Página 9 dos resultados de 179249 itens digitais encontrados em 0.338 segundos

## A texture tensor to quantify deformations: the example of two-dimensional flowing foams

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 02/01/2003

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In a continuum description of materials, the stress tensor field $\bar{%
\bar{\sigma}}$ quantifies the internal forces the neighbouring regions exert on
a region of the material. The classical theory of elastic solids assumes that
$\bar{\bar{\sigma}}$ determines the strain, while hydrodynamics assumes that
$\bar{\bar{\sigma}}$ determines the strain rate. To extend both successful
theories to more general materials, which display both elastic and fluid
properties, we recently introduced a descriptor generalizing the classical
strain to include plastic deformations: the ``statistical strain'', based on
averages on microscopic details (``A texture tensor to quantify deformations''
M.Au., Y.J., J.A.G., F.G, companion paper, {\em Granular Matter}, same issue).
Here, we apply such a statistical analysis to a two-dimensional foam steadily
flowing through a constriction, a problem beyond reach of both theories, and
prove that the foam has the elastic properties of a (linear and isotropic)
continuous medium.; Comment: 4 pages, 5 figures. To appear in Granular Matter

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## Inverse Optimization Techniques for Targeted Self-Assembly

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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This article reviews recent inverse statistical-mechanical methodologies that
we have devised to optimize interaction potentials in soft matter systems that
correspond to stable "target" structures. We are interested in finding the
interaction potential, not necessarily pairwise additive or spherically
symmetric, that stabilizes a targeted many-body system by generally
incorporating complete configurational information. Unlike previous work, our
primary interest is in the possible many-body structures that may be generated,
some of which may include interesting but known structures, while others may
represent entirely new structural motifs. Soft matter systems, such as colloids
and polymers, offer a versatile means of realizing the optimized interactions.
It is shown that these inverse approaches hold great promise for controlling
self-assembly to a degree that surpasses the less-than-optimal path that nature
has provided. Indeed, we envision being able to "tailor" potentials that
produce varying degrees of disorder, thus extending the traditional idea of
self-assembly to incorporate both amorphous and crystalline structures as well
as quasicrystals. The notion of tailoring potentials that correspond to
targeted structures is motivated by the rich fundamental statistical-mechanical
issues and questions offered by this fascinating inverse problem as well as our
recent ability to identify structures that have optimal bulk properties or
desirable performance characteristics. Recent results have already led to a
deeper basic understanding of the mathematical relationship between the
collective structural behavior of many-body systems and their interactions...

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## $\mu$-model for the statics of dry granular medium

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 22/03/2006

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We propose the description of the granular matter which is based on
distribution of dry friction coefficients. Using such a concept and a simple
one-dimensional packing of grains we solve the silo problem. The friction
coefficients at contacts are determined both by geometry of packing
configuration and the stress distribution in a medium. Within such an approach
the Janssen coefficient is determined and its dependence on the
particle-particle and boundary-particle friction coefficients is obtained. Also
we investigate the conditions for the appearance of the maximum in the pressure
distribution with the depth with overweight on top. As an outcome of our work
we propose the general framework to the description of the granular matter as a
continual medium which is characterized by the field of the dry friction
tensor.; Comment: 9 pages, 7 postscript figures

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## Active colloids in liquid crystals

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 14/12/2015

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Active colloids in liquid crystals (ACLCs) is an active matter with
qualitatively new facets of behavior as compared to active matter that becomes
isotropic when relaxed into an equilibrium state. We discuss two classes of
ACLCs: (i) externally driven ACLCs, in which the motion of colloidal particles
is powered by an externally applied electric field, and (ii) internally driven
ACLCs, formed by self-propelled particles such as bacteria. The liquid crystal
(LC) medium is of a thermotropic type in the first case and lyotropic (water
based) in the second case. In the absence of external fields and self-propelled
particles, the ACLCs are inactive, with the equilibrium LC state exhibiting
long-range orientational order. The external electric field causes ACLCs of
type (i) to experience translations, rotations, and orbiting, powered by
mechanisms such as LC-enabled electrokinetics, Quincke rotations and entrapment
at the defects of LC order. A dense system of Quincke rotators, orbiting along
circularly shaped smectic defects, undergoes a transition into a collective
coherent orbiting when their activity increases. An example of internally
driven ACLCs of type (ii) is living liquid crystals, representing swimming
bacteria placed in an otherwise passive lyotropic chromonic LC. The LC strongly
affects many aspects of bacterial behavior...

