Bimodal Mixture Research Articles

Impact-induced compaction of primitive solar system solids: The need for mesoscale modelling and experiments

Primitive solar system solids were accreted as highly porous bimodal mixtures of mm-sized chondrules and sub-μm matrix grains. To understand the compaction and lithification of these materials by shock, it is necessary to investigate the process at the mesoscale; i.e., the scale of individual chondrules. Here we document simulations of hypervelocity compaction of primitive materials using the iSALE shock physics model. We compare the numerical methods employed here with shock compaction experiments involving bimodal mixtures of glass beads and silica powder and find good agreement in bulk material response between the experiments and models. The heterogeneous response to shock of bimodal porous mixtures with a composition more appropriate for primitive solids was subsequently investigated: strong temperature dichotomies between the chondrules and matrix were observed (non-porous chondrules remained largely cold, while the porous matrix saw temperature increases of 100’s K). Matrix compaction was heterogeneous, and post-shock porosity was found to be lower on the lee-side of chondrules. The strain in the matrix was shown to be higher near the chondrule rims, in agreement with observations from meteorites. Chondrule flattening in the direction of the shock increases with increasing impact velocity, with flattened chondrules oriented with their semi-minor axis parallel to the shock direction.

Experimental study of the effect of grain sizes in a bimodal mixture on bed slope, bed texture, and the transition to washload

AbstractWhen fine sediment is added to a coarse‐grained system, the mobility and composition of the bed can change dramatically. We conducted a series of flume experiments to determine how the size of fine particles introduced to an active gravel bed influences the mobility and composition of the bed. We initiated our experiments using a constant water discharge and feed rate of gravel. After the system reached steady state, we doubled the feed rate by supplying a second sediment of equal or lesser size, creating size ratios from 1:1 to 1:150. As we decreased the relative size of the fine particles, the system transitioned among three regimes: (1) For particle size ratios close to one, the bed slope increased to transport the additional load of similar‐sized particles. The bed surface remained planar and unchanged. (2) For intermediate particle size ratios, the bed slope decreased with the additional fines. The bed surface became patchy with regions of fine and coarse grains. (3) For the largest particle size ratios (the smallest fines), the bed slope remained relatively unchanged. The subsurface became clogged with fine sediment, but fine particles were not present in the surface layer. This third regime constitutes washload, defined by those fractions that do not affect bed‐material transport conditions. Our results indicate washload should be defined in terms of three conditions: small grain size relative to that of the bed material, full suspension based on the Rouse number, and a small rate of fine sediment supply relative to transport capacity.

On the effect of segregation on intense bimodal bed load

Open-channel two-phase flow above a granular mobile bed is studied experimentally and theoretically. In the two-phase flow, water serves as a carrying liquid for plastic grains transported as collisional contact load in the upper-stage plane bed regime. The investigation evaluates friction- and transport characteristics of the flow under the condition of intense collisional transport of grains and links them with the internal structure of the two-phase flow. The paper focusses on the effect of bimodal solids (mixed two fractions of grains of similar density and different size and shape) on the flow characteristics and internal structure. Hence, experimental results obtained for the bimodal mixture are compared with results for individual grain fractions. The experiments show that the bimodal character of the transported solids affects the layered internal structure of the flow as a result of fraction segregation due primarily to gravity (kinetic) sieving during transport. The segregation also affects the friction- and transport characteristics of intense bed load. In the paper, the effects are described and quantified.

Micromechanics of complex granular materials: a focus on small strain behavior

We study the bulk properties of isotropic and anisotropic granular assemblies using discrete element simulations and experiments. The focus is on the influence of the microstructure on the elastic properties of the aggregate. By studying three selected examples, namely assemblies of monodisperse glass beads, bidisperse mixtures in size and soft-stiff bimodal mixtures, we show that the effective moduli of a dense granular assembly can be manipulated by properly combining material characteristics and system properties.

