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## From Over-charging to Like-charge Attraction in the Weak Coupling Regime

Xing, Xiangjun; Xu, Zhenli; Ma, Hongru
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
Despite decades of intensive studies, the effective interactions between strongly charged colloids still remain elusive. Here we show that a strongly charged surface with a layer of condensed counter- ions behaves effectively as a conductor, due to the mobile nature of the condensed ions. An external source charge in its vicinity is therefore attracted towards the surface, due to the image charge effect. This mechanism leads to correlational energies for counter-ions condensed on two distinct surfaces, as well as for free ions in the bulk. Generalizing Debye-Huckel theory and image charge methods, we analytically calculate these correlation energies for the two-plates problem, at the iso-electric point, where condensed counterions precisely balance the bare surface charges. At this point, the effective interaction between two plates is always attractive at small separation and repulsive at large separation.; Comment: 5 pages, 3 eps figures

## Polymers as compressible soft spheres

D'Adamo, Giuseppe; Pelissetto, Andrea; Pierleoni, Carlo
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
We consider a coarse-grained model in which polymers under good-solvent conditions are represented by soft spheres whose radii, which should be identified with the polymer radii of gyrations, are allowed to fluctuate. The corresponding pair potential depends on the sphere radii. This model is a single-sphere version of the one proposed in Vettorel et al., Soft Matter 6, 2282 (2010), and it is sufficiently simple to allow us to determine all potentials accurately from full-monomer simulations of two isolated polymers (zero-density potentials). We find that in the dilute regime (which is the expected validity range of single-sphere coarse-grained models based on zero-density potentials) this model correctly reproduces the density dependence of the radius of gyration. However, for the thermodynamics and the intermolecular structure, the model is largely equivalent to the simpler one in which the sphere radii are fixed to the average value of the radius of gyration and radiiindependent potentials are used: for the thermodynamics there is no advantage in considering a fluctuating sphere size.; Comment: 21 pages, 7 figures

## Orientational Order in Liquids upon Condensation in Nanochannels: An Optical Birefringence Study on Rodlike and Disclike Molecules in Monolithic Mesoporous Silica

Wolff, Matthias; Knorr, Klaus; Huber, Patrick; Kityk, Andriy V.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
We present high-resolution optical birefringence measurements upon sequential filling of an array of parallel-aligned nanochannels (14~nm mean diameter) with rod-like (acetonitrile) and disc-like (hexafluorobenzene) molecules. We will demonstrate that such birefringence isotherms, when performed simultaneously with optically isotropic and index-matched counterparts (neopentane and hexafluoromethane), allow one to characterize the orientational state of the confined liquids with a high accuracy as a function of pore filling. The pore condensates are almost bulk-like, optically isotropic liquids. For both anisotropic species we find, however, a weak orientational order (of a few percent at maximum) upon film-condensation in the monolithic mesoporous membrane. It occurs upon formation of the second and third adsorbed layer, only, and vanishes gradually upon onset of capillary condensation. Presumably, it originates in the breaking of the full rotational symmetry of the interaction potential at the cylindrical, free liquid-vapor interface in the film-condensed state rather than at the silica-liquid interface. This conclusion is corroborated by comparisons of our experimental results with molecular dynamics simulations reported in the literature.; Comment: 6 pages...

## Phase behavior of hard spheres confined between parallel hard plates: Manipulation of colloidal crystal structures by confinement

Fortini, Andrea; Dijkstra, Marjolein
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
We study the phase behavior of hard spheres confined between two parallel hard plates using extensive computer simulations. We determine the full equilibrium phase diagram for arbitrary densities and plate separations from one to five hard-sphere diameters using free energy calculations. We find a first-order fluid-solid transition, which corresponds to either capillary freezing or melting depending on the plate separation. The coexisting solid phase consists of crystalline layers with either triangular or square symmetry. Increasing the plate separation, we find a sequence of crystal structures from n triangular to (n+1) square to (n+1) triangular, where n is the number of crystal layers, in agreement with experiments on colloids. At high densities, the transition between square to triangular phases are intervened by intermediate structures, e.g., prism, buckled, and rhombic phases.; Comment: 9 pages, 4 figures. Accepted for publication in J. Phys.: Condens. Matter

