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Simple models for granular force networks

Wambaugh, John F.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
A remarkable feature of static granular matter is the distribution of force along intricate networks. Even regular inter-particle contact networks produce wildly inhomogeneous force networks where certain "chains" of particles carry forces far larger than the mean. In this paper, we briefly review past theoretical approaches to understanding the geometry of force networks. We then investigate the structure of experimentally-obtained granular force networks using a simple algorithm to obtain corresponding graphs. We compare our observations with the results of geometric models, including random bond percolation, which show similar spatial distributions without enforcing vector force balance. Our findings suggest that some aspects of the mean geometry of granular force networks may be captured by these simple descriptions.; Comment: 10 pages, 18 figures. Revised version

Optimal Packings of Superballs

Jiao, Yang; Stillinger, Frank; Torquato, Sal
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 09/02/2009
Relevância na Pesquisa
55.53%
Dense hard-particle packings are intimately related to the structure of low-temperature phases of matter and are useful models of heterogeneous materials and granular media. Most studies of the densest packings in three dimensions have considered spherical shapes, and it is only more recently that nonspherical shapes (e.g., ellipsoids) have been investigated. Superballs (whose shapes are defined by |x1|^2p + |x2|^2p + |x3|^2p <= 1) provide a versatile family of convex particles (p >= 0.5) with both cubic- and octahedral-like shapes as well as concave particles (0 < p < 0.5) with octahedral-like shapes. In this paper, we provide analytical constructions for the densest known superball packings for all convex and concave cases. The candidate maximally dense packings are certain families of Bravais lattice packings. The maximal packing density as a function of p is nonanalytic at the sphere-point (p = 1) and increases dramatically as p moves away from unity. The packing characteristics determined by the broken rotational symmetry of superballs are similar to but richer than their two-dimensional "superdisk" counterparts, and are distinctly different from that of ellipsoid packings. Our candidate optimal superball packings provide a starting point to quantify the equilibrium phase behavior of superball systems...

The Influence of the Degree of Heterogeneity on the Elastic Properties of Random Sphere Packings

Schenker, Iwan; Filser, Frank T.; Hütter, Markus; Gauckler, Ludwig J.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
The macroscopic mechanical properties of colloidal particle gels strongly depend on the local arrangement of the powder particles. Experiments have shown that more heterogeneous microstructures exhibit up to one order of magnitude higher elastic properties than their more homogeneous counterparts at equal volume fraction. In this paper, packings of spherical particles are used as model structures to computationally investigate the elastic properties of coagulated particle gels as a function of their degree of heterogeneity. The discrete element model comprises a linear elastic contact law, particle bonding and damping. The simulation parameters were calibrated using a homogeneous and a heterogeneous microstructure originating from earlier Brownian dynamics simulations. A systematic study of the elastic properties as a function of the degree of heterogeneity was performed using two sets of microstructures obtained from Brownian dynamics simulation and from the void expansion method. Both sets cover a broad and to a large extent overlapping range of degrees of heterogeneity. The simulations have shown that the elastic properties as a function of the degree of heterogeneity are independent of the structure generation algorithm and that the relation between the shear modulus and the degree of heterogeneity can be well described by a power law. This suggests the presence of a critical degree of heterogeneity and...

Polyelectrolyte Persistence Length: Attractive Effect of Counterion Correlations and Fluctuations

Ariel, Gil; Andelman, David
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
The persistence length of a single, strongly charged, stiff polyelectrolyte chain is investigated theoretically. Path integral formulation is used to obtain the effective electrostatic interaction between the monomers. We find significant deviations from the classical Odijk, Skolnick and Fixman (OSF) result. An induced attraction between monomers is due to thermal fluctuations and correlations between bound counterions. The electrostatic persistence length is found to be smaller than the OSF value and indicates a possible mechanical instability (collapse) for highly charged polyelectrolytes with multivalent counterions. In addition, we calculate the amount of condensed counterions on a slightly bent polyelectrolyte. More counterions are found to be adsorbed as compared to the Manning condensation on a cylinder.; Comment: 5 pages, 1 ps figure

Lattice Boltzmann simulations of soft matter systems

Duenweg, Burkhard; Ladd, Anthony J. C.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
This article concerns numerical simulations of the dynamics of particles immersed in a continuum solvent. As prototypical systems, we consider colloidal dispersions of spherical particles and solutions of uncharged polymers. After a brief explanation of the concept of hydrodynamic interactions, we give a general overview over the various simulation methods that have been developed to cope with the resulting computational problems. We then focus on the approach we have developed, which couples a system of particles to a lattice Boltzmann model representing the solvent degrees of freedom. The standard D3Q19 lattice Boltzmann model is derived and explained in depth, followed by a detailed discussion of complementary methods for the coupling of solvent and solute. Colloidal dispersions are best described in terms of extended particles with appropriate boundary conditions at the surfaces, while particles with internal degrees of freedom are easier to simulate as an arrangement of mass points with frictional coupling to the solvent. In both cases, particular care has been taken to simulate thermal fluctuations in a consistent way. The usefulness of this methodology is illustrated by studies from our own research, where the dynamics of colloidal and polymeric systems has been investigated in both equilibrium and nonequilibrium situations.; Comment: Review article...

