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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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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

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## Swimmer-tracer scattering at low Reynolds number

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 02/07/2010

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Statistical Mechanics#Physics - Biological Physics

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

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## Probabilistic Phase Space Trajectory Description for Anomalous Polymer Dynamics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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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

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## Fluctuations of the Casimir-like force between two membrane inclusions

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

55.53%

#Condensed Matter - Soft Condensed Matter#Condensed Matter - Statistical Mechanics#Physics - Biological Physics

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

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## Theory of competitive counterion adsorption on flexible polyelectrolytes : Divalent salts

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 17/01/2008

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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

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## Spinning up and down a Boltzmann gas

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 02/03/2000

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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

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## Spinor condensates and light scattering from Bose-Einstein condensates

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 28/04/2000

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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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 01/05/2000

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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

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## Environment effects on the electric conductivity of the DNA

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 24/06/2009

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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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 28/04/2015

Relevância na Pesquisa

55.53%

#Condensed Matter - Soft Condensed Matter#Condensed Matter - Statistical Mechanics#Physics - Biological Physics#Physics - Fluid Dynamics

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

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## Stress response inside perturbed particle assemblies

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 26/10/2009

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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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

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...

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## Order and mobility of solid vortex matter in oscillatory driving currents

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 12/11/2002

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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

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## Phase transitions in simple and not so simple binary fluids

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/11/2002

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...

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## Viscoelastic fractures in stratified composite materials: "lenticular trumpet"

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 04/12/2002

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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.

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## A giant overshoot effect in the Janssen granular column

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 10/12/2002

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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.

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## Debye-Hueckel-Bjerrum theory for charged colloids

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 12/02/1998

Relevância na Pesquisa

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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

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## When Do Like Charges Attract?

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 13/02/1998

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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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 27/03/2008

Relevância na Pesquisa

55.53%

Due to the non-linearity of Hertzian contacts, the speed of sound in granular
matter increases with pressure. Under gravity, the non-linear elastic
description predicts that acoustic propagation is only possible through surface
modes, called Rayleigh-Hertz modes and guided by the index gradient. Here we
directly evidence these modes in a controlled laboratory experiment and use
them to probe the elastic properties of a granular packing under vanishing
confining pressure. The shape and the dispersion relation of both transverse
and sagittal modes are compared to the prediction of non-linear elasticity that
includes finite size effects. This allows to test the existence of a shear
stiffness anomaly close to the jamming transition.; Comment: 4 pages 4 figures

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## On the effect of the thermal gas component to the stability of vortices in trapped Bose-Einstein condensates

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 12/03/2002

Relevância na Pesquisa

55.53%

We study the stability of vortices in trapped single-component Bose-Einstein
condensates within self-consistent mean-field theories--especially we consider
the Hartree-Fock-Bogoliubov-Popov theory and its recently proposed gapless
extensions. It is shown that for sufficiently repulsively interacting systems
the anomalous negative-energy modes related to vortex instabilities are lifted
to positive energies due to partial filling of the vortex core with
noncondensed gas. Such a behavior implies that within these theories the vortex
states are eventually stable against transfer of condensate matter to the
anomalous core modes. This self-stabilization of vortices, shown to occur under
very general circumstances, is contrasted to the predictions of the
non-self-consistent Bogoliubov approximation, which is known to exhibit
anomalous modes for all vortex configurations and thus implying instability of
these states. In addition, the shortcomings of these approximations in
describing the properties of vortices are analysed, and the need of a
self-consistent theory taking properly into account the coupled dynamics of the
condensate and the noncondensate atoms is emphasized.; Comment: 8 pages

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