## Correlation between crystalline order and vitrification in colloidal monolayers

Tamborini, Elisa; Royall, C. Patrick; Cicuta, Pietro
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
We investigate experimentally the relationship between local structure and dynamical arrest in a quasi-2d colloidal model system which approximates hard discs. We introduce polydispersity to the system to suppress crystallisation. Upon compression, the increase in structural relaxation time is accompanied by the emergence of local hexagonal symmetry. Examining the dynamical heterogeneity of the system, we identify three types of motion : "zero-dimensional" corresponding to beta-relaxation, "one-dimensional" or stringlike motion and "two-dimensional" motion. The dynamic heterogeneity is correlated with the local order, that is to say locally hexagonal regions are more likely to be dynamically slow. However we find that lengthscales corresponding to dynamic heterogeneity and local structure do not appear to scale together approaching the glass transition.; Comment: 13 papes, to appear in J. Phys.: Condens. Matter

## Aging as dynamics in configuration space

Kob, Walter; Sciortino, Francesco; Tartaglia, Piero
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
The relaxation dynamics of many disordered systems, such as structural glasses, proteins, granular materials or spin glasses, is not completely frozen even at very low temperatures. This residual motion leads to a change of the properties of the material, a process commonly called aging. Despite recent advances in the theoretical description of such aging processes, the microscopic mechanisms leading to the aging dynamics are still a matter of dispute. In this Letter we investigate the aging dynamics of a simple glass former by means of molecular dynamics computer simulation. Using the concept of the inherent structure we give evidence that aging dynamics can be understood as a decrease of the effective configurational temperature T of the system. From our results we conclude that the equilibration process is faster when the system is quenched to T_c, the critical T of mode-coupling theory, and that thermodynamic concepts are useful to describe the out-of-equilibrium aging process.; Comment: Latex 4 figures

## Self-assembling DNA-caged particles: nanoblocks for hierarchical self-assembly

Licata, Nicholas A.; Tkachenko, Alexei V.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
DNA is an ideal candidate to organize matter on the nanoscale, primarily due to the specificity and complexity of DNA based interactions. Recent advances in this direction include the self-assembly of colloidal crystals using DNA grafted particles. In this article we theoretically study the self-assembly of DNA-caged particles. These nanoblocks combine DNA grafted particles with more complicated purely DNA based constructs. Geometrically the nanoblock is a sphere (DNA grafted particle) inscribed inside a polyhedron (DNA cage). The faces of the DNA cage are open, and the edges are made from double stranded DNA. The cage vertices are modified DNA junctions. We calculate the equilibriuim yield of self-assembled, tetrahedrally caged particles, and discuss their stability with respect to alternative structures. The experimental feasability of the method is discussed. To conclude we indicate the usefulness of DNA-caged particles as nanoblocks in a hierarchical self-assembly strategy.; Comment: v2: 21 pages, 8 figures; revised discussion in Sec. 2, replaced 2 figures, added new references

## Nematic twist-bend phase with nanoscale modulation of molecular orientation

Borshch, Volodymyr; Kim, Young-Ki; Xiang, Jie; Gao, Min; Jákli, Antal; Panov, Vitaly P.; Vij, Jagdish K.; Imrie, Corrie T.; Tamba, Maria-Gabriela; Mehl, Georg H.; Lavrentovich, Oleg D.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
A state of matter in which molecules show a long-range orientational order and no positional order is called a nematic liquid crystal. The best known and most widely used (for example, in modern displays) is the uniaxial nematic, with the rod-like molecules aligned along a single axis, called the director. When the molecules are chiral, the director twists in space, drawing a right-angle helicoid and remaining perpendicular to the helix axis; the structure is called a chiral nematic. In this work, using transmission electron and optical microscopy, we experimentally demonstrate a new nematic order, formed by achiral molecules, in which the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend. The oblique helicoids have a nanoscale pitch. The new twist-bend nematic represents a structural link between the uniaxial nematic (no tilt) and a chiral nematic (helicoids with right-angle tilt).; Comment: 31 pages: 8 Figures and Supplementary Information with 3 Figures