Timescales in creep and yielding of attractive gels

Grenard, Vincent; Divoux, Thibaut; Taberlet, Nicolas; Manneville, Sébastien
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
The stress-induced yielding scenario of colloidal gels is investigated under rough boundary conditions by means of rheometry coupled to local velocity measurements. Under an applied shear stress, the fluidization of gels made of attractive carbon black particles suspended in a mineral oil is shown to involve a previously unreported shear rate response $\dot \gamma (t)$ characterized by two well-defined and separated timescales $\tau_c$ and $\tau_f$. First $\dot \gamma(t)$ decreases as a weak power law strongly reminiscent of the primary creep observed in numerous crystalline and amorphous solids, coined the "Andrade creep." We show that the bulk deformation remains homogeneous at the micron scale, which demonstrates that if plastic events take place or if any shear transformation zone exists, such phenomena occur at a smaller scale. As a key result of this paper, the duration $\tau_c$ of this creep regime decreases as a power law of the viscous stress, defined as the difference between the applied stress and the yield stress with an exponent ranging between 2 and 3 depending on the gel concentration. The end of this first regime is marked by a jump of the shear rate by several orders of magnitude, while the gel slowly slides as a solid block experiencing strong wall slip at both walls...

Monodisperse cluster crystals: classical and quantum dynamics

Diaz-Mendez, Rogelio; Mezzacapo, Fabio; Cinti, Fabio; Lechner, Wolfgang; Pupillo, Guido
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
We study the phases and dynamics of a gas of monodisperse particles interacting via soft-core potentials in two spatial dimensions, which is of interest for soft-matter colloidal systems and quantum atomic gases. Using exact theoretical methods, we demonstrate that the equilibrium low-temperature classical phase simultaneously breaks continuous translational symmetry and dynamic space-time homogeneity, whose absence is usually associated with out-of-equilibrium glassy phenomena. This results in an exotic self-assembled cluster crystal with coexisting liquid-like long-time dynamical properties, which corresponds to a classical analog of supersolid behavior. We demonstrate that the effects of quantum fluctuations and bosonic statistics on cluster-glassy crystals are separate and competing: zero-point motion tends to destabilize crystalline order, which can be restored by bosonic statistics.; Comment: 8 pages, 7 figures

Bose-Einstein condensation thermodynamics of a trapped gas with attractive interaction

Adhikari, Sadhan K.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 07/06/2000
Relevância na Pesquisa
55.53%
We study the Bose-Einstein condensation of an interacting gas with attractive interaction confined in a harmonic trap using a semiclassical two-fluid mean-field model. The condensed state is described by converged numerical solution of the Gross-Pitaevskii equation. By solving the system of coupled equations of this model iteratively we obtain converged results for the temperature dependencies of the condensate fraction, chemical potential, and internal energy for the Bose-Einstein condensate of $^7$Li atoms.; Comment: Five latex pages, four postscript figures, Accepted in Physica A

Observation of Superfluid Flow in a Bose-Einstein Condensed Gas

Onofrio, R.; Raman, C.; Vogels, J. M.; Abo-Shaeer, J.; Chikkatur, A. P.; Ketterle, W.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 07/06/2000
Relevância na Pesquisa
55.53%
We have studied the hydrodynamic flow in a Bose-Einstein condensate stirred by a macroscopic object, a blue detuned laser beam, using nondestructive {\em in situ} phase contrast imaging. A critical velocity for the onset of a pressure gradient has been observed, and shown to be density dependent. The technique has been compared to a calorimetric method used previously to measure the heating induced by the motion of the laser beam.; Comment: 4 pages, 5 figures

Reply to the ``Comment on `quantum backaction of optical observations on Bose-Einstein condensates' ''

Leonhardt, U.; Kiss, T.; Piwnicki, P.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 28/06/2000
Relevância na Pesquisa
55.53%
In our paper we estimated the quantum backaction of dispersive imaging with off-resonant light on Bose-Einstein condensates. We have calculated the rates of the two processes involved, phase diffusion and depletion of the condensate. We compare here the depletion rate obtained within our model limitations to the Rayleigh scattering rate, both having the same physical origin: dispersive interaction of light with matter. We show that residual absorption sets indeed the limit of dispersive imaging.; Comment: 1 page (Reply to comment)