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## Condensation transition in DNA-polyaminoamide dendrimer fibers studied using optical tweezers

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 30/05/2006

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Statistical Mechanics#Physics - Biological Physics#Quantitative Biology - Biomolecules

When mixed together, DNA and polyaminoamide (PAMAM) dendrimers form fibers
that condense into a compact structure. We use optical tweezers to pull
condensed fibers and investigate the decondensation transition by measuring
force-extension curves (FECs). A characteristic plateau force (around 10 pN)
and hysteresis between the pulling and relaxation cycles are observed for
different dendrimer sizes, indicating the existence of a first-order transition
between two phases (condensed and extended) of the fiber. The fact that we can
reproduce the same FECs in the absence of additional dendrimers in the buffer
medium indicates that dendrimers remain irreversibly bound to the DNA backbone.
Upon salt variation FECs change noticeably confirming that electrostatic forces
drive the condensation transition. Finally, we propose a simple model for the
decondensing transition that qualitatively reproduces the FECs and which is
confirmed by AFM images.; Comment: Latex version, 4 pages+3 color figures

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## Self-assembly of Chiral Tubules

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 19/11/2013

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The efficient and controlled assembly of complex structures from
macromolecular building blocks is a critical open question in both biological
systems and nanoscience. Using molecular dynamics simulations we study the
self-assembly of tubular structures from model macromolecular monomers with
multiple binding sites on their surfaces [Cheng et al., Soft Matter 8,
5666-5678 (2012)]. In this work we add chirality to the model monomer and a
lock-and-key interaction. The self-assembly of free monomers into tubules
yields a pitch value that often does not match the chirality of the monomer
(including achiral monomers). We show that this mismatch occurs because of a
twist deformation that brings the lateral interaction sites into alignment when
the tubule pitch differs from the monomer chirality. The energy cost for this
deformation is small as the energy distributions substantially overlap for
small differences in the pitch and chirality. In order to control the tubule
pitch by preventing the twist deformation, the interaction between the vertical
surfaces must be increased without resulting in kinetically trapped structures.
For this purpose, we employ the lock-and-key interactions and obtain good
control of the self-assembled tubule pitch. These results explain some
fundamental features of microtubules. The vertical interaction strength is
larger than the lateral in microtubules because this yields a controlled
assembly of tubules with the proper pitch. We also generally find that the
control of the assembly into tubules is difficult...

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## Jamming II: Edwards' statistical mechanics of random packings of hard spheres

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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The problem of finding the most efficient way to pack spheres has an
illustrious history, dating back to the crystalline arrays conjectured by
Kepler and the random geometries explored by Bernal in the 60's. This problem
finds applications spanning from the mathematician's pencil, the processing of
granular materials, the jamming and glass transitions, all the way to fruit
packing in every grocery. There are presently numerous experiments showing that
the loosest way to pack spheres gives a density of ~55% (RLP) while filling all
the loose voids results in a maximum density of ~63-64% (RCP). While those
values seem robustly true, to this date there is no physical explanation or
theoretical prediction for them. Here we show that random packings of
monodisperse hard spheres in 3d can pack between the densities 4/(4 + 2 \sqrt
3) or 53.6% and 6/(6 + 2 \sqrt 3) or 63.4%, defining RLP and RCP, respectively.
The reason for these limits arises from a statistical picture of jammed states
in which the RCP can be interpreted as the ground state of the ensemble of
jammed matter with zero compactivity, while the RLP arises in the infinite
compactivity limit. We combine an extended statistical mechanics approach 'a la
Edwards' (where the role traditionally played by the energy and temperature in
thermal systems is substituted by the volume and compactivity) with a
constraint on mechanical stability imposed by the isostatic condition.
Ultimately...

