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PubMed1995

SMC2, a Saccharomyces cerevisiae gene essential for chromosome segregation and condensation, defines a subgroup within the SMC family.

We characterized the SMC2 (structural maintenance of chromosomes) gene that encodes a new Saccharomyces cerevisiae member of the growing family of SMC proteins. This family of evolutionary conserved proteins was introduced with identification of SMC1, a gene essential for chromosome segregation in budding yeast. The analysis of the putative structure of the Smc2 protein (Smc2p) suggests that it defines a distinct subgroup within the SMC family. This subgroup includes the ScII, XCAPE, and cut14 proteins characterized concurrently. Smc2p is a nuclear, 135-kD protein that is essential for vegetative growth. The temperature-sensitive mutation, smc2-6, confers a defect in chromosome segregation and causes partial chromosome decondensation in cells arrested in mitosis. The Smc2p molecules are able to form complexes in vivo both with Smc1p and with themselves, suggesting that they can assemble into a multimeric structure. In this study we present the first evidence that two proteins belonging to two different subgroups within the SMC family carry nonredundant biological functions. Based on genetic, biochemical, and evolutionary data we propose that the SMC family is a group of prokaryotic and eukaryotic chromosomal proteins that are likely to be one of the key components in establishing the ordered structure of chromosomes.

Amino Acid Sequence↗
PubMed1999

Mechanism of the onset of axial segregation in a rotating cylindrical drum filled with binary granular mixtures.

A phenomenon of axial band formation in a long rotating cylindrical drum filled with a binary granular mixture is described as a two-stage process. At the first stage, due to different mobility of the large and small particles, a central core occupied by a small-size fraction is formed along the axis of the drum. The axial segregation, i.e., transverse band formation, is found to be associated with the instability of the radially segregated state caused by an asymmetry of the free surface of the granular material. This instability is caused by the difference in the repose angles of two granular fractions and deviation of the filling level of the drum from 0.5.

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PubMed1999

Continuum theory of axial segregation in a long rotating drum.

We develop a continuum description for the axial segregation of granular materials in a long rotating drum based on the dynamics of the thin near-surface granular flow coupled to bulk flow. The equations of motion are reduced to the one-dimensional system for two local variables only, the concentration difference and the dynamic angle of repose, or the average slope of the free surface. The parameters of the system are established from comparison with experimental data. The resulting system describes both initial transient traveling wave dynamics and the formation of quasi-stationary bands of segregated materials. A long-term evolution proceeds through slow logarithmic coarsening of the band structure which is analogous to the spinoidal decomposition described by the Cahn-Hilliard equation.

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PubMed2002

Gravity-assisted segregation of granular materials of equal mass and size.

High-resolution segregation is demonstrated for elastic granular materials of the same mass and size. Each grain starts at a randomly selected position in the entrance facet of a cylinder, accelerates downwards due to gravity, and then bounces against a massive obstacle with a collision cross section that is proportional to the facet size. Bounce dynamics of the falling grain is a function of its relative elasticity with the obstacle. Subsequent collisions of the grain with the wall are assumed to be perfectly elastic. In the absence of interparticle collisions, grain focusing occurs at points along the cylinder axis. In the absence of rotation, focusing occurs regardless of the initial locations and (downward) velocities of the grains at the entrance facet. The focus location depends only on the coefficient of restitution of the falling particle and the obstacle size. Grains arrive at the focus in temporally localized bursts even if released simultaneously from the facet. Efficient segregation is, therefore, achieved without additional mechanical work (e.g., shaking, spinning) on the system configuration.

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PubMed2004

Surface segregation in liquid mixtures with strong interspecies attraction.

Simulations are carried out to investigate surface segregation in liquid mixtures with strong interspecies attraction. The simulations show that the majority species of particles segregates to the surface, even if the other species has the lower pure-component surface tension. This behavior is expected in metal-metalloid mixtures, such as Ni-P, and mixtures in which hydrogen bonds can form only between unlike species, such as acetone-chloroform.

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PubMed2004

Similarity-based cooperation and spatial segregation.

