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At least 883 records · Page 49Linked to original sources
PubMed2005

Phase diagrams for cohesive particle mixing and segregation.

By taking a discrete view of cohesion, we develop a particle-level model which can accurately predict the extent of particle mixing and segregation in cohesive (wet) granular systems. Our model is based on a discrete characterization tool and is used to generate phase diagrams of the predicted particle behavior. These phase diagrams exhibit both mixed and segregated phases where the boundary is determined by the mechanical and surface properties of the particles, such that manipulation of surface properties and/or size/density ratios provides a method to control cohesive particle mixing and segregation. A detailed description of the phase diagram development process as well as quantitative validation of the theoretical results are reported here.

Journal Article↗
PubMed2005

Segregation mechanisms in a numerical model of a binary granular mixture.

A simple phenomenological numerical model of a binary granular mixture is developed and investigated numerically. We attempt to model a recently reported experimental system where a horizontally vibrated binary monolayer was found to exhibit a transition from a mixed to a segregated state as the filling fraction of the mixture was increased. This numerical model is found to reproduce much of the experimentally observed behavior, most importantly the transition from the mixed to the segregated state. We use the numerical model to investigate granular segregation mechanisms and explain the experimentally observed behavior.

Journal Article↗
PubMed2006

Segregation in multicomponent ceramic colloids during drying of droplets.

Studies of drying of colloidal droplets focus on unary particle systems. We report here the drying of binary and ternary powder suspensions. When multicomponent ceramic suspensions are deposited in the form of small drops (5 microl), particle segregation can occur on drying so that the upper surface of the powder residue does not match that of the bulk composition. We show that the segregation effect and the shape of the droplet residues are both related to the participation of particles in two types of flow during drying; radial flow toward the rim where the three-phase boundary becomes locked by a pile up of particles and secondly, recirculation flows in the remaining liquid driven by Marangoni stresses. Methods to control both shape and segregation are described. The phenomenon described is general and independent of the method of preparing the drops but the motivation is to obtain uniform drop shape and composition in thick film ceramic libraries in combinatorial ink-jet printing.

Journal Article↗
PubMed2006

Surface roughness effects in granular matter: influence on angle of repose and the absence of segregation.

We investigate the effect of nanoscale variations in the surface roughness of individual particles on macroscale granular flow characteristics. Experiments were conducted in circular rotating tumblers with smooth and rough 2 and 3 mm steel particles. The smooth beads had a rms surface roughness of approximately 30 to 60 nm; rough beads had a surface roughness of approximately 240 to 350 nm. The dynamic angle of repose for rough particles increased by 10 degrees to 25 degrees over that of smooth particles over a wide range of rotation speeds. Even though surface roughness affects the angle of repose, we were unable to detect any segregation of bidisperse mixtures of rough and smooth particles in the radial direction in two-dimensional (2D) tumblers. Furthermore, no axial banding segregation occurred in 3D tumblers, both cylindrical and spherical. For mixtures of smooth and rough particles, the angle of repose increased monotonically with increasing concentration of rough particles. Particle dynamics simulations verified that the dependence of the angle of repose on the concentration of rough particles can be directly related to the coefficient of friction of the particles. Simulations over a broad range of friction parameters failed to induce segregation solely from differences in the angle of repose. These results indicate that nanoscale surface roughness can affect the flowability and angle of repose of granular matter without driving demixing of the bulk granular material.

Journal Article↗
PubMed2006

Interstitial gas and density segregation of vertically vibrated granular media.

We report experimental studies of the effect of interstitial gas on mass-density segregation in a vertically vibrated mixture of equal-sized bronze and glass spheres. Sufficiently strong vibration in the presence of interstitial gas induces vertical segregation into sharply separated bronze and glass layers. We find that the segregated steady state (i.e., bronze or glass layer on top) is a sensitive function of gas pressure and viscosity, as well as vibration frequency and amplitude. In particular, we identify distinct regimes of behavior that characterize the change from bronze-on-top to glass-on-top steady state.

