Página 19 dos resultados de 179249 itens digitais encontrados em 0.393 segundos

Probing relevant ingredients in mean-field approaches for the athermal rheology of yield stress materials

Puosi, Francesco; Olivier, Julien; Martens, Kirsten
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Although the notion of mechanical noise is expected to play a key role in the non-linear rheology of athermally sheared amorphous systems, its characterization has so far remained elusive. Here, we show using molecular dynamic simulations that in spite of the presence of strong spatio-temporal correlations in the system, the local stress exhibits normal diffusion under the effect of the mechanical noise in the finite driving regime. The diffusion constant appears to be proportional to the mean plastic activity. Our data suggests that the corresponding proportionality constant is density independent, and can be directly related to the specific form of the rheological flow curve, pointing the way to a generic way of modeling mechanical noise in mean-field equations.; Comment: 8 pages, 5 figures, 1 table in Soft Matter (2015)

Universal two-step crystallization of DNA-functionalized nanoparticles

Dai, Wei; Kumar, Sanat K.; Starr, Francis W.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 12/08/2010
Relevância na Pesquisa
55.53%
We examine the crystallization dynamics of nanoparticles reversibly tethered by DNA hybridization. We show that the crystallization happens readily only in a narrow temperature "slot," and always proceeds via a two-step process, mediated by a highly-connected amorphous intermediate. For lower temperature quenches, the dynamics of unzipping strands in the amorphous state is sufficiently slow that crystallization is kinetically hindered. This accounts for the well-documented difficulty of forming crystals in these systems. The strong parallel to the crystallization behavior of proteins and colloids suggests that these disparate systems crystallize in an apparently universal manner.; Comment: Accepted for publication in Soft Matter

Self-propelled particles with selective attraction-repulsion interaction - From microscopic dynamics to coarse-grained theories

Grossmann, Robert; Schimansky-Geier, Lutz; Romanczuk, Pawel
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
In this work we derive and analyze coarse-grained descriptions of self-propelled particles with selective attraction-repulsion interaction, where individuals may respond differently to their neighbours depending on their relative state of motion (approach versus movement away). Based on the formulation of a nonlinear Fokker-Planck equation, we derive a kinetic description of the system dynamics in terms of equations for the Fourier modes of a one-particle density function. This approach allows effective numerical investigation of the stability of possible solutions of the system. The detailed analysis of the interaction integrals entering the equations demonstrates that divergences at small wavelengths can appear at arbitrary expansion orders. Further on, we also derive a hydrodynamic theory by performing a closure at the level of the second Fourier mode of the one-particle density function. We show that the general form of equations is in agreement with the theory formulated by Toner and Tu. Finally, we compare our analytical predictions on the stability of the disordered homogeneous solution with results of individual-based simulations. They show good agreement for sufficiently large densities and non-negligible short-ranged repulsion. Disagreements of numerical results and the hydrodynamic theory for weak short-ranged repulsion reveal the existence of a previously unknown phase of the model consisting of dense...

Numerical treatment of the Boltzmann equation for self-propelled particle systems

Thüroff, Florian; Weber, Christoph A.; Frey, Erwin
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 17/11/2014
Relevância na Pesquisa
55.53%
Kinetic theories constitute one of the most promising tools to decipher the characteristic spatio-temporal dynamics in systems of actively propelled particles. In this context, the Boltzmann equation plays a pivotal role, since it provides a natural translation between a particle-level description of the system's dynamics and the corresponding hydrodynamic fields. Yet, the intricate mathematical structure of the Boltzmann equation substantially limits the progress toward a full understanding of this equation by solely analytical means. Here, we propose a general framework to numerically solve the Boltzmann equation for self-propelled particle systems in two spatial dimensions and with arbitrary boundary conditions. We discuss potential applications of this numerical framework to active matter systems, and use the algorithm to give a detailed analysis to a model system of self-propelled particles with polar interactions. In accordance with previous studies, we find that spatially homogeneous isotropic and broken symmetry states populate two distinct regions in parameter space, which are separated by a narrow region of spatially inhomogeneous, density-segregated moving patterns. We find clear evidence that these three regions in parameter space are connected by first order phase transitions...