We analyze a cooperative game, where the cooperative act is not based on the previous behavior of the coplayer, but on the similarity between the players. This system has been studied in a mean-field description recently [Phys. Rev. E 68, 046129 (2003)]]. Here, the spatial extension to a two-dimensional lattice is studied, where each player interacts with eight players in a Moore neighborhood. The system shows a strong segregation independent of parameters. The introduction of a local conversion mechanism towards tolerance allows for four-state cycles and the emergence of spiral waves in the spatial game. In the case of asymmetric costs of cooperation a rich variety of complex behavior is observed depending on both cooperation costs. Finally, we study the stabilization of a cooperative fixed point of a forecast rule in the symmetric game, which corresponds to cooperation across segregation borders. This fixed point becomes unstable for high cooperation costs, but can be stabilized by a linear feedback mechanism.

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PubMed2006

Segregation of large granules from close-packed cluster of small granules due to buoyancy.

Segregation of large granules in a vibrofluidized granular bed with inhomogeneous granular number density distribution is studied by an event-driven algorithm. Simulation results show that the mean vertical position of large granules decreases with the increase of the density ration of the large granules to the small ones. This conclusion is consistent with the explanation that the net pressure due to the small surrounding particle impacts balances the large granular weight, and indict that the upward movement of the large granules is driven by the buoyancy. The values of temperature, density, and pressure of the systems are also computed by changing the conditions such as heating temperature on the bottom and restitution coefficient of particles. These results indicate that the segregation of large granules also happen in the systems with density inversion or even close-packed cluster of particles floating on a low-density fluid, due to the buoyancy. An equation of state is proposed to explain the buoyancy.

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PubMed2006

Dynamics of axial segregation in granular slurries: parallel experiments and influence of aspect ratio and periodic tilting.

An efficient technique for conducting rotating tumbler experiments in parallel is introduced and used to study the effect of tumbler length and periodic tilting of the tumbler on axial segregation. When rotated, bidisperse granular slurries segregate into what appear at the surface to be alternating bands of larger and smaller particles. The number of bands increases linearly with tumbler length while the fractional area occupied by each type of band is constant. Periodic tilting of the rotation axis induces a periodic axial flow of particles in the flowing layer. For the range of tilt angle amplitudes investigated (0 degrees -3.5 degrees), the number of bands decreases with increasing angle, but the rate of merging and the fractional area of bands rich in smaller particles are unaffected.

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PubMed2006

Mechanisms in the size segregation of a binary granular mixture.

A granular mixture of particles of two sizes that is shaken vertically will in most cases segregate. If the larger particles accumulate at the top of the sample, this is called the Brazil-nut effect (BNE); if they accumulate at the bottom, it is called the reverse Brazil-nut effect (RBNE). While this process is of great industrial importance in the handling of bulk solids, it is not well understood. In recent years ten different mechanisms have been suggested to explain when each type of segregation is observed. However, the dependence of the mechanisms on driving conditions and material parameters and hence their relative importance is largely unknown. In this paper we present experiments and simulations where both types of particles are made from the same material and shaken under low air pressure, which reduces the number of mechanisms to be considered to seven. We observe both BNE and RBNE by varying systematically the driving frequency and amplitude, diameter ratio, ratio of total volume of small to large particles, and overall sample volume. All our results can be explained by a combination of three mechanisms: a geometrical mechanism called void filling, transport of particles in sidewall-driven convection rolls, and thermal diffusion, a mechanism predicted by kinetic theory.

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PubMed2000

Origin of antimony segregation in GaInSb/InAs strained-layer superlattices

We show how cross-sectional scanning tunneling microscopy may be used to reconstruct the Sb segregation profiles in GaInSb /InAs strained-layer superlattices. These profiles are accurately described by a one-dimensional model parametrizing the spatial evolution of an Sb seed at the InAs-on-GaInSb interface in terms of two-anion-layer exchange. We argue that the segregation seed, which decreases from 2 / 3 to 1 / 2 monolayer when growth conditions are made less anion rich, has its origin in the Sb-bilayer reconstruction maintained during GaInSb epitaxy.

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PubMed2002

Lateral versus perpendicular segregation in mixed polymer brushes.