Effects of Mixture Distribution on Localized Forced Ignition of Stratified Mixtures: A Direct Numerical Simulation Study

ABSTRACTThe influences of initial mixture distribution on localized forced ignition of globally stoichiometric stratified mixtures have been analyzed using three-dimensional compressible direct numerical simulations. The globally stoichiometric mixtures (i.e., ) for different root-mean-square (rms) values of equivalence ratio (i.e., = 0.2, 0.4, and 0.6) and the Taylor micro-scale of equivalence ratio variation (i.e., 2.1, 5.5, and 8.3 with being the Zel’dovich flame thickness of stoichiometric mixture) have been analyzed for different initial rms values of turbulent velocity . The equivalence ratio variation is initialized following both Gaussian and bi-modal distributions for a given set of values of and in order to analyze the effects of mixture distribution. The localized forced ignition is accounted for by considering a source term in the energy conservation equation that deposits energy for a stipulated time interval. It has been demonstrated that the initial equivalence ratio distribution has significant effects on the extent of burning of stratified mixtures following successful localized forced ignition. It has been found that an increase in ) has adverse effects on the burned gas mass, whereas the effects of on the extent of burning are non-monotonic and dependent on for initial bi-modal mixture distribution. The initial Gaussian mixture distribution exhibits an increase in burned gas mass with decreasing , but these cases are more prone to flame extinction for high values of than the corresponding bi-modal distribution cases. Detailed physical explanations have been provided for the observed mixture distribution, , , and dependences on the extent of burning following localized forced ignition of stratified mixtures.

Simple stochastic cellular automaton model for starved beds and implications about formation of sand topographic features in terms of sand flux

A simple stochastic cellular automaton model is proposed for simulating bedload transport, especially for cases with a low transport rate and where available sediments are very sparse on substrates in a subaqueous system. Numerical simulations show that the bed type changes from sheet flow through sand patches to ripples as the amount of sand increases; this is consistent with observations in flume experiments and in the field. Without changes in external conditions, the sand flux calculated for a given amount of sand decreases over time as bedforms develop from a flat bed. This appears to be inconsistent with the general understanding that sand flux remains unchanged under the constant-fluid condition, but it is consistent with the previous experimental data. For areas of low sand abundance, the sand flux versus sand amount (flux–density relation) in the simulation shows a single peak with an abrupt decrease, followed by a long tail; this is very similar to the flux–density relation seen in automobile traffic flow. This pattern (the relation between segments of the curve and the corresponding bed states) suggests that sand sheets, sand patches, and sand ripples correspond respectively to the free-flow phase, congested phase, and jam phase of traffic flows. This implies that sand topographic features on starved beds are determined by the degree of interference between sand particles. Although the present study deals with simple cases only, this can provide a simplified but effective modeling of the more complicated sediment transport processes controlled by interference due to contact between grains, such as the pulsatory migration of grain-size bimodal mixtures with repetition of clustering and scattering.

Effect of nanopowder ratio in bimodal powder mixture on powder injection molding

One of the significant advantages of nanopowder is activated sintering and better surface finish of the final components. However, because of its drawbacks like higher cost and low formability, the mixture of nano and micro powder emerged as an effective solution. In this research, effects of nanopowder in nano/micro bimodal feedstock were studied. Bimodal powder in feedstock was prepared with nano and micro sized powders. These powders were mixed with different nanopowder volume ratio from 0 to 50%. The effects of nanopowder vol.% on the critical solids loading was investigated with rheometer experiments. From debinding behavior, apparent debinding activation energy for each feedstock was determined. Sintering behavior of micro and bimodal feedstocks were analyzed by dilatometer and compared with die compacted specimen. Nanopowder effect on sintered density and hardness of the samples were verified as well.

Armor breakup and reformation in a degradational laboratory experiment

Abstract. Armor breakup and reformation was studied in a laboratory experiment using a trimodal mixture composed of a 1 mm sand fraction and two gravel fractions (6 and 10 mm). The initial bed was characterized by a stepwise downstream fining pattern (trimodal reach) and a downstream sand reach, and the experiment was conducted under conditions without sediment supply. In the initial stage of the experiment an armor formed over the trimodal reach. The formation of the armor under partial transport conditions led to an abrupt spatial transition in the bed slope and in the mean grain size of the bed surface, as such showing similar results to a previous laboratory experiment conducted with a bimodal mixture. The focus of the current analysis is to study the mechanisms of armor breakup. After an increase in flow rate the armor broke up and a new coarser armor quickly formed. The breakup initially induced a bed surface fining due to the exposure of the finer substrate, which was accompanied by a sudden increase in the sediment transport rate, followed by the formation of an armor that was coarser than the initial one. The reformation of the armor was enabled by the supply of coarse material from the upstream degrading reach and the presence of gravel in the original substrate sediment. Here armor breakup and reformation enabled slope adjustment such that the new steady state was closer to normal flow conditions.

When past is present: Substitutions of long-term memory for sensory evidence in perceptual judgments.