## Connecting short and long time dynamics in hard-sphere-like colloidal glasses

Pastore, Raffaele; Ciamarra, Massimo Pica; Pesce, Giuseppe; Sasso, Antonio
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
Glass-forming materials are characterized by an intermittent motion at the microscopic scale. Particles spend most of their time rattling within the cages formed by their neighbors, and seldom jump to a different cage. In molecular glass formers the temperature dependence of the jump features, such as the average caging time and jump length, characterizes the relaxation processes and allows for a short-time prediction of the diffusivity. Here we experimentally investigate the cage-jump motion of a two-dimensional hard-sphere-like colloidal suspension, where the volume fraction is the relevant parameter controlling the slowing down of the dynamics. We characterize the volume fraction dependence of the cage-jump features and show that, as in molecular systems, they allow for a short time prediction of the diffusivity.; Comment: 5 pages, 6 figures, Soft Matter 2015

## Yielding dynamics of a Herschel-Bulkley fluid: a critical-like fluidization behaviour

Divoux, Thibaut; Tamarii, David; Barentin, Catherine; Teitel, Stephen; Manneville, Sébastien
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
The shear-induced fluidization of a carbopol microgel is investigated during long start-up experiments using combined rheology and velocimetry in Couette cells of varying gap widths and boundary conditions. As already described in [Divoux et al., {\it Phys. Rev. Lett.}, 2010, {\bf 104}, 208301], we show that the fluidization process of this simple yield stress fluid involves a transient shear-banding regime whose duration $\tau_f$ decreases as a power law of the applied shear rate $\gp$. Here we go one step further by an exhaustive investigation of the influence of the shearing geometry through the gap width $e$ and the boundary conditions. While slip conditions at the walls seem to have a negligible influence on the fluidization time $\tau_f$, different fluidization processes are observed depending on $\gp$ and $e$: the shear band remains almost stationary for several hours at low shear rates or small gap widths before strong fluctuations lead to a homogeneous flow, whereas at larger values of $\gp$ or $e$, the transient shear band is seen to invade the whole gap in a much smoother way. Still, the power-law behaviour appears as very robust and hints to critical-like dynamics. To further discuss these results, we propose (i) a qualitative scenario to explain the induction-like period that precedes full fluidization and (ii) an analogy with critical phenomena that naturally leads to the observed power laws if one assumes that the yield point is the critical point of an underlying out-of-equilibrium phase transition.; Comment: 16 pages...

## Capillary leveling of stepped films with inhomogeneous molecular mobility

McGraw, Joshua D.; Salez, Thomas; Bäumchen, Oliver; Raphaël, Elie; Dalnoki-Veress, Kari
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
A homogeneous thin polymer film with a stepped height profile levels due to the presence of Laplace pressure gradients. Here we report on studies of polymeric samples with precisely controlled, spatially inhomogeneous molecular weight distributions. The viscosity of a polymer melt strongly depends on the chain length distribution; thus, we learn about thin-film hydrodynamics with viscosity gradients. These gradients are achieved by stacking two films with different molecular weights atop one another. After a sufficient time these samples can be well described as having one dimensional viscosity gradients in the plane of the film, with a uniform viscosity normal to the film. We develop a hydrodynamic model that accurately predicts the shape of the experimentally observed self-similar profiles. The model allows for the extraction of a capillary velocity, the ratio of the surface tension and the viscosity, in the system. The results are in excellent agreement with capillary velocity measurements of uniform mono- and bi-disperse stepped films and are consistent with bulk polymer rheology.; Comment: Accepted for publication in Soft Matter, Themed Issue on "The Geometry and Topology of Soft Materials"

## The Geometry of Soft Materials: A Primer

Kamien, Randall D.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
We present an overview of the differential geometry of curves and surfaces using examples from soft matter as illustrations. The presentation requires a background only in vector calculus and is otherwise self-contained.; Comment: 45 pages, RevTeX, 12 eps figures

## A micromechanical model of collapsing quicksand

Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
The discrete element method constitutes a general class of modeling techniques to simulate the microscopic behavior (i.e. at the particle scale) of granular/soil materials. We present a contact dynamics method, accounting for the cohesive nature of fine powders and soils. A modification of the model adjusted to capture the essential physical processes underlying the dynamics of generation and collapse of loose systems is able to simulate "quicksand" behavior of a collapsing soil material, in particular of a specific type, which we call "living quicksand". We investigate the penetration behavior of an object for varying density of the material. We also investigate the dynamics of the penetration process, by measuring the relation between the driving force and the resulting velocity of the intruder, leading to a "power law" behavior with exponent 1/2, i.e. a quadratic velocity dependence of the drag force on the intruder.; Comment: 5 pages, 4 figures, accepted for granular matter