Journal Article↗
PubMed2000

Indium-indium pair correlation and surface segregation in InGaAs alloys

In-In pair correlations and In surface segregation in In xGa 1-xAs alloys are studied by first-principles total-energy calculations. By calculating the substitution energy of a single In atom, we find that the near-surface energetics explains the observed In segregation on InGaAs(001)-beta2(2x4) surfaces. Indium surface segregation further enhances the In site selectivity, thus the long-range ordering. We find that the [110] and [001] In-In pair correlations are repulsive and nearly isotropic in bulk but are highly anisotropic near the (001) surface. The sign of the [110] In-In interaction energies vs the distance from the surface is oscillatory. These findings explain the recent puzzling cross-sectional x-STM results.

Journal Article↗
PubMed2002

Induced anticlinic ordering and nanophase segregation of bow-shaped molecules in a smectic solvent.

Recent experiments indicate that doping low concentrations of bent-core molecules into calamitic smectic solvents can induce anticlinic and biaxial smectic phases. We have carried out Monte Carlo simulations of mixtures of rodlike molecules (hard spherocylinders with length/breadth ratio L(rod)/D = 5) and bow-shaped molecules (hard spherocylinder dimers with length/breadth ratio L(ban)/D = 5 or 2.5 and opening angle psi). We find that a low concentration ( 3%) of L(ban)/D = 5 dimers induces anticlinic ( SmC(A)) ordering in an untilted smectic ( SmA) phase for 100 < or = psi < 150. For L(ban)/D = 2.5, no tilted phases are induced. However, with decreasing psi we observe a sharp transition from intralamellar nanophase segregation (bow-shaped molecules segregated within smectic layers) to interlamellar nanophase segregation (bow-shaped molecules concentrated between smectic layers) near psi = 130.

Journal Article↗
PubMed2002

Granular segregation as a critical phenomenon.

We present the results of an experimental study of patterned segregation in a horizontally shaken shallow layer of a binary mixture of dry particles. An order parameter for the segregated domains is defined and the effect of the variation of the combined filling fraction, C, of the mixture on the observed pattern formation is systematically studied. We find that there is a critical event associated with the onset of segregation, at C(c)=0.647+/-0.049, which has the characteristics of a continuous phase transition, including critical slowing down.

Journal Article↗
PubMed2003

Cu-Ag (111) polymorphism induced by segregation and advacancies.

Chemical and structural phase transitions induced by Ag surface segregation in the dilute Cu(Ag) (111) system have been investigated by Monte Carlo simulations. The polymorphism observed when depositing Ag on Cu (111) is proven to exist also in equilibrium segregation. If the segregation isotherms are not very sensitive to the superstructures, we show that the superstructure observed in the high part of the isotherm depends strongly on the number of advacancies.

Journal Article↗
PubMed2004

Understanding the magnetocatalytic effect: magnetism as a driving force for surface segregation.

The magnetocatalytic (or Hedvall) effect refers to a change in the rate of a chemical reaction on a magnetic surface at the Curie point T(C). For Ni catalysts, experiments suggest the effect is related to a sudden increase in segregated surface C, a strong catalytic poison, at temperatures below T(C). However, the connection between magnetism and surface segregation is not understood. Using density functional theory and spin-dynamics simulations, we show that the solubility of C in Ni is significantly reduced in the ferromagnetic state, because C suppresses Ni magnetism and thereby increases the heat of solution. This explains the observed increase in C segregation and the reduced catalytic activity below T(C).

Journal Article↗
PubMed2004

Vibration-induced granular segregation: a phenomenon driven by three mechanisms.

The segregation of large spheres in a granular bed under vertical vibrations is studied. In our experiments, we systematically measure rise times as a function of density, diameter, and depth, for two different sinusoidal excitations. The measurements reveal that, at low frequencies, inertia and convection are the only mechanisms behind segregation. Inertia (convection) dominates when the relative density is greater (less) than one. At high frequencies, where convection is suppressed, fluidization of the granular bed causes either buoyancy or sinkage and segregation occurs.

Journal Article↗
PubMed2006

Granular species segregation under vertical tapping: effects of size, density, friction, and shaking amplitude.

We present extensive molecular dynamics simulations on species segregation in a granular mixture subject to vertical taps. We discuss how grain properties, e.g., size, density, friction, as well as shaking properties, e.g., amplitude and frequency, affect such a phenomenon. Both the Brazil nut effect (larger particles on the top, BN) and the reverse Brazil nut effect (larger particles on the bottom, RBN) are found and we derive the system comprehensive "segregation diagram" and the BN to RBN crossover line. We also discuss the role of friction and show that particles which differ only for their frictional properties segregate in states depending on the tapping acceleration and frequency.