When perception is underdetermined by current sensory inputs, memories for related experiences in the past might fill in missing detail. To evaluate this possibility, we measured the likelihood of relying on long-term memory versus sensory evidence when judging the appearance of an object near the threshold of awareness. Specifically, we associated colors with shapes in long-term memory and then presented the shapes again later in unrelated colors and had observers judge the appearance of the new colors. We found that responses were well characterized as a bimodal mixture of original and current-color representations (vs. an integrated unimodal representation). That is, although irrelevant to judgments of the current color, observers occasionally anchored their responses on the original colors in memory. Moreover, the likelihood of such memory substitutions increased when sensory input was degraded. In fact, they occurred even in the absence of sensory input when observers falsely reported having seen something. Thus, although perceptual judgments intuitively seem to reflect the current state of the environment, they can also unknowingly be dictated by past experiences.

Effect of polymer molecular weight distribution on solute sequestration in two-phase partitioning bioreactors

Polymeric solids are effective absorbents in two-phase partitioning bioreactors (TPPBs) when they provide adequate absorptive capacity for the target solute, as well as the physical properties required by solid–liquid TPPB operations. This study demonstrates the influence of molecular weight distribution (MWD) on solute uptake, as measured by solute partition coefficient (PC), and mechanical strength, as measured by the polymer’s complex modulus (G∗). Experimental PC data for n-octanol absorption from aqueous solution by poly(dimethyl siloxane) (PDMS) demonstrate a decline in absorptive capacity with increasing number average molecular weight (Mn), in agreement with Flory–Huggins solution theory predictions. Importantly, MWD is shown to have no effect on solute uptake, with both unimodal and bimodal distributions generating the same PC at a given Mn. This is in contrast to G∗, whose MWD sensitivity is exploited to formulate bimodal mixtures of poly(isobutylene) (PIB) that provide high n-octanol PC values as well as satisfactory material strength. This bimodal MWD strategy to TPPB absorbent design is extended to miscible solutions of high MW PIB and cyclohexylbenzene, which are shown to generate superior n-octanol PC at a given G∗.

MESOSCALE MODELING OF IMPACT COMPACTION OF PRIMITIVE SOLAR SYSTEM SOLIDS

ABSTRACT We have developed a method for simulating the mesoscale compaction of early solar system solids in low-velocity impact events using the iSALE shock physics code. Chondrules are represented by non-porous disks, placed within a porous matrix. By simulating impacts into bimodal mixtures over a wide range of parameter space (including the chondrule-to-matrix ratio, the matrix porosity and composition, and the impact velocity), we have shown how each of these parameters influences the shock processing of heterogeneous materials. The temperature after shock processing shows a strong dichotomy: matrix temperatures are elevated much higher than the chondrules, which remain largely cold. Chondrules can protect some matrix from shock compaction, with shadow regions in the lee side of chondrules exhibiting higher porosity that elsewhere in the matrix. Using the results from this mesoscale modeling, we show how the ε − α porous-compaction model parameters depend on initial bulk porosity. We also show that the timescale for the temperature dichotomy to equilibrate is highly dependent on the porosity of the matrix after the shock, and will be on the order of seconds for matrix porosities of less than 0.1, and on the order of tens to hundreds of seconds for matrix porosities of ∼0.3–0.5. Finally, we have shown that the composition of the post-shock material is able to match the bulk porosity and chondrule-to-matrix ratios of meteorite groups such as carbonaceous chondrites and unequilibrated ordinary chondrites.

Effect of (0–40) wt. % Si addition to Al on the thermal conductivity and thermal expansion of diamond/Al composites by pressure infiltration

This study pertained to the effect of (0–40) wt.% Si addition to Al on the thermal conductivities (TC) and the coefficient of thermal expansion (CTE) of diamond/Al composites by pressure infiltration. The fracture surfaces and interface microstructures by metal electro-etching were discussed with respect to Al matrix chemistry and diamond particles size. The results revealed that the diamond (single size)/Al–12.2Si (in wt.-pct) composites exhibited higher thermal conductivity of 532 W/m K due to the better interface bonding, which is over 90% of the theoretical prediction by the Hasselman–Johnson Scheme. At the interfaces, SiC was found and the undesired interface formation of Al4C3 was reduced. The CTE of diamond (single size)/Al–40Si (in wt.-pct) composites exhibited the average value of 4.5 × 10−6/K, but the TC value declined to 314 W/m K due to the inhomogeneous enrichment of Si. The bimodal mixtures of diamond particles (106–125 μm: 45–38 μm = 3:1) could improve TC and CTE of composites.

