Página 9 dos resultados de 179249 itens digitais encontrados em 0.423 segundos

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

Asipauskas, Marius; Aubouy, Miguel; Glazier, James A.; Graner, François; Jiang, Yi
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 02/01/2003
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55.54%
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

Inverse Optimization Techniques for Targeted Self-Assembly

Torquato, Salvatore
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
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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...

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

Glavatskiy, K. S.; Kulinskii, V. L.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 22/03/2006
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55.54%
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

Active colloids in liquid crystals

Lavrentovich, Oleg D.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 14/12/2015
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55.54%
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...

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

Ritort, F.; Mihardja, S.; Smith, S. B.; Bustamante, C.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 30/05/2006
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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

Self-assembly of Chiral Tubules

Cheng, Shengfeng; Stevens, Mark J.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 19/11/2013
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55.54%
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...

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

Wang, Ping; Song, Chaoming; Jin, Yuliang; Makse, Hernan A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.54%
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...

Plug flow formation and growth in da Vinci Fluids

Schwartz, Moshe; Blumenfeld, Raphael
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.54%
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

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

Solis, Francisco J.; de la Cruz, Monica Olvera
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.55%
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

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

Chigvinadze, J. G.; Acrivos, J. V.; Ashimov, S. M.; Iashvili, A. A.; Machaidze, T. V.; Wolf, Th.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 03/04/2007
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55.55%
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

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

Gammal, A.; Frederico, T.; Tomio, Lauro; Chomaz, Ph.
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

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

Komatsu, Yuta; Tanaka, Hajime
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...

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

Ohashi, Y.; Griffin, A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 10/02/2003
Relevância na Pesquisa
55.56%
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...

Colloidal gelation and non-ergodicity transitions

Bergenholtz, J.; Fuchs, M.; Voigtmann, Th.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 15/12/1999
Relevância na Pesquisa
55.56%
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

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

Baule, Adrian; Makse, Hernán A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 24/02/2014
Relevância na Pesquisa
55.55%
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

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

Varagnolo, Silvia; Mistura, Giampaolo; Pierno, Matteo; Sbragaglia, Mauro
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 25/09/2015
Relevância na Pesquisa
55.56%
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

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

Tang, Evelyn; Fu, Liang
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
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

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

Wei, Yu-Fu; Hsiao, Pai-Yi
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...

Soft quasicrystals - Why are they stable?

Lifshitz, Ron; Diamant, Haim
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 05/11/2006
Relevância na Pesquisa
55.56%
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.

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

Joshi, Yogesh M.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 24/02/2015
Relevância na Pesquisa
55.56%
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...