Journal Article↗
PubMed1993

Photosynthetic Characteristics of Segregates from Hybrids between Flaveria brownii (C4 Like) and Flaveria linearis (C3-C4).

Characteristics related to C4 photosynthesis were studied in reciprocal F1 hybrids and F2 plants from Flaveria brownii (C4 like) and Flaveria linearis (C3-C4). The reciprocal F1 plants differed in 13C/12C ratios of leaves and the percentage of 14C initially incorporated into C4 acids, being more like the pollen parents in these traits. They did not differ in apparent photosynthesis or in O2 inhibition of apparent photosynthesis and differed only slightly in CO2 compensation concentration at 175 [mu]mol quanta m-2 s-1 and 400 mL L-1 O2. The 13C/12C ratios of 78 F2 progeny from the two F1 plants exhibited a normal distribution centered between those of the parents, with a few values slightly higher and lower than the parents. Apparent photosynthesis at 130 [mu]L L-1 CO2 and inhibition of photosynthesis by O2 was nearly normally distributed in the F2 population, but no values for F2 plants approached those for F. brownii (15.4 [mu]mol m-2 s-1 and 7.8%, respectively). Distribution of the CO2 compensation concentration measured at 1000 [mu]mol quanta m-2 s-1 and 400 mL L-1 of O2 in the F2 population was skewed toward F. brownii with 72% of the progeny having values <9 [mu]L of CO2 L-1 compared to 1.5 and 27.2 [mu]L L-1 for F. brownii and F. linearis, respectively. Correlations among traits of F2 plants were low (coefficients of 0.30 to -0.49), indicating that the C4- related traits are not closely linked in segregating populations. Plants in the F2 population selected for high or low apparent photosynthesis at 130 [mu]L of CO2 L-1 (six each) did not rank consistently high or low for 13C/12C ratios, O2 inhibition of apparent photosynthesis, CO2 compensation concentration, or activities of phosphoenolpyruvate carboxylase or NADP-malic enzyme. This study confirms results of earlier work that indicates independent segregation of C4 traits and also shows that the C4-like parental type can be recovered, at least for some characteristics (13C/12C ratio), in segregating populations. Recovery of fully functional C4 plants awaits further experimentation with C4 x C3 or C4 x C3-C4 hybrid plants that produce fertile progeny.

Journal Article↗
PubMed2003

Persistence of unselected transgenic DNA during a plastid transformation and segregation approach to herbicide resistance.

The use of a nonlethal selection scheme, most often using the aadA gene that confers resistance to spectinomycin and streptomycin, has been considered critical for recovery of plastid transformation events. In this study, the plastid-lethal markers, glyphosate or phosphinothricin herbicides, were used to develop a selection scheme for plastids that circumvents the need for integration of an antibiotic resistance marker. The effect of selective agents on tobacco (Nicotiana tabacum) mesophyll chloroplasts was first examined by transmission electron microscopy. We found that at concentrations typically used for selection of nuclear transformants, herbicides caused rapid disintegration of plastid membranes, whereas antibiotics had no apparent effect. To overcome this apparent herbicide lethality to plastids, a "transformation segregation" scheme was developed that used two independent transformation vectors for a cotransformation approach and two different selective agents in a phased selection scheme. One transformation vector carried an antibiotic resistance (aadA) marker used for early nonlethal selection, and the other transformation vector carried the herbicide (CP4 or bar) resistance marker for use in a subsequent lethal selection phase. Because the two markers were carried on separate plasmids and were targeted to different locations on the plastid genome, we reasoned that segregation of the two markers in some transplastomic lines could occur. We report here a plastid cotransformation frequency of 50% to 64%, with a high frequency (20%) of these giving rise to transformation segregants containing exclusively the initially nonselected herbicide resistance marker. Our studies indicate a high degree of persistence of unselected transforming DNA, providing useful insights into plastid chromosome dynamics.

Aminobutyrates↗
PubMed1987

Expression and genetic segregation of parental BoLA serotypes in bovine embryos.

The detection of parentally derived BoLA serotypes was attempted in 68 bovine embryos. 23 bovine embryos were tested for genetic segregation of maternally derived BoLA serotypes. 45 bovine embryos were tested for genetic segregation of paternally derived BoLA serotypes. The expected parentally derived BoLA gene products were detectable on approximately 50% of the embryos tested. A 1:1 segregation ratio of expression or non-expression of parental BoLA serotypes in 7-day-old preimplantation bovine embryos, which is expected for codominant alleles, could not be rejected.

Animals↗
PubMed2005

The prevalence of the cysteine1584 variant of von Willebrand factor is increased in type 1 von Willebrand disease: co-segregation with increased susceptibility to ADAMTS13 proteolysis but not clinical phenotype.

The molecular pathogenesis of type 1 von Willebrand disease (VWD) is uncertain in most patients. We examined 30 type 1 VWD families in the UK Haemophilia Centre Doctors' Organization study. Heterozygosity for Y/C1584 was present in eight of 30 (27%) families and 19 of 76 (25%) individuals with type 1 VWD recruited into the study. Eighteen (95%) of these 19 individuals were blood group O. C1584 did not co-segregate with VWD in four families, and co-segregated in one family; the results were equivocal in three families. In all families increased susceptibility of von Willebrand factor (VWF) to a disintegrin and metalloprotease with thrombospondin motifs (ADAMTS) 13 proteolysis co-segregated with C1584 in affected and unaffected individuals. These data show that C1584, associated with blood group O, is prevalent among patients with type 1 VWD but not necessarily causative of disease and should not be used in isolation to diagnose VWD. Increased susceptibility of C1584 VWF to ADAMTS13 proteolysis may be physiologically significant and increase an individual's risk of bleeding and presenting with VWD.

ABO Blood-Group System↗
PubMed2005

Effects of integrated or segregated sport participation on the physical self for adolescents with intellectual disabilities.

BACKGROUND: The purpose was to examine the effects of the type of athletic programme (integrated vs. segregated) on the athletic domain of perceived competence and on general self-worth. METHODS: Participants were 32 adolescent females with intellectual disabilities (ID), divided equally into four groups: (1) segregated swimming; (2) integrated swimming; (3) adapted physical activity; and (4) sedentary. The experimental treatment was 32 months long; for sport groups, this involved 2 h of training each week and 12 competitive meets. The Harter Self-Perception Profile for Children 11 times was administrated to determine changes in perceived physical competence and general self-worth. RESULTS: Results indicated (1) no change in perceived general self-worth for the four groups; (2) significantly lower perceived athletic competence only for the integrated swimming group, despite the increase in athletic performance. CONCLUSIONS: The integrated sport participation for adolescents with ID placed in segregated school is useful but needs to be well controlled by professional in physical education.

Adaptation, Psychological↗
PubMed2006

Young people with intellectual disabilities attending mainstream and segregated schooling: perceived stigma, social comparison and future aspirations.

BACKGROUND: Mainstream schooling is a key policy in the promotion of social inclusion of young people with learning disabilities. Yet there is limited evidence about the school experience of young people about to leave mainstream as compared with segregated education, and how it impacts on their relative view of self and future aspirations. METHODS: Sixty young people with mild to moderate intellectual disabilities in their final year of secondary school participated in this study. Twenty-eight individuals came from mainstream schools and 32 attended segregated school. They completed a series of self-report measures on perceptions of stigma, social comparison to a more disabled and non-disabled peer and the likelihood involved in attaining their future goals. RESULTS: The majority of participants from both groups reported experiencing stigmatized treatment in the local area where they lived. The mainstream group reported significant additional stigma at school. In terms of social comparisons, both groups compared themselves positively with a more disabled peer and with a non-disabled peer. While the mainstream pupils had more ambitious work-related aspirations, both groups felt it equally likely that they would attain their future goals. Although the participants from segregated schools came from significantly more deprived areas and had lower scores on tests of cognitive functioning, neither of these factors appeared to have an impact on their experience of stigma, social comparisons or future aspirations. CONCLUSIONS: Irrespective of schooling environment, the young people appeared to be able to cope with the threats to their identities and retained a sense of optimism about their future. Nevertheless, negative treatment reported by the children was a serious source of concern and there is a need for schools to promote the emotional well-being of pupils with intellectual disabilities.

Adaptation, Psychological↗