Force Induced DNA Melting

Santosh, Mogurampelly; Maiti, Prabal K
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 10/09/2008
Relevância na Pesquisa
55.53%
When pulled along the axis, double-strand DNA undergoes a large conformational change and elongates roughly twice its initial contour length at a pulling force about 70 pN. The transition to this highly overstretched form of DNA is very cooperative. Applying force perpendicular to the DNA axis (unzipping), double-strand DNA can also be separated into two single-stranded DNA which is a fundamental process in DNA replication. We study the DNA overstretching and unzipping transition using fully atomistic molecular dynamics (MD) simulations and argue that the conformational changes of double strand DNA associated with either of the above mentioned processes can be viewed as force induced DNA melting. As the force at one end of the DNA is increased the DNA start melting abruptly/smoothly after a critical force depending on the pulling direction. The critical force fm, at which DNA melts completely decreases as the temperature of the system is increased. The melting force in case of unzipping is smaller compared to the melting force when the DNA is pulled along the helical axis. In the cases of melting through unzipping, the double-strand separation has jumps which correspond to the different energy minima arising due to different base pair sequence. The fraction of Watson-Crick base pair hydrogen bond breaking as a function of force does not show smooth and continuous behavior and consists of plateaus followed by sharp jumps.; Comment: 23 pages...

Questioning the relationship between the $\chi$4 susceptibility and the dynamical correlation length in a glass former

Colin, Rémy; Alsayed, Ahmed; Gay, Cyprien; Abou, Bérengère
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Clusters of fast and slow correlated particles, identified as dynamical heterogeneities (DHs), con-stitute a central aspect of glassy dynamics. A key ingredient of the glass transition scenario is asignificant increase of the cluster size $\xi$4 as the transition is approached. In need of easy-to-computetools to measure $\xi$4 , the dynamical susceptibility $\chi$4 was introduced recently, and used in various ex-perimental works to probe DHs. Here, we investigate DHs in dense microgel suspensions using imagecorrelation analysis, and compute both $\chi$4 and the four-point correlation function G4 . The spatialdecrease of G4 provides a direct access to $\xi$4 , which is found to grow significantly with increasingvolume fraction. However, this increase is not captured by $\chi$4 . We show that the assumptions thatvalidate the connection between $\chi$4 and $\xi$4 are not fulfilled in our experiments.; Comment: The present version was accepted for publication in Soft Matter (http://pubs.rsc.org/en/journals/journalissues/sm)

Nonequilibrium thermodynamics and glassy rheology

Bouchbinder, Eran; Langer, J. S.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 21/05/2013
Relevância na Pesquisa
55.53%
Mechanically driven glassy systems and complex fluids exhibit a wealth of rheological behaviors that call for theoretical understanding and predictive modeling. A distinct feature of these nonequilibrium systems is their dynamically evolving state of structural disorder, which determines their rheological responses. Here we highlight a recently developed nonequilibrium thermodynamic framework in which the structural state is characterized by an evolving effective disorder temperature that may differ from the ordinary thermal temperature. The specific properties of each physical system of interest are described by a small set of coarse-grained internal state variables and their associated energies and entropies. The dynamics of the internal variables, together with the flow of energy and entropy between the different parts of the driven system, determine continuum-level rheological constitutive laws. We conclude with brief descriptions of several successful applications of this framework.; Comment: An invited Highlight article submitted to "Soft Matter"

Structure and Stability of Vortices in Dilute Bose-Einstein Condensates at Ultralow Temperatures

Virtanen, S. M. M.; Simula, T. P.; Salomaa, M. M.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 02/02/2001
Relevância na Pesquisa
55.53%
We compute the structure of a quantized vortex line in a harmonically trapped dilute atomic Bose-Einstein condensate using the Popov version of the Hartree-Fock-Bogoliubov mean-field theory. The vortex is shown to be (meta)stable in a nonrotating trap even in the zero-temperature limit, thus confirming that weak particle interactions induce the condensed gas a fundamental property characterizing ``classical'' superfluids. We present the structure of the vortex at ultralow temperatures and discuss the crucial effect of the thermal gas component to its energetic stability.; Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Lett

Weak violation of universality for Polyelectrolyte Chains: Variational Theory and Simulations

Migliorini, G.; Rostiashvili, V. G.; Vilgis, T. A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 07/02/2001
Relevância na Pesquisa
55.53%
A variational approach is considered to calculate the free energy and the conformational properties of a polyelectrolyte chain in $d$ dimensions. We consider in detail the case of pure Coulombic interactions between the monomers, when screening is not present, in order to compute the end-to-end distance and the asymptotic properties of the chain as a function of the polymer chain length $N$. We find $R \simeq N^{\nu}(\log N)^{\gamma}$ where $\nu = \frac{3}{\lambda+2}$ and $\lambda$ is the exponent which characterize the long-range interaction $U \propto 1/r^{\lambda}$. The exponent $\gamma$ is shown to be non-universal, depending on the strength of the Coulomb interaction. We check our findings, by a direct numerical minimization of the variational energy for chains of increasing size $2^4