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## Plug flow formation and growth in da Vinci Fluids

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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A new, da Vinci, fluid is described as a model for flow of dense granular
matter. We postulate local properties of the fluid, which are generically
different from ordinary fluids in that energy is dissipated by solid friction.
We present the equation of flow of such a fluid and show that it gives rise to
formation and growth of plug flow regions, which is characteristic of flow of
granular matter. Simple explicit examples are presented to illustrate the
evolution of plug flow regions.; Comment: 5 pages, 1 figure, submitted to Phys. Rev. Lett., Corrected reference

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## Attractive Interactions Between Rod-like Polyelectrolytes: Polarization, Crystallization, and Packing

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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We study the attractive interactions between rod-like charged polymers in
solution that appear in the presence of multi-valence counterions. The
counterions condensed to the rods exhibit both a strong transversal
polarization and a longitudinal crystalline arrangement. At short distances
between the rods, the fraction of condensed counterions increases, and the
majority of these occupy the region between the rods, where they minimize their
repulsive interactions by arranging themselves into packing structures. The
attractive interaction is strongest for multivalent counterions. Our model
takes into account the hard-core volume of the condensed counterions and their
angular distribution around the rods. The hard core constraint strongly
suppresses longitudinal charge fluctuations.; Comment: 4 figures, uses revtex, psfig and epsf. The new version contains a
different introduction, and the bibliography has been expanded

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## Investigation of relaxation phenomena in high-temperature superconductors HoBa2Cu3O7-d at the action of pulsed magnetic fields

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 03/04/2007

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It is used the mechanical method of Abrikosov vortex stimulated dynamics
investigation in superconductors. With its help it was studied relaxation
phenomena in vortex matter of high-temperature superconductors. It established
that pulsed magnetic fields change the course of relaxation processes taking
place in vortex matter. The study of the influence of magnetic pulses differing
by their durations and amplitudes on vortex system of isotropic
high-temperature superconductors system HoBa2Cu3O7-d showed the presence of
threshold phenomena. The small duration pulses does not change the course of
relaxation processes taking place in vortex matter. When the duration of pulses
exceeds some critical value (threshold), then their influence change the course
of relaxation process which is revealed by stepwise change of relaxing
mechanical moment . These investigations showed that the time for formatting of
Abrikosov vortex lattice in HoBa2Cu3O7-d is of the order of 20 microsec. which
on the order of value exceeds the time necessary for formation of a single
vortex observed in type II superconductors.; Comment: 6 pages, 0 figures

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## Atomic Bose-Einstein Condensation with Three-Body Interactions and Collective Excitations

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 05/11/1999

Relevância na Pesquisa

55.55%

The stability of a Bose-Einstein condensed state of trapped ultra-cold atoms
is investigated under the assumption of an attractive two-body and a repulsive
three-body interaction. The Ginzburg-Pitaevskii-Gross (GPG) nonlinear
Schr\"odinger equation is extended to include an effective potential dependent
on the square of the density and solved numerically for the s-wave. The lowest
frequency of the collective mode is determined and its dependences on the
number of atoms and on the strength of the three-body force are studied. We
show that the addition of three-body dynamics can allow the number of condensed
atoms to increase considerably, even when the strength of the three-body force
is very small compared with the strength of the two-body force. We also observe
a first-order liquid-gas phase transition for the condensed state up to a
critical strength of the effective three-body force.; Comment: 7 eps-figures

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## Roles of energy dissipation in a liquid-solid transition of out-of-equilibrium systems

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 11/09/2015

Relevância na Pesquisa

55.56%

Self-organization of active matter as well as driven granular matter in
non-equilibrium dynamical states has attracted considerable attention not only
from the fundamental and application viewpoints but also as a model to
understand the occurrence of such phenomena in nature. These systems share
common features originating from their intrinsically out-of-equilibrium nature.
It remains elusive how energy dissipation affects the state selection in such
non-equilibrium states. As a simple model system, we consider a non-equilibrium
stationary state maintained by continuous energy input, relevant to industrial
processing of granular materials by vibration and/or flow. More specifically,
we experimentally study roles of dissipation in self-organization of a driven
granular particle monolayer. We find that the introduction of strong
inelasticity entirely changes the nature of the liquid-solid transition from
two-step (nearly) continuous transitions (liquid-hexatic-solid) to a strongly
discontinuous first-order-like one (liquid-solid), where the two phases with
different effective temperatures can coexist, unlike thermal systems, under a
balance between energy input and dissipation. Our finding indicates a pivotal
role of energy dissipation and suggests a novel principle in the
self-organization of systems far from equilibrium. A similar principle may
apply to active matter...

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## Superfluidity and collective modes in a uniform gas of Fermi atoms with a Feshbach resonance

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 10/02/2003

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Materials Science#Condensed Matter - Superconductivity

We investigate strong-coupling superfluidity in a uniform gas of Fermi atoms
attractively interacting via quasi-molecular bosons associated with a Feshbach
resonance. This interaction is tunable by the threshold energy $2\nu$ of the
Feshbach resonance, becoming large as $2\nu$ is decreased. In recent work, we
showed that the enhancement of this tunable pairing interaction naturally leads
to the BCS-BEC crossover, where the character of the superfluid phase
transition changes from the BCS-type to a BEC of composite bosons consisting of
preformed Cooper-pairs and Feshbach-induced molecules. In this paper, we extend
our previous work and study both single quasi-particles and the collective
dynamics of the superfluid phase below Tc. We show how the superfluid order
parameter changes from the Cooper-pair amplitude to the square root of the
number of condensed molecules associated with the Feshbach resonance, as the
threshold energy $2\nu$ is lowered. We also discuss the Goldstone mode
associated with superfluidity, and show how its character smoothly changes from
the Anderson-Bogoliubov phonon in the BCS regime to the Bogoliubov phonon in
the BEC regime in the BCS-BEC crossover. This Goldstone mode is shown to appear
as a resonance in the spectrum of the density-density correlation function...

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## Colloidal gelation and non-ergodicity transitions

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/12/1999

Relevância na Pesquisa

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Disordered Systems and Neural Networks#Condensed Matter - Materials Science

Within the framework of the mode coupling theory (MCT) of structural
relaxation, mechanisms and properties of non-ergodicity transitions in rather
dilute suspensions of colloidal particles characterized by strong short-ranged
attractions are studied. Results building on the virial expansion for particles
with hard cores and interacting via an attractive square well potential are
presented, and their relevance to colloidal gelation is discussed.; Comment: 10 pages, 4 figures; Talk at the Conference: "Unifying Concepts in
Glass Physics" ICTP Trieste, September 1999; to be published in J. Phys.:
Condens. Matter

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## Fundamental challenges in packing problems: from spherical to non-spherical particles

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 24/02/2014

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Random packings of objects of a particular shape are ubiquitous in science
and engineering. However, such jammed matter states have eluded any systematic
theoretical treatment due to the strong positional and orientational
correlations involved. In recent years progress on a fundamental description of
jammed matter could be made by starting from a constant volume ensemble in the
spirit of conventional statistical mechanics. Recent work has shown that this
approach, first introduced by S. F. Edwards more than two decades ago, can be
cast into a predictive framework to calculate the packing fractions of both
spherical and non-spherical particles.; Comment: Highlight article in Soft Matter

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## Sliding droplets of Xanthan solutions: a joint experimental and numerical study

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/09/2015

Relevância na Pesquisa

55.56%

#Condensed Matter - Soft Condensed Matter#Condensed Matter - Mesoscale and Nanoscale Physics#Condensed Matter - Materials Science

We have investigated the sliding of droplets made of solutions of Xanthan, a
stiff rodlike polysaccharide exhibiting a non-newtonian behavior, notably
characterized by a shear-rate dependence of the viscosity. These experimental
results are quantitatively compared with those of newtonian fluids (water). The
impact of the non-newtonian behavior on the sliding process was shown through
the relation between the average dimensionless velocity (i.e. the Capillary
number) and the dimensionless volume forces (i.e. the Bond number). To this
aim, it is needed to define operative strategies to compute the Capillary
number for the shear thinning fluids and compare with the corresponding
newtonian case. Results from experiments were complemented with lattice
Boltzmann numerical simulations of sliding droplets, aimed to disentangle the
influence that non-newtonian flow properties have on the sliding.; Comment: Submitted to: The European Physics Journal E - Special Issue
"Multi-scale phenomena in complex flows and flowing matter". 8 pages, 7
figures

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## Strain-induced partially flat band, helical snake states, and interface superconductivity in topological crystalline insulators

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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#Condensed Matter - Superconductivity#Condensed Matter - Mesoscale and Nanoscale Physics#Condensed Matter - Materials Science#Condensed Matter - Strongly Correlated Electrons

Topological crystalline insulators in IV-VI compounds host novel topological
surface states consisting of multi-valley massless Dirac fermions at low
energy. Here we show that strain generically acts as an effective gauge field
on these Dirac fermions and creates pseudo-Landau orbitals without breaking
time-reversal symmetry. We predict the realization of this phenomenon in IV-VI
semiconductor heterostructures, due to a naturally occurring misfit dislocation
array at the interface that produces a periodically varying strain field.
Remarkably, the zero-energy Landau orbitals form a flat band in the vicinity of
the Dirac point, and coexist with a network of snake states at higher energy.
We propose that the high density of states of this flat band gives rise to
interface superconductivity observed in IV-VI semiconductor multilayers at
unusually high temperatures, with non-BCS behavior. Our work demonstrates a new
route to altering macroscopic electronic properties to achieve a partially flat
band, and paves the way for realizing novel correlated states of matter.; Comment: Accepted by Nature Physics

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## Effect of chain stiffness on ion distributions around a polyelectrolyte in multivalent salt solutions

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 20/12/2009

Relevância na Pesquisa

55.56%

Ion distributions in dilute polyelectrolyte solutions are studied by means of
Langevin dynamics simulations. We show that the distributions depend on the
conformation of a chain while the conformation is determined by the chain
stiffness and the salt concentration. We observe that the monovalent
counterions originally condensed on a chain can be replaced by the multivalent
ones dissociated from the added salt due to strong electrostatic interaction.
These newly condensed ions give an important impact on the chain structure. At
low and at high salt concentrations, the conformation of a semiflexible chain
is rodlike. The ion distributions show similarity to those for a rigid chain,
but difference to those for a flexible chain whose conformation is a coil. In
the mid-salt region, the flexible chain and the semiflexible chain collapse but
the collapsed chain structures are, respectively, disordered and ordered
structures. The ion distributions hence show different profiles for these three
chain stiffness with the curves for the semiflexible chain lying between those
for the flexible and the rigid chains. The number of the condensed multivalent
counterions, as well as the effective chain charge, also shows similar
behavior, demonstrating a direct connection with the chain morphology.
Moreover...

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## Soft quasicrystals - Why are they stable?

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 05/11/2006

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In the last two years we have witnessed the exciting experimental discovery
of soft matter with nontrivial quasiperiodic long-range order - a new form of
matter termed a soft quasicrystal. Two groups have independently discovered
such order in soft matter: Zeng et al. [Nature 428 (2004) 157] in a system of
dendrimer liquid crystals; and Takano et al. [J. Polym. Sci. Polym. Phys. 43
(2005) 2427] in a system of ABC star-shaped polymers. These newly discovered
soft quasicrystals not only provide exciting platforms for the fundamental
study of both quasicrystals and of soft matter, but also hold the promise for
new applications based on self-assembled nanomaterials with unique physical
properties that take advantage of the quasiperiodicity, such as complete and
isotropic photonic band-gap materials. Here we provide a concise review of the
emerging field of soft quasicrystals, suggesting that the existence of two
natural length-scales, along with 3-body interactions, may constitute the
underlying source of their stability.

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## A Model for Aging under Deformation Field, Residual Stresses and Strains in Soft Glassy Materials

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 24/02/2015

Relevância na Pesquisa

55.56%

#Condensed Matter - Soft Condensed Matter#Condensed Matter - Mesoscale and Nanoscale Physics#Condensed Matter - Materials Science

A model is proposed that considers aging and rejuvenation in a soft glassy
material as respectively a decrease and an increase in free energy. The aging
term is weighted by inverse of characteristic relaxation time suggesting
greater mobility of the constituents induce faster aging in a material. A
dependence of relaxation time on free energy is proposed, which under quiescent
conditions, leads to power law dependence of relaxation time on waiting time as
observed experimentally. The model considers two cases namely, a constant
modulus when aging is entropy controlled and a time dependent modulus. In the
former and the latter cases the model has respectively two and three
experimentally measurable parameters that are physically meaningful. Overall
the model predicts how material undergoes aging and approaches rejuvenated
state under application of deformation field. Particularly model proposes
distinction between various kinds of rheological effects for different
combinations of parameters. Interestingly, when relaxation time evolves
stronger than linear, the model predicts various features observed in soft
glassy materials such as thixotropic and constant yield stress, thixotropic
shear banding, and presence of residual stress and strain.; Comment: 50 pages...

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