Rotational Diffusion in a Chain of Particles

Stark, Holger; Reichert, Michael; Bibette, Jerome
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 24/10/2005
Relevância na Pesquisa
55.53%
We study the coupled rotational diffusion in a two-particle chain on the basis of a Smoluchowski equation and calculate time-correlation functions that are measurable in an experiment. This might be used to explore hydrodynamic interactions in the limit where lubrication theory is valid.; Comment: 7 pages, 2 figures, to be published in J. Phys.: Condens. Matter

Geometric partition functions of cellular systems: Explicit calculation of the entropy in two and three dimensions

Blumenfeld, Raphael; Edwards, Sam F.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 22/11/2005
Relevância na Pesquisa
55.53%
A method is proposed for the characterisation of the entropy of cellular structures, based on the compactivity concept for granular packings. Hamiltonian-like volume functions are constructed both in two and in three dimensions, enabling the identification of a phase space and making it possible to take account of geometrical correlations systematically. Case studies are presented for which explicit calculations of the mean vertex density and porosity fluctuations are given as functions of compactivity. The formalism applies equally well to two- and three-dimensional granular assemblies.; Comment: 14 pages, 4 figures, to appear in The European Physical Journal E - Soft Matter

Salt-Induced Counterion-Mobility Anomaly in Polyelectrolyte Electrophoresis

Fischer, Sebastian; Naji, Ali; Netz, Roland R.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 23/05/2009
Relevância na Pesquisa
55.53%
We study the electrokinetics of a single polyelectrolyte chain in salt solution using hydrodynamic simulations. The salt-dependent chain mobility compares well with experimental DNA data. The mobility of condensed counterions exhibits a salt-dependent change of sign, an anomaly that is also reflected in the counterion excess conductivity. Using Green's function techniques this anomaly is explained by electrostatic screening of the hydrodynamic interactions between chain and counterions.; Comment: 4 pages, 3 figures

DNA toroid condensation as analytic solutions

Ishimoto, Yukitaka; Kikuchi, Norio
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 30/01/2007
Relevância na Pesquisa
55.53%
It now becomes apparent that condensed DNA toroid which emerges in a poor solvent condition can be realised in the framework of the non-linear sigma model on a line segment. In fact, the classical solutions of the model, i.e., of the bending potential exhibit toroidal forms, and fit well with the delta-function term of the attractive interaction. The proposed theory is in good agreement with experimental observations that the toroid is the ground state. In this paper, we give a rigorous proof that the solutions are indeed the exact solutions of the equations of motion with the first derivatives of the attractive interaction term. We also show a refined mapping to experimental data, considering the finite size effects of the cross section and of the chain length.; Comment: 11 pages, 3 figures (colour)

Condensate localization in a quasi-periodic structure

Eksioglu, Y.; Vignolo, P.; Tosi, M. P.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 19/05/2004
Relevância na Pesquisa
55.53%
We propose a set-up of optical laser beams by which one may realize a quasi-one-dimensional Fibonacci array of potential wells for a Bose-Einstein condensate. We use a Bose-Hubbard tight-binding model to evaluate the transport of superfluid Rb87 atoms driven by a constant force through such an array. We show that the minigaps that are generated in the spectral density-of-states by the quasi-periodic disorder give rise to prominent localization effects, which can be observed by measuring the tunnel output of matter into vacuum as a function of the intensity of the applied force.; Comment: 9 pages, 4 figures

Nucleation of colloids and macromolecules: does the nucleation pathway matter?

Lutsko, James F.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/02/2012
Relevância na Pesquisa
55.53%
A recent description of diffusion-limited nucleation based on fluctuating hydrodynamics that extends classical nucleation theory predicts a very non-classical two-step scenario whereby nucleation is most likely to occur in spatially-extended, low-amplitude density fluctuations. In this paper, it is shown how the formalism can be used to determine the maximum probability of observing \emph{any} proposed nucleation pathway, thus allowing one to address the question as to their relative likelihood, including of the newly proposed pathway compared to classical scenarios. Calculations are presented for the nucleation of high-concentration bubbles in a low-concentration solution of globular proteins and it is found that the relative probabilities (new theory compared to classical result) for reaching a critical nucleus containing $N_c$ molecules scales as $e^{-N_c/3}$ thus indicating that for all but the smallest nuclei, the classical scenario is extremely unlikely.; Comment: 7 pages, 5 figures