Transport coefficients of multi-particle collision algorithms with velocity-dependent collision rules

Ihle, Thomas
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 15/04/2008
Relevância na Pesquisa
55.53%
Detailed calculations of the transport coefficients of a recently introduced particle-based model for fluid dynamics with a non-ideal equation of state are presented. Excluded volume interactions are modeled by means of biased stochastic multiparticle collisions which depend on the local velocities and densities. Momentum and energy are exactly conserved locally. A general scheme to derive transport coefficients for such biased, velocity dependent collision rules is developed. Analytic expressions for the self-diffusion coefficient and the shear viscosity are obtained, and very good agreement is found with numerical results at small and large mean free paths. The viscosity turns out to be proportional to the square root of temperature, as in a real gas. In addition, the theoretical framework is applied to a two-component version of the model, and expressions for the viscosity and the difference in diffusion of the two species are given.; Comment: 31 pages, 8 figures, accepted by J. Phys. Cond. Matter

A closed form for the electrostatic interaction between two rod-like charged objects

Askari, M.; Abouie, J.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 02/03/2011
Relevância na Pesquisa
55.53%
We have calculated the electrostatic interaction between two rod-like charged objects with arbitrary orientations in three dimensions. we obtained a closed form formula expressing the interaction energy in terms of the separation distance between the centers of the two rod-like objects, $r$, their lengths (denoted by $2l_1$ and $2l_2$), and their relative orientations (indicated by $\theta$ and $\phi$). When the objects have the same length ($2l_1=2l_2=l$), for particular values of separations, i.e for $r\leq0.8 l$, two types of minimum are appeared in the interaction energy with respect to $\theta$. By employing the closed form formula and introducing a scaled temperature $t$, we have also studied the thermodynamic properties of a one dimensional system of rod-like charged objects. For different separation distances, the dependence of the specific heat of the system to the scaled temperature has been studied. It is found that for $r<0.8 l$, the specific heat has a maximum.; Comment: 10 pages, 9 figures, 1 table, Accepted by J. Phys.: Condens. Matter

Computer-Generated Holographic Optical Tweezer Arrays

Dufresne, Eric R.; Spalding, Gabriel C.; Dearing, Matthew T.; Sheets, Steven A.; Grier, David G.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 28/08/2000
Relevância na Pesquisa
55.53%
Holographic techniques significantly extend the capabilities of laser tweezing, making possible extended trapping patterns for manipulating large numbers of particles and volumes of soft matter. We describe practical methods for creating arbitrary configurations of optical tweezers using computer-generated diffractive optical elements. While the discussion focuses on ways to create planar arrays of identical tweezers, the approach can be generalized to three-dimensional arrangements of heterogeneous tweezers and extended trapping patterns.; Comment: 8 pages, 9 Postscript figures, REVTeX source. For related materials, see http://griergroup.uchicago.edu/~grier/

Long-lived Giant Number Fluctuations in a Swarming Granular Nematic

Narayan, Vijay; Ramaswamy, Sriram; Menon, Narayanan
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Coherently moving flocks of birds, beasts or bacteria are examples of living matter with spontaneous orientational order. How do these systems differ from thermal equilibrium systems with such liquid-crystalline order? Working with a fluidized monolayer of macroscopic rods in the nematic liquid crystalline phase, we find giant number fluctuations consistent with a standard deviation growing linearly with the mean, in contrast to any situation where the Central Limit Theorem applies. These fluctuations are long-lived, decaying only as a logarithmic function of time. This shows that flocking, coherent motion and large-scale inhomogeneity can appear in a system in which particles do not communicate except by contact.; Comment: This is the author's version of the work. It is posted here by permission of the AAAS. The definitive version is to appear in SCIENCE

Capillary Filling of Anodized Alumina Nanopore Arrays

Alvine, K. J.; Shpyrko, O. G.; Pershan, P. S.; Shin, K.; Russell, T. P.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 05/12/2006
Relevância na Pesquisa
55.53%
The filling behavior of a room temperature solvent, perfluoromethylcyclohexane, in approximately 20 nm nanoporous alumina membranes was investigated in situ with small angle x-ray scattering. Adsorption in the pores was controlled reversibly by varying the chemical potential between the sample and a liquid reservoir via a thermal offset, $\Delta$T. The system exhibited a pronounced hysteretic capillary filling transition as liquid was condensed into the nanopores. These results are compared with Kelvin-Cohan theory, with a modified Derjaguin approximation, as well as with predictions by Cole and Saam.; Comment: 4 pages, 3 figures, pre-proofs