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## Simple models for granular force networks

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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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

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## Optimal Packings of Superballs

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 09/02/2009

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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...

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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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...

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## Polyelectrolyte Persistence Length: Attractive Effect of Counterion Correlations and Fluctuations

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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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

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## Lattice Boltzmann simulations of soft matter systems

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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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...

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## Timescales in creep and yielding of attractive gels

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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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...

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## Monodisperse cluster crystals: classical and quantum dynamics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 07/06/2000

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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

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## Observation of Superfluid Flow in a Bose-Einstein Condensed Gas

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 07/06/2000

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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

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

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)

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## Force Induced DNA Melting

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 10/09/2008

Relevância na Pesquisa

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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...

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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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)

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## Nonequilibrium thermodynamics and glassy rheology

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 21/05/2013

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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"

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 02/02/2001

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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

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

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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/04/2008

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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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 02/03/2011

Relevância na Pesquisa

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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

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## Computer-Generated Holographic Optical Tweezer Arrays

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 28/08/2000

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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/

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## Long-lived Giant Number Fluctuations in a Swarming Granular Nematic

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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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

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## Capillary Filling of Anodized Alumina Nanopore Arrays

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 05/12/2006

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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

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