretically predict the water droplet growth time for the water droplets to grow to the ideal size for uniform packing that is necessary for fabrication of the defect-free honeycomb-patterned polymer film. With the use of these techniques, A4-sized, defect-free, honeycomb-patterned polymer films can be fabricated.

Oxygen carrier dispersion in inert packed beds to improve performance in chemical looping combustion

Various packed beds of copper-based oxygen carriers (CuO on Al2O3) were tested over 100 cycles of low temperature (673K) Chemical Looping Combustion (CLC) with H2 as the fuel gas. The oxygen carriers were uniformly mixed with alumina (Al2O3) in order to investigate the level of separation necessary to prevent agglomeration. It was found that a mass ratio of 1:6 oxygen carrier to alumina gave the best performance in terms of stable, repeating hydrogen breakthrough curves over 100 cycles. In order to quantify the average separation achieved in the mixed packed beds, two sphere-packing models were developed. The hexagonal close-packing model assumed a uniform spherical packing structure, and based the separation calculations on a hypergeometric probability distribution. The more computationally intensive full-scale model used discrete element modelling to simulate random packing arrangements governed by gravity and contact dynamics. Both models predicted that average ‘nearest neighbour’ particle separation drops to near zero for oxygen carrier mass fractions of x⩾0.25. For the packed bed systems studied, agglomeration was observed when the mass fraction of oxygen carrier was above this threshold.

Colloidal processing and partial sintering of high-performance porous zirconia nanoceramics with hierarchical heterogeneities

High-performance, porous, Y-TZP nanoceramics with hierarchical heterogeneities originating from uniform, intra- and inter-particle packing were prepared by colloidal processing and partial sinterin ...

The Four Canonical TPR Subunits of Human APC/C Form Related Homo-Dimeric Structures and Stack in Parallel to Form a TPR Suprahelix

The anaphase-promoting complex or cyclosome (APC/C) is a large E3 RING-cullin ubiquitin ligase composed of between 14 and 15 individual proteins. A striking feature of the APC/C is that only four proteins are involved in directly recognizing target proteins and catalyzing the assembly of a polyubiquitin chain. All other subunits, which account for >80% of the mass of the APC/C, provide scaffolding functions. A major proportion of these scaffolding subunits are structurally related. In metazoans, there are four canonical tetratricopeptide repeat (TPR) proteins that form homo-dimers (Apc3/Cdc27, Apc6/Cdc16, Apc7 and Apc8/Cdc23). Here, we describe the crystal structure of the N-terminal homo-dimerization domain of Schizosaccharomyces pombe Cdc23 (Cdc23Nterm). Cdc23Nterm is composed of seven contiguous TPR motifs that self-associate through a related mechanism to those of Cdc16 and Cdc27. Using the Cdc23Nterm structure, we generated a model of full-length Cdc23. The resultant “V”-shaped molecule docks into the Cdc23-assigned density of the human APC/C structure determined using negative stain electron microscopy (EM). Based on sequence conservation, we propose that Apc7 forms a homo-dimeric structure equivalent to those of Cdc16, Cdc23 and Cdc27. The model is consistent with the Apc7-assigned density of the human APC/C EM structure. The four canonical homo-dimeric TPR proteins of human APC/C stack in parallel on one side of the complex. Remarkably, the uniform relative packing of neighboring TPR proteins generates a novel left-handed suprahelical TPR assembly. This finding has implications for understanding the assembly of other TPR-containing multimeric complexes.

A sedimentation device to produce uniform sand packs

In physical modelling using sand, the construction of a sand pack is one of the main experimental difficulties. Existing solutions are generally based either on delicate manipulations (pouring and scraping), or on motorized sedimentation devices, and the qualities of packs thus produced are known to affect experimental outcomes. However, uniformity, planarity, reproducibility and efficiency can be achieved with a simple sedimentation device without motors. A rectangular reservoir pierced with holes rests on a support plate pierced with wider holes. Sand flows when the reservoir is displaced so that its holes match those of the support plate. Sand jets are diffused by planar horizontal sieves, below which the sand sets into the experimental box. Tests on a 250μm median grain size sand show that the density is at its maximum value, reproducible, and uniform. The spatial variations are only of ±0.4% of the mean density. Thickness of the sand layers shows spatial variations around ±2% of the mean thickness, and 6% near the side walls. Very fine grains (90μm median grain size) produce less uniform and less planar packs because of their greater sensitivity to air currents caused by the sedimentation. According to direct shear tests the sand pack has a well defined static friction coefficient decreasing to a lower dynamic value after about 3mm of slip with dilatancy. In contrast, poured sand packs, which are initially less dense, develop only the dynamic friction coefficient (no peak shear stress during slip), without dilatancy. Hole diameters, hole spacing, and the number and openings of the sieves are the parameters controlling the qualities of the sand pack for a given grain size distribution.

Structural morphology study of Cd2+ induced Langmuir Blodgett multilayer films of arachidic acid

The organization and headgroup co-ordination of Cadmium Arachidate (CdAA) molecule in Langmuir–Blodgett (LB) multilayer films deposited on hydrophilic Glass (SiO2) and Silicon (100) substrate at normal subphase pH (6.8) are studied. X-ray diffraction (XRD) and X-ray reflectivity (XRR) study reveals ordered layer by layer organization with uniform packing of CdAA molecules, and with a small tilt angle of alkyl chain of CdAA molecule equal to 6.8°±1.75°. Electron density profiles (EDPs) shows that the coverage of films remains almost constant with increase in bilayer thickness which indicate very little presence of pinhole defects. AFM study for 25 ML shows that coverage of the film remain intact upto 22nd ML and then decreases sharply due to presence of pinhole defects. Fourier transform infrared spectroscopy (FTIR) study is also consistent with XRD and XRR study of ordered deposition of CdAA molecule. FTIR and X-ray photoelectron spectroscopy (XPS) study indicates the formation of unidentate bridging metal-carboxylate coordination type headgroups consistent with one cadmium metal ion between two carboxylate (COO) groups in each headgroup structure.

Packing of charged chains on toroidal geometries

We study a strongly adsorbed flexible polyelectrolyte chain on tori. In this generalized Thomson problem, the patterns of the adsorbed chain are analyzed in the space of the toroidal coordinates and in terms of the orientation of each chain segment. Various patterns are found, including double spirals, disclination-like structures, Janus tori, and uniform wrappings, arising from the long-range electrostatic interaction and the toroidal geometry. Their broken mirror symmetry is quantitatively characterized by introducing an order parameter, an integral of the torsion. The uniform packing, which breaks the mirror symmetry the least, has the lowest value of the order parameter. In addition, it is found that the electrostatic energy of confined chains on tori conforms to a power law regardless of the screening effect in some typical cases studied. Furthermore, we study random walks on tori that generate chain configurations in the large screening limit or at large thermal fluctuation; some features associated with the toroidal geometry are discussed.

From Kepler's conjecture and fcc lattice to modelling of crowding in living matter.

Up to now, sphere packing has been investigated without any reference to living matter. This study focuses on the void space (VS) of sphere packing to mimic the extracellular spaces of living tissues. It was inspired by the importance of the extracellular matrix, the vehicle of micro and macromolecules involved in cell metabolism, intercellular communication and drug delivery. The analysis of sphere packing evidenced that in uniform random packing VS is about 1.9 times greater than in the face centered cubic (fcc) lattice (thus being very close to the 1.9 volume ratio of the cube to the sphere). This datum is a good reference for cell packing in vivo. The disproportionate increase of VS per sphere in loose packing in vitro is analyzed having in mind the variability in volume and composition of the interstitial spaces in vivo and cell trafficking. Arrangements of lymphocytes mimicking a two-dimensional hexagonal pattern and dense packing of disks generated by numerical procedures, are described in 7 micro m-thick haematoxylin and eosin-stained histological slices from a human lymph node. In narrow tubes simulating roundish cells arranged in limited compartments of the interstice, sphere packing is characterized by noticeable increases of VS. The VS of this packing in vitro is compatible with variability in volume and composition of the interstitial spaces and with cell trafficking in vivo. This paper stresses that in mammalian tissues and organs cells can be packed quite more densely than spheres in the fcc lattice. As to pathology, attention is focused: (i) on overcrowding of cell organelles in some diseases, (ii) on shrinking or swelling of high amplitude, whose opposite effects are to concentrate or dilute intracellular structures and crowding of macromolecules, and (iii) on neoplastic tissues.

Pore Space Microstructure Evolution of Regular Sphere Packings Undergoing Compaction and Cementation

We examine the pore space structure evolution of ordered uniform sphere packs: simple cubic (SC), body centered cubic (BCC), and face centered cubic (FCC), undergoing simple diagenetic processes that reduce their pore spaces. Focus is on the occurrence of pore space microstructure changes or transitions, which are followed through their characteristic or critical pore lengths (lc). For almost all the cubic packings undergoing either compaction or cementation there are no singularities in lc. This is a consequence of having a single pore shape controlling flow at all stages of the process. However, this is not so for the BCC packing under cementation, for which lc is non-monotonic exhibiting a kink at \({\phi \approx 0.1452}\) , the porosity at which the pore shape controlling flow switches to a different form and position. These results for uniform compaction/cementation complement our previous works on pore networks under random shrinkage. Kinks in lc as porosity decreases signal pore space microstructure transitions that anticipate sudden changes in the permeability–porosity relation as porosity decreases. The consequences are great; clearly lc is not a constant unless the diagenetic process is mild. A lc function of compaction/cementation advancement should be used above a transition and a different lc function below. For the sphere packs here, once the diagenetic process has reduced the pore space substantially, a lc function of compaction/cementation advancement is mandatory if we are to capture all significant flow features.

Electron tomography of fiber cell cytoplasm and dense cores of multilamellar bodies from human age-related nuclear cataracts

Human nuclear cataract formation is a multi-factorial disease with contributions to light scattering from many cellular sources that change their scattering properties over decades. The aging process produces aggregation of cytoplasmic crystallin proteins, which alters the protein packing and texture of the cytoplasm. Previous studies of the cytoplasmic texture quantified increases in density fluctuations in protein packing and theoretically predicted the corresponding scattering. Multilamellar bodies (MLBs) are large particles with a core of crystallin cytoplasm that have been suggested to be major sources of scattering in human nuclei. The core has been shown to condense over time such that the refractive index increases compared to the adjacent aged and textured cytoplasm. Electron tomography is used here to visualize the 3D arrangement of protein aggregates in aged and cataractous lens nuclear cytoplasm compared to the dense protein packing in the cores of MLBs. Thin sections, 70 nm thick, were prepared from epoxy-embedded human transparent donor lenses and nuclear cataracts. Tilt series were collected on an FEI T20 transmission electron microscope (TEM) operated at 200 kV using 15 nm gold particles as fiducial markers. Images were aligned and corrected with FEI software and reconstructed with IMOD and other software packages to produce animated tilt series and stereo anaglyphs. The 3D views of protein density showed the relatively uniform packing of proteins in aged transparent lens nuclear cytoplasm and less dense packing of aged cataractous cytoplasm where many low-density regions can be appreciated in the absence of the TEM projection artifacts. In contrast the cores of the MLBs showed a dense packing of protein with minimal density fluctuations. These observations support the conclusion that, during the nuclear cataract formation, alterations in protein packing are extensive and can result in pronounced density fluctuations. Aging causes the MLB cores to become increasingly different in their protein packing from the adjacent cytoplasm. These results support the hypothesis that the MLBs increase their scattering with age and nuclear cataract formation.

An Amino Acid Packing Code for α-Helical Structure and Protein Design

This work demonstrates that all packing in α-helices can be simplified to repetitive patterns of a single motif: the knob–socket. Using the precision of Voronoi Polyhedra/Delauney Tessellations to identify contacts, the knob–socket is a four-residue tetrahedral motif: a knob residue on one α-helix packs into the three-residue socket on another α-helix. The principle of the knob–socket model relates the packing between levels of protein structure: the intra-helical packing arrangements within secondary structure that permit inter-helix tertiary packing interactions. Within an α-helix, the three-residue sockets arrange residues into a uniform packing lattice. Inter-helix packing results from a definable pattern of interdigitated knob–socket motifs between two α-helices. Furthermore, the knob–socket model classifies three types of sockets: (1) free, favoring only intra-helical packing; (2) filled, favoring inter-helical interactions; and (3) non, disfavoring α-helical structure. The amino acid propensities in these three socket classes essentially represent an amino acid code for structure in α-helical packing. Using this code, we used a novel yet straightforward approach for the design of α-helical structure to validate the knob–socket model. Unique sequences for three peptides were created to produce a predicted amount of α-helical structure: mostly helical, some helical, and no helix. These three peptides were synthesized, and helical content was assessed using CD spectroscopy. The measured α-helicity of each peptide was consistent with the expected predictions. These results and analysis demonstrate that the knob–socket motif functions as the basic unit of packing and presents an intuitive tool to decipher the rules governing packing in protein structure.

Macrosegregation in horizontal direct chill casting of ternary Al alloys: Investigation of solid motion

Macrosegregation in direct chill casting processes is controlled by fluid flow due to the thermosolutal natural and forced convection, shrinkage, and transport of unattached solid grains. Because grain refinement is usually used in aluminum direct chill casting, some effort must be made to model free-floating solid grains, and their attachment to a rigid mushy zone. Criteria for attachment vary, but many are based on using a critical solid packing fraction, which is treated as uniform and constant throughout the domain. In the case of horizontal casting (HDC), gravity acts perpendicularly to the casting direction, and the assumption of a uniform packing fraction cannot be applied because the solid particles attach to some surfaces by settling and others by being swept into the rigid solid from below. In this simulation of HDC casting of an Al-Cu-Mg alloy, the rigid and unattached solid is tracked separately, and a rule set is developed to determine the attachment of free-floating solid. Comparison between cases with and without unattached solid movement shows qualitatively different results, particularly in bottom part of slab. Non-uniform packing fractions cause very different segregation patterns in the lower half of the ingot compared to the cases with no solid movement, less segregation near centerline compared to uniform packing fraction cases, and positive segregation near the place where inlet jet impinges on the mushy zone.