## Non--Newtonian viscosity of interacting Brownian particles: comparison of theory and data

Fuchs, Matthias; Cates, Michael E.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
A recent first-principles approach to the non-linear rheology of dense colloidal suspensions is evaluated and compared to simulation results of sheared systems close to their glass transitions. The predicted scenario of a universal transition of the structural dynamics between yielding of glasses and non-Newtonian (shear-thinning) fluid flow appears well obeyed, and calculations within simplified models rationalize the data over variations in shear rate and viscosity of up to 3 decades.; Comment: 6 pages, 2 figures; J. Phys. Condens. Matter to be published (Jan. 2003)

## Dynamical heterogeneity in aging colloidal glasses of Laponite

Jabbari-Farouji, Sara; Zargar, Rojman; Wegdam, Gerard; Bonn, Daniel
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
Glasses behave as solids due to their long relaxation time; however the origin of this slow response remains a puzzle. Growing dynamic length scales due to cooperative motion of particles are believed to be central to the understanding of both the slow dynamics and the emergence of rigidity. Here, we provide experimental evidence of a growing dynamical heterogeneity length scale that increases with increasing waiting time in an aging colloidal glass of Laponite. The signature of heterogeneity in the dynamics follows from dynamic light scattering measurements in which we study both the rotational and translational diffusion of the disk-shaped particles of Laponite in suspension. These measurements are accompanied by simultaneous microrheology and macroscopic rheology experiments. We find that rotational diffusion of particles slows down at a faster rate than their translational motion. Such decoupling of translational and orientational degrees of freedom finds its origin in the dynamic heterogeneity since rotation and translation probe different length scales in the sample. The macroscopic rheology experiments show that the low frequency shear viscosity increases at a much faster rate than both rotational and translational diffusive relaxation times.; Comment: 12 pages...

## The nature of the glass and gel transitions in sticky spheres

Royall, C. Patrick; Williams, Stephen R.; Tanaka, Hajime
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
Glasses and gels are the two dynamically arrested, disordered states of matter. Despite their importance, their similarities and differences remain elusive, especially at high density. We identify dynamical and structural signatures which distinguish the gel and glass transitions in a colloidal model system of hard and "sticky" spheres. Gelation is induced by crossing the gas-liquid phase-separation line and the resulting rapid densification of the colloid-rich phase leads to a sharp change in dynamics and local structure. Thus, we find that gelation is first-order-like and can occur at much higher densities than previously thought: far from being low-density networks, gels have a clear "thermodynamic" definition which nevertheless leads to a non-equilibrium state with a distinct local structure characteristic of a rapidly quenched glass. In contrast, approaching the glass transition, the dynamics slow continuously accompanied by the emergence of local five-fold symmetric structure. Our findings provide a general thermodynamic, kinetic, and structural basis upon which to distinguish gelation from vitrification.; Comment: 12 pages

## Wet to dry crossover and a flow vortex-lattice in active nematics

Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
Active systems, from bacterial suspensions to vibrated granular matter, are continuously driven out of equilibrium by local injection of energy from their constituent elements. The energy input leads to exotic behaviour such as collective motion, pattern formation, topological defects and active turbulence, but theories that link the different manifestations of activity across systems and length scales are lacking. Here we unify two different classes of active matter by using friction as a control parameter to interpolate between wet active systems, whose behaviour is dominated by hydrodynamics, and dry active matter where any flow is screened. At the wet-dry crossover, we find a novel lattice of flow vortices interleaved with an ordered network of topological defects which arises from the competition between friction and viscous dissipation. Our results contribute to understanding the physics of matter operating out-of-equilibrium, with its potential in the design of active micro- and nano-machines.

## Absence of `fragility' and mechanical response of jammed granular materials

Pastore, Raffaele; Ciamarra, Massimo Pica; Coniglio, Antonio
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.56%
We perform molecular dynamic (MD) simulations of frictional non-thermal particles driven by an externally applied shear stress. After the system jams following a transient flow, we probe its mechanical response in order to clarify whether the resulting solid is 'fragile'. We find the system to respond elastically and isotropically to small perturbations of the shear stress, suggesting absence of fragility. These results are interpreted in terms of the energy landscape of dissipative systems. For the same values of the control parameters, we check the behaviour of the system during a stress cycle. Increasