Página 14 dos resultados de 179249 itens digitais encontrados em 0.406 segundos

Stable crystalline lattices in two-dimensional binary mixtures of dipolar particles

Assoud, Lahcen; Messina, René; Löwen, Hartmut
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
The phase diagram of binary mixtures of particles interacting via a pair potential of parallel dipoles is computed at zero temperature as a function of composition and the ratio of their magnetic susceptibilities. Using lattice sums, a rich variety of different stable crystalline structures is identified including $A_mB_n$ structures. [$A$ $(B)$ particles correspond to large (small) dipolar moments.] Their elementary cells consist of triangular, square, rectangular or rhombic lattices of the $A$ particles with a basis comprising various structures of $A$ and $B$ particles. For small (dipolar) asymmetry there are intermediate $AB_2$ and $A_2B$ crystals besides the pure $A$ and $B$ triangular crystals. These structures are detectable in experiments on granular and colloidal matter.; Comment: 6 pages - 2 figs - phase diagram updated

Swimmer-tracer scattering at low Reynolds number

Dunkel, Jörn; Putz, Victor B.; Zaid, Irwin M.; Yeomans, Julia M.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Understanding the stochastic dynamics of tracer particles in active fluids is important for identifying the physical properties of flow generating objects such as colloids, bacteria or algae. Here, we study both analytically and numerically the scattering of a tracer particle in different types of time-dependent, hydrodynamic flow fields. Specifically, we compare the tracer motion induced by an externally driven colloid with the one generated by various self-motile, multi-sphere swimmers. Our results suggest that force-free swimmers generically induce loop-shaped tracer trajectories. The specific topological structure of these loops is determined by the hydrodynamic properties of the microswimmer. Quantitative estimates for typical experimental conditions imply that the loops survive on average even if Brownian motion effects are taken into account.; Comment: 14 pages, to appear in Soft Matter

Probabilistic Phase Space Trajectory Description for Anomalous Polymer Dynamics

Panja, Debabrata
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
It has been recently shown that the phase space trajectories for the anomalous dynamics of a tagged monomer of a polymer --- for single polymeric systems such as phantom Rouse, self-avoiding Rouse, Zimm, reptation, and translocation through a narrow pore in a membrane; as well as for many-polymeric system such as polymer melts in the entangled regime --- is robustly described by the Generalized Langevin Equation (GLE). Here I show that the probability distribution of phase space trajectories for all these classical anomalous dynamics for single polymers is that of a fractional Brownian motion (fBm), while the dynamics for polymer melts between the entangled regime and the eventual diffusive regime exhibits small, but systematic deviations from that of a fBm.; Comment: 8 pages, two figures, 3 eps figure files, minor changes, supplementary material included moved to the appendix, references expanded, to appear in J. Phys.: Condens. Matter

Fluctuations of the Casimir-like force between two membrane inclusions

Bitbol, Anne-Florence; Dommersnes, Paul G.; Fournier, Jean-Baptiste
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Although Casimir forces are inseparable from their fluctuations, little is known about these fluctuations in soft matter systems. We use the membrane stress tensor to study the fluctuations of the membrane-mediated Casimir-like force. This method enables us to recover the Casimir force between two inclusions and to calculate its variance. We show that the Casimir force is dominated by its fluctuations. Furthermore, when the distance d between the inclusions is decreased from infinity, the variance of the Casimir force decreases as -1/d^2. This distance dependence shares a common physical origin with the Casimir force itself.; Comment: 5 pages, 3 figures

Theory of competitive counterion adsorption on flexible polyelectrolytes : Divalent salts

Kundagrami, Arindam; Muthukumar, M.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Counterion distribution around an isolated flexible polyelectrolyte in the presence of a divalent salt is evaluated using the adsorption model [M. Muthukumar, J. Chem. Phys. {\bf 120}, 9343 (2004)] that considers Bjerrum length, salt concentration, and local dielectric heterogeneity as physical variables in the system. Self consistent calculations of effective charge and size of polymer show that divalent counterions replace condensed monovalent counterions in competitive adsorption. The theory further predicts that at modest physical conditions, polymer charge is compensated and reversed with increasing divalent salt. Consequently, the polyelectrolyte collapses and reswells, respectively. Lower temperatures and higher degrees of dielectric heterogeneity enhance condensation of all species of ions. Complete diagram of states for the effective charge calculated as functions of Coulomb strength and salt concentration suggest that (a) overcharging requires a minimum Coulomb strenth, and (b) progressively higher presence of salt recharges the polymer due to either electrostatic screening (low Coulomb strength) or negative coion condensation (high Coulomb strength). A simple theory of ion-bridging is also presented which predicts a first-order collapse of polyelectrolytes. The theoretical predictions are in agreement with generic results from experiments and simulations.; Comment: 30 double-spaced pages (JCP type) and 12 figures

Spinning up and down a Boltzmann gas

Guery-Odelin, David
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Using the average method, we derive a close set of linear equations that describes the spinning up of an harmonically trapped gas by a rotating anisotropy. We find explicit expressions for the needed to transfer angular momentum as well as the decay time induced by a static residual anisotropy. These different time scales are compared with the measured nucleation time and lifetime of vortices by the ENS group. We find a good agreement that may emphasize the role played by the non-condensed component in thoses experiments.; Comment: 6 pages, 5 figures, submitted to Phys. Rev. A

Spinor condensates and light scattering from Bose-Einstein condensates

Stamper-Kurn, Dan M.; Ketterle, Wolfgang
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
These notes discuss two aspects of the physics of atomic Bose-Einstein condensates: optical properties and spinor condensates. The first topic includes light scattering experiments which probe the excitations of a condensate in both the free-particle and phonon regime. At higher light intensity, a new form of superradiance and phase-coherent matter wave amplification were observed. We also discuss properties of spinor condensates and describe studies of ground--state spin domain structures and dynamical studies which revealed metastable excited states and quantum tunneling.; Comment: 58 pages, 33 figures, to appear in Proceedings of Les Houches 1999 Summer School, Session LXXII

Elastic scattering loss of atoms from colliding Bose-Einstein condensate wavepackets

Band, Y. B.; Trippenbach, M.; Burke Jr., J. P.; Julienne, P. S.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Bragg diffraction of atoms by light waves has been used to create high momentum components in a Bose-Einstein condensate. Collisions between atoms from two distinct momentum wavepackets cause elastic scattering that can remove a significant fraction of atoms from the wavepackets and cause the formation of a spherical shell of scattered atoms. We develop a slowly varying envelope technique that includes the effects of this loss on the condensate dynamics described by the Gross-Pitaevski equation. Three-dimensional numerical calculations are presented for two experimental situations: passage of a moving daughter condensate through a non-moving parent condensate, and four-wave mixing of matter waves.; Comment: Phys. Rev. Lett, in press

Environment effects on the electric conductivity of the DNA

Malyshev, A. V.; Diaz, E.; Dominguez-Adame, F.; Malyshev, V. A.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
We present a theoretical analysis of the environment effects on charge transport in double-stranded synthetic poly(G)-poly(C) DNA molecules attached to two ideal leads. Coupling of the DNA to the environment results in two effects: (i) localization of carrier functions due to the static disorder and (ii) phonon-induced scattering of the carrier between these localized states, resulting in hopping conductivity. A nonlinear Pauli master equation for populations of localized states is used to describe the hopping transport and calculate the electric current as a function of the applied bias. We demonstrate that, although the electronic gap in the density of states shrinks as the disorder increases, the voltage gap in the $I-V$ characteristics becomes wider. Simple physical explanation of this effect is provided.; Comment: 8 pages, 2 figures, to appear in J. Phys.: Condens. Matter

Active Model H: Scalar Active Matter in a Momentum-Conserving Fluid

Tiribocchi, Adriano; Wittkowski, Raphael; Marenduzzo, Davide; Cates, Michael E.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
We present a continuum theory of self-propelled particles, without alignment interactions, in a momentum-conserving solvent. To address phase separation we introduce a scalar concentration field $\phi$ with advective-diffusive dynamics. Activity creates a contribution $\Sigma_{ij}=-\zeta((\partial_i\phi)(\partial_j\phi)-(\nabla\phi)^{2}\delta_{ij}/d)$ to the deviatoric stress, where $\zeta$ is odd under time reversal and $d$ is the number of spatial dimensions; this causes an effective interfacial tension contribution that is negative for contractile swimmers. We predict that domain growth then ceases at a length scale where diffusive coarsening is balanced by active stretching of interfaces, and confirm this numerically. Thus the interplay of activity and hydrodynamics is highly nontrivial, even without alignment interactions.; Comment: 7 pages, 4 figures

Stress response inside perturbed particle assemblies

Silbert, Leonardo E.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
The effect of structural disorder on the stress response inside three dimensional particle assemblies is studied using computer simulations of frictionless sphere packings. Upon applying a localised, perturbative force within the packings, the resulting {\it Green's} function response is mapped inside the different assemblies, thus providing an explicit view as to how the imposed perturbation is transmitted through the packing. In weakly disordered arrays, the resulting transmission of forces is of the double-peak variety, but with peak widths scaling linearly with distance from the source of the perturbation. This behaviour is consistent with an anisotropic elasticity response profile. Increasing the disorder distorts the response function until a single-peak response is obtained for fully disordered packings consistent with an isotropic description.; Comment: 8 pages, 7 figure captions To appear in Granular Matter

Stress overshoot in a simple yield stress fluid: an extensive study combining rheology and velocimetry

Divoux, Thibaut; Barentin, Catherine; Manneville, Sebastien
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
We report a large amount of experimental data on the stress overshoot phenomenon which takes place during start-up shear flows in a simple yield stress fluid, namely a carbopol microgel. A combination of classical rheological measurements and ultrasonic velocimetry makes it possible to get physical insights on the transient dynamics of both the stress $\sigma(t)$ and the velocity field across the gap of a rough cylindrical Couette cell during the start-up of shear under an applied shear rate $\dot\gamma$. (i) At small strains ($\gamma <1$), $\sigma(t)$ increases linearly and the microgel undergoes homogeneous deformation. (ii) At a time $t_m$, the stress reaches a maximum value $\sigma_m$ which corresponds to the failure of the microgel and to the nucleation of a thin lubrication layer at the moving wall. (iii) The microgel then experiences a strong elastic recoil and enters a regime of total wall slip while the stress slowly decreases. (iv) Total wall slip gives way to a transient shear-banding phenomenon, which occurs on timescales much longer than that of the stress overshoot and has been described elsewhere [Divoux \textit{et al., Phys. Rev. Lett.}, 2010, \textbf{104}, 208301]. This whole sequence is very robust to concentration changes in the explored range ($0.5 \le C \le 3%$ w/w). We further demonstrate that the maximum stress $\sigma_m$ and the corresponding strain $\gamma_m=\dot\gamma t_m$ both depend on the applied shear rate $\dot \gamma$ and on the waiting time $t_w$ between preshear and shear start-up: they remain roughly constant as long as $\dot\gamma$ is smaller than some critical shear rate $\dot\gamma_w\sim 1/t_w$ and they increase as weak power laws of $\dot \gamma$ for $\dot\gamma> \dot\gamma_w$ [...].; Comment: 18 pages...

Order and mobility of solid vortex matter in oscillatory driving currents

Valenzuela, Sergio O.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
We study numerically the evolution of the degree order and mobility of the vortex lattice under steady and oscillating applied forces. We show that the oscillatory motion of vortices can favor an ordered structure, even when the motion of the vortices is plastic when the same force is applied in a constant way. Our results relate the spatial order of the vortex lattice with its mobility and they are in agreement with recent experiments. We predict that, in oscillating applied forces, the lattice orients with a principal axis perpendicular to the direction of motion.; Comment: 4 pages; 3 figures

Phase transitions in simple and not so simple binary fluids

Parola, A.; Pini, D.; Reatto, L.; Tau, M.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Compared to pure fluids, binary mixtures display a very diverse phase behavior, which depends sensitively on the parameters of the microscopic potential. Here we investigate the phase diagrams of simple model mixtures by use of a microscopic implementation of the renormalization group technique. First, we consider a symmetric mixture with attractive interactions, possibly relevant for describing fluids of molecules with internal degrees of freedom. Despite the simplicity of the model, slightly tuning the strength of the interactions between unlike species drastically changes the topology of the phase boundary, forcing or inhibiting demixing, and brings about several interesting features such as double critical points, tricritical points, and coexistence domains enclosing `islands' of homogeneous, mixed fluid. Homogeneous phase separation in mixtures can be driven also by purely repulsive interactions. As an example, we consider a model of soft particles which has been adopted to describe binary polymer solutions. This is shown to display demixing (fluid-fluid) transition at sufficiently high density. The nature and the physical properties of the corresponding phase transition are investigated.; Comment: 6 pages + 3 figures, presented at the 5th EPS Liquid Matter Conference...

Viscoelastic fractures in stratified composite materials: "lenticular trumpet"

Okumura, Ko
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
We consider fractures in a stratified composite material with solid layers separated by thin slices of extremely soft matter. Viscoelastic effects associated with the soft layers are taken into account via the simplest model for weakly cross-linked polymers. We find that certain small cracks running along layers take a new "trumpet" shape quite different from previously known shapes.

A giant overshoot effect in the Janssen granular column

Ovarlez, G.; Fond, C.; Clément, E.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
We present new experimental results on the mechanical behavior of static granular assemblies confined in a vertical column. Our measurements confirm, for the first time, the universal Janssen's scaling for the stress saturation curve. We show consequently, in the context of isotropic elasticity, a relation between the Poisson ratio and granular packing fraction. Moreover, using a systematic study of the overshoot effect created by a top mass equal to the saturation mass, we show behaviors reproduced qualitatively by isotropic elastic materials but in the case of a granular assembly of a spectacular amplitude. These experimental results are strong tests for any theory of granular matter.

Debye-Hueckel-Bjerrum theory for charged colloids

Tamashiro, M. N.; Levin, Yan; Barbosa, Marcia C.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
We formulate an extension of the Debye-Hueckel-Bjerrum theory [M. E. Fisher and Y. Levin, Phys. Rev. Lett. 71, 3826 (1993)] to the fluid state of a highly asymmetric charged colloid. Allowing for the formation of clusters consisting of one polyion and n condensed counterions, the total Helmholtz free energy of the colloidal suspension is constructed. The thermodynamic properties, such as the cluster-density distribution and the pressure, are obtained by the minimization of the free energy under the constraints of fixed number of polyions and counterions. In agreement with the current experimental and Monte Carlo results, no evidence of any phase transition is encountered.; Comment: 7 pages, 3 Postscript figures, uses multicol.sty, submitted to Physica A

When Do Like Charges Attract?

Levin, Yan
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
We study the interaction potential between two polyions inside a colloidal suspension. It is shown that at large separation the interaction potential is purely repulsive, with the induced attractive interactions being doubly screened. For short separations the condensed counterions become correlated, what leads to an effective attraction between the two macromolecules.; Comment: 4 pages, 1 Postscript figure, uses multicol.sty, axodraw.sty, submitted to Phys. Rev. Lett

Evidence of Raleigh-Hertz surface waves and shear stiffness anomaly in granular media

Bonneau, L.; Andreotti, B.; Clement, E.
Tipo: Artigo de Revista Científica