Controlled self-assembly of periodic and aperiodic cluster crystals

Barkan, Kobi; Engel, Michael; Lifshitz, Ron
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 17/01/2014
Relevância na Pesquisa
55.53%
Soft particles are known to overlap and form stable clusters that self-assemble into periodic crystalline phases with density-independent lattice constants. We use molecular dynamics simulations in two dimensions to demonstrate that, through a judicious design of an isotropic pair potential, one can control the ordering of the clusters and generate a variety of phases, including decagonal and dodecagonal quasicrystals. Our results confirm analytical predictions based on a mean-field approximation, providing insight into the stabilization of quasicrystals in soft macromolecular systems, and suggesting a practical approach for their controlled self-assembly in laboratory realizations using synthesized soft-matter particles.; Comment: Supplemental Material can be obtained through the author's website at: http://www.tau.ac.il/~ronlif/pubs/ClusterCrystals-Supp.pdf

Shear Banding and Spatiotemporal Oscillations in Vortex Matter in Nanostructured Superconductors

Reichhardt, C.; Reichhardt, C. J. Olson
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 16/12/2009
Relevância na Pesquisa
55.53%
We propose a simple nanostructured pinning array geometry where a rich variety of complex vortex shear banding phenomena can be realized. A single row of pinning sites is removed from a square pinning array. Shear banding effects arise when vortex motion in the pin-free channel nucleates motion of vortices in the surrounding pinned regions, creating discrete steps in the vortex velocity profile away from the channel. Near the global depinning transition, the width of the band of moving vortices undergoes oscillations or fluctuations that can span the entire system. We use simulations to show that these effects should be observable in the transport properties of the system. Similar large oscillations and shear banding effects are known to occur for sheared complex fluids in which different dynamical phases coexist.; Comment: 4 pages, 4 postscript figures

Effects of particle-size ratio on jamming of binary mixtures

Xu, Ning; Ching, Emily S. C.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 17/12/2009
Relevância na Pesquisa
55.53%
We perform a systematic numerical study of the effects of the particle-size ratio $R \ge 1$ on the properties of jammed binary mixtures. We find that changing $R$ does not qualitatively affect the critical scaling of the pressure and coordination number with the compression near the jamming transition, but the critical volume fraction at the jamming transition varies with $R$. Moreover, the static structure factor (density correlation) $S(k)$ strongly depends on $R$ and shows distinct long wave-length behaviors between large and small particles. Thus the previously reported behavior of $S(k)\sim k$ in the long wave-length limit is only a special case in the $R\to 1$ limit, and cannot be simply generalized to jammed systems with $R>1$.; Comment: 5 pages and 4 figures, submitted to Soft Matter, special issue on Granular and Jammed Materials

Slow relaxation and aging phenomena at nano-scale in granular materials

Zaitsev, V. Y.; Gusev, V. E.; Tournat, V.; Richard, P.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Complex systems having metastable elements often demonstrate nearly log-time relaxations and a kind of aging: repeated stimuli weaken the system's relaxational response. Granular matter is known to exhibit a wealth of such behaviors, for which the role of thermal fluctuations is usually ignored. However, we demonstrate that the latter can pronouncedly affect contacting mesoscopic-scale asperities and be macroscopically observed via appropriate acoustic effects. We also propose a mechanism comprising slow relaxations and aging as intrinsic properties of a wide class of systems with metastable states.; Comment: 5 pages, 4 figures

Excitation energy after a smooth quench in a Luttinger liquid

Dziarmaga, Jacek; Tylutki, Marek
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Low energy physics of quasi-one-dimensional ultracold atomic gases is often described by a gapless Luttinger liquid (LL). It is nowadays routine to manipulate these systems by changing their parameters in time but, no matter how slow the manipulation is, it must excite a gapless system. We study a smooth change of parameters of the LL (a smooth "quench") with a variable quench time and find that the excitation energy decays with an inverse power of the quench time. This universal exponent is -2 at zero temperature, and -1 for slow enough quenches at finite temperature. The smooth quench does not excite beyond the range of validity of the low energy LL description.; Comment: 5 pages, 2 figures; minor changes, version published in Phys. Rev. B

Ejection dynamics of a ring polymer out of a nanochannel

Sheng, Junfang; Luo, Kaifu
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 18/09/2011
Relevância na Pesquisa
55.53%
We investigate the ejection dynamics of a ring polymer out of a cylindrical nanochannel using both theoretical analysis and three dimensional Langevin dynamics simulations. The ejection dynamics for ring polymers shows two regimes like for linear polymers, depending on the relative length of the chain compared with the channel. For long chains with length $N$ larger than the critical chain length $N_{c}$, at which the chain just fully occupies the nanochannel, the ejection for ring polymers is faster compared with linear chains of identical length due to a larger entropic pulling force; while for short chains ($N

Surveying the Free Energy Landscapes of Continuum Models: Application to Soft Matter Systems

Kusumaatmaja, Halim
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 13/02/2015
Relevância na Pesquisa
55.53%
A variety of methods are developed for characterising the free energy landscapes of continuum, Landau-type free energy models. Using morphologies of lipid vesicles and a multistable liquid crystal device as examples, I show that the methods allow systematic study of not only the most relevant minimum energy configurations, but also the transition pathways between any two minima, as well as their corresponding energy barriers and transition state configurations. A global view of the free energy landscapes can then be visualized using either a disconnectivity graph or a network representation. Different forms of free energy functionals and boundary conditions can be readily implemented, thus allowing these tools to be utilised for a broad range of problems.

Effective non-additive pair potential for lock-and-key interacting particles: the role of the limited valence

Largo, Julio; Tartaglia, Piero; Sciortino, Francesco
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 14/03/2007
Relevância na Pesquisa
55.53%
Theoretical studies of self-assembly processes and condensed phases in colloidal systems are often based on effective inter-particle potentials. Here we show that developing an effective potential for particles interacting with a limited number of ``lock-and-key'' selective bonds (due to the specificity of bio-molecular interactions) requires -- beside the non-sphericity of the potential -- a (many body) constraint that prevent multiple bonding on the same site. We show the importance of retaining both valence and bond-selectivity by developing, as a case study, a simple effective potential describing the interaction between colloidal particles coated by four single-strand DNA chains.; Comment: 4 pages, 5 figures

Phase Separation of Charge-Stabilized Colloids: A Gibbs Ensemble Monte Carlo Simulation Study

Lu, Ben; Denton, Alan R.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
Fluid phase behavior of charge-stabilized colloidal suspensions is explored by applying a new variant of the Gibbs ensemble Monte Carlo simulation method to a coarse-grained one-component model with implicit microions and solvent. The simulations take as input linear-response approximations for effective electrostatic interactions -- hard-sphere-Yukawa pair potential and one-body volume energy. The conventional Gibbs ensemble trial moves are supplemented by exchange of (implicit) salt between coexisting phases, with acceptance probabilities influenced by the state dependence of the effective interactions. Compared with large-scale simulations of the primitive model, with explicit microions, our computationally practical simulations of the one-component model closely match the pressures and pair distribution functions at moderate electrostatic couplings. For macroion valences and couplings within the linear-response regime, deionized aqueous suspensions with monovalent microions exhibit separation into macroion-rich and macroion-poor fluid phases below a critical salt concentration. The resulting pressures and phase diagrams are in excellent agreement with predictions of a variational free energy theory based on the same model.; Comment: 11 pages...

Local origin of global contact numbers in frictional ellipsoid packings

Schaller, Fabian M.; Neudecker, Max; Saadatfar, Mohammad; Delaney, Gary; Schröder-Turk, Gerd E.; Schröter, Matthias
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.53%
In particulate soft matter systems the average number of contacts $Z$ of a particle is an important predictor of the mechanical properties of the system. Using X-ray tomography, we analyze packings of frictional, oblate ellipsoids of various aspect ratios $\alpha$, prepared at different global volume fractions $\phi_g$. We find that $Z$ is a monotonously increasing function of $\phi_g$ for all $\alpha$. We demonstrate that this functional dependence can be explained by a local analysis where each particle is described by its local volume fraction $\phi_l$ computed from a Voronoi tessellation. $Z$ can be expressed as an integral over all values of $\phi_l$: $Z(\phi_g, \alpha, X) = \int Z_l (\phi_l, \alpha, X) \; P(\phi_l | \phi_g) \; d\phi_l$. The local contact number function $ Z_l (\phi_l, \alpha, X)$ describes the relevant physics in term of locally defined variables only, including possible higher order terms $X$. The conditional probability $P(\phi_l | \phi_g)$ to find a specific value of $\phi_l$ given a global packing fraction $\phi_g$ is found to be independent of $\alpha$ and $X$. Our results demonstrate that for frictional particles a local approach is not only a theoretical requirement but also feasible.; Comment: more revisions...