Effects of Bimodal and Monomodal SiC Particle on the Thermal Properties of SiC-Particle-Dispersed Al-Matrix Composite Fabricated by SPS

Silicon carbide (SiC)‐particle‐dispersed‐aluminum (Al) matrix composites were fabricated in continuous solid‐liquid co‐existent state by spark plasma sintering (SPS) process from the mixture of SiC powders, Al powders and Al‐5mass%Si alloy powders. As the SiC powders, two kinds of powders, monomodal SiC powders of 109.8μm in diameter and a bimodal SiC powder mixture of 109.8μm and 14.3μm in diameter, were used. The microstructures and thermal conductivities of the composites fabricated were examined. These composites were all well consolidated by heating at a temperature range between 798K and 876K for 1.56ks during SPS process. No reaction at the interface between the SiC particle and the Al matrix was observed by scanning electron microscopy for the composites fabricated under the sintering conditions employed in the present study. The relative packing density of the monomodal composite decreased from 99.8% to 95.1% with increasing SiC volume fraction in a range of 55% and 65%, whereas that of the bimodal composite was 97% or higher than that in a SiC volume fraction range up to 70%. The thermal conductivity of the bimodal composite was higher than that of the monomodal composite in a SiC volume fraction range higher than 55%. The coefficient of thermal expansion of the composites falls in the upper line of Kerner’s model, indicating strong bonding between the SiC particle and the Al matrix in the composite.

Size-based characterization of nanoparticle mixtures by the inline coupling of capillary electrophoresis to Taylor dispersion analysis

Separation of closely related nanoparticles is still a challenging issue for the characterization of complex mixtures for industrial/research applications or regulatory purposes. In this work, the remarkable separating performances of CE were complemented with the absolute size-based determination provided by Taylor dispersion analysis (TDA) for the characterization of nanoparticle mixtures. The inline hyphenation of CE to TDA was successfully implemented for the baseline separation followed by a size-based characterization of a bimodal mixture containing two closely size-related nanolatexes (70nm and 56nm radii). A pixel sensor UV area imager providing three detection points along the capillary was used for a differential measurement of the peak broadening during the Taylor dispersion step. Comparison of this new technique with dynamic light scattering and hydrodynamic chromatography is also discussed.

Ligand-Unsymmetrical Phenoxyiminato Dinickel Catalyst for High Molecular Weight Long-Chain Branched Polyethylenes.

An unsymmetrical bimetallic catalyst, (CF3/SO2)FI2-Ni2, having a CF3-functionalized phenoxyiminato Ni(II) center, which produces linear high-Mw polyolefin (CF3/Ni) joined to an adjacent SO2-functionalized phenoxyiminato Ni(II) center, which produces highly branched low-Mw polyolefin (SO2/Ni), was synthesized and fully characterized. In ethylene homopolymerizations, (CF3/SO2)FI2-Ni2 affords monomodal (Đ = 1.7), highly, long-chain branched (69 branches/1000 C) polyethylenes with Mw = 24 kg/mol. In contrast, bimetallic (CF3)FI2-Ni2 and (SO2)FI2-Ni2 produce high-Mw (25 kg/mol) exclusively methyl-branched (40 branches/1000 C) and low-Mw (4.5 kg/mol) highly branched (105 branches/1000 C) polyethylenes, respectively, while tandem monometallic (CF3)FI-Ni + (SO2)FI-Ni catalyst mixtures yield a bimodal polyolefin mixture (Đ = 6.4).

Segregation in heaps and silos: Comparison between experiment, simulation and continuum model

Segregation is a very common phenomenon in industrial processes wherever granular materials are handled. It can occur at different stages in an industrial process and it is usually undesired. To support the industrial design, it is necessary to understand the mechanisms of segregation and to develop a tool which is able to predict the propensity of segregation depending on product and process properties.The underlying mechanisms of segregation during filling and discharge of silos are discussed in detail. To study the effects for different configurations of silos (flat bottom vs. inclined hopper; with and without belt conveyor) a bimodal mixture of differently sized particles is employed. A useful tool to support the analysis of segregation processes is the discrete element method (DEM). A calibrated and validated DEM simulation has been used to simulate a pilot scale silo with a belt conveyor at the outlet. In this context, the influence of some common procedures in DEM, which are used to reduce the computational effort (way of filling, upscaling, periodic boundary condition) as well as their effect on the measured segregation is presented.Moreover, a continuum approach is presented which fits the segregation effects using a simplified model to experimental data. This approach is useful in a first stage to predict the segregation behaviour based on a convective/diffusive model.

A study of the fixed points and spurious solutions of the deflation-based FastICA algorithm

The FastICA algorithm is one of the most popular algorithms in the domain of independent component analysis (ICA). Despite its success, it is observed that FastICA occasionally yields outcomes that do not correspond to any true solutions (known as demixing vectors) of the ICA problem. These outcomes are commonly referred to as spurious solutions. Although FastICA is a well-studied ICA algorithm, the occurrence of spurious solutions is not yet completely understood by the ICA community. In this contribution, we aim at addressing this issue. In the first part of this work, we are interested in characterizing the relationship between demixing vectors, local optimizers of the contrast function and (attractive or unattractive) fixed points of FastICA algorithm. We will show that there exists an inclusion relationship between these sets. In the second part, we investigate the possible scenarios where spurious solutions occur. It will be shown that when certain bimodal Gaussian mixture distributions are involved, there may exist spurious solutions that are attractive fixed points of FastICA. In this case, popular nonlinearities such as "Gauss" or "tanh" tend to yield spurious solutions, whereas "kurtosis" gives much more reliable results.

Improvement of Axle Load Spectra Characterization by a Mixture of Three Distributions

AbstractTraffic is one of the most important variables used in pavement design methods. There are currently two methods for characterizing wheel load repetitions: equivalent single axle load (ESAL) and load spectra, which is a more precise characterization of traffic but relies on the same input data used to calculate ESALs. Although the load spectra are described using a bimodal mixture distribution, samples taken in Colombia indicate that the load spectra exhibit an intermediate region that can be modelled with an additional distribution. This research proposes the use of a mixture of three distributions: two log-normal distributions for the description of the peaks and a normal distribution for the intermediate region, finding a closed statistical solution for the formal evaluation of its properties, and deduces the expression of the nth moment that facilitates obtaining important features of the proposed distribution. In a case of application in two geographic regions of Colombia, with more than 53,00...

Experimental investigations of graded sediment transport under unsteady flow hydrographs

Natural fluvial channels can experience significant variations in sediment transport rates under unsteady flow conditions, especially during flood hydrograph events. At present, however, there is a distinct lack of understanding of the interaction between unsteady hydrograph flow properties and temporal variability in graded sediment transport rates. In the current study, a series of parametric experiments were conducted to investigate the response of two-graded sediment beds to a range of different unsteady hydrograph flow conditions. Investigations of the total and fractional bed-load sediment transport rates revealed strong temporal variations in transport over the hydrographs, with size-dependent temporal lag effects observed between peak flow conditions and peak bed-load transport rates. Specifically, coarse gravels had increased mobility during the rising limb of the hydrographs, attaining their peak bed-load transport rate either prior to, or near, peak flow conditions. By contrast, the finer grades tended to have enhanced mobility during the receding limb of the hydrographs, with peak transport rates measured after peak flow conditions had passed. Grain size distributions measured from the collected bed-load samples also indicated material coarsening over the rising limb and fining during the receding limb, while corresponding image analysis measurements of bed surface composition showed only marginal variation over the hydrographs. Computation of total and fractional sediment yields revealed that the bimodal sediment mixture tested was transported at significantly higher rates than the uni-modal mixture over all hydrograph conditions tested. This finding indicated that the uni-modal sediment bed was inherently more stable than the bimodal bed due to the increased abundance of medium-sized gravels present in the uni-modal sediment grade. The parametric dependences established in the study have clear implications for improved understanding of fractional inter-granular effects at the bed surface and their influence on graded sediment transport processes within natural fluvial channels under flood flow hydrographs.

Impact of Particle Size of Ceramic Granule Blends on Mechanical Strength and Porosity of 3D Printed Scaffolds.

3D printing is a promising method for the fabrication of scaffolds in the field of bone tissue engineering. To date, the mechanical strength of 3D printed ceramic scaffolds is not sufficient for a variety of applications in the reconstructive surgery. Mechanical strength is directly in relation with the porosity of the 3D printed scaffolds. The porosity is directly influenced by particle size and particle-size distribution of the raw material. To investigate this impact, a hydroxyapatite granule blend with a wide particle size distribution was fractioned by sieving. The specific fractions and bimodal mixtures of the sieved granule blend were used to 3D print specimens. It has been shown that an optimized arrangement of fractions with large and small particles can provide 3D printed specimens with good mechanical strength due to a higher packing density. An increase of mechanical strength can possibly expand the application area of 3D printed hydroxyapatite scaffolds.