Grafting of incompatible polymers on a substrate prevents macrophase separation and the chains self-assemble laterally. Mixed brushes are exposed to different solvents and the morphology is observed via atomic force and x-ray photoemission microscopy. In a nonselective solvent the different species segregate into parallel cylinders ("ripple structure"). Upon exposure to a selective solvent, we encounter a transition to a "dimple" structure, in which the unfavored component forms clusters. Simultaneously, we observe an enhanced perpendicular segregation. The experimental observations are compared to self-consistent field calculations, where qualitative agreement is found.

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PubMed2002

Variational approach to hard sphere segregation under gravity.

It is demonstrated that the minimization of the free energy functional for hard spheres and hard disks yields the result that excited granular materials under gravity segregate not only in the widely known "Brazil nut" fashion, i.e., with the larger particles rising to the top, but also in reverse "Brazil nut" fashion. Specifically, the local density approximation is used to investigate the crossover between the two types of segregation occurring in the liquid state, and the results are found to agree qualitatively with previously published results of simulation and of a simple model based on condensation.

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PubMed2002

Surface segregation of Ge at SiGe(001) by concerted exchange pathways.

The segregation of Ge during growth on SiGe(001) surfaces was investigated by ab initio calculations. Four processes involving adatoms rather than ad-dimers were considered. The two most efficient channels proceed by the concerted exchange mechanism and involve a swap between an incorporated Ge and a Si adatom, or between Si and Ge in the first and the second surface layers, respectively. The calculated activation energies of approximately 1.5 eV explain well the high-temperature experimental data. Segregation mechanisms involving step edges are much less efficient.

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PubMed2002

Segregation in binary mixtures under gravity.

We employ kinetic theory for a binary mixture to study segregation by size and/or mass in a gravitational field. Simple segregation criteria are obtained for spheres and disks that are supported by numerical simulations.

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PubMed2002

Hydrodynamics of binary fluid phase segregation.

Starting with the Vlasov-Boltzmann equation for a binary fluid mixture, we derive an equation for the velocity field u when the system is segregated into two phases (at low temperatures) with a sharp interface between them. u satisfies the incompressible Navier-Stokes equations together with a jump boundary condition for the pressure across the interface which, in turn, moves with a velocity given by the normal component of u. Numerical simulations of the Vlasov-Boltzmann equations for shear flows parallel and perpendicular to the interface in a phase segregated mixture support this analysis. We expect similar behavior in real fluid mixtures.

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PubMed2002

Segregation in strongly ordering compounds: a key role of constitutional defects.

For the example of the B2 CoAl(100) surface, we demonstrate that even slight deviations from an ordered alloy's ideal stoichiometry in a subsurface region or in the bulk can drastically affect its surface composition. By experimental surface analysis and first-principles calculations, we show that Co antisite atoms segregate to the very surface, driven by the same strong interactions which enforce order in the bulk. Our findings are consistent with the lack of antisite segregation we found earlier for the much weaker ordering FeAl(100), and resolve contradictory reports for NiAl(100).

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PubMed2003

Controlling cohesive particle mixing and segregation.

We observe experimentally that particle systems that would otherwise mix can be made to segregate and vice versa simply by adding moisture. Using a newly developed theoretical approach, we generate phase diagrams that exhibit both mixed and segregated phases and show how the location of phase boundaries may be manipulated via modifying the mechanical and surface properties of the particles. These results have implications for industrial mixing/separation processes as well as novel particle production methods (e.g., engineered agglomerates with precisely prescribed compositions).

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PubMed2005

Size segregation in granular media induced by phase transition.

In order to study analytically the nature of the size segregation in granular mixtures, we introduce a mean field theory in the framework of a statistical mechanics approach, based on Edwards' original ideas. For simplicity we apply the theory to a lattice model for a hard sphere binary mixture under gravity, and we find a new purely thermodynamic mechanism that gives rise to the size segregation phenomenon. By varying the number of small grains and the mass ratio, we find a crossover from the Brazil nut to the reverse Brazil nut effect, which becomes a true phase transition when the number of small grains is larger then a critical value. We suggest that this transition is induced by the effective attraction between large grains due to the presence of small ones (depletion force). Finally the theoretical results are confirmed by numerical simulations of the 3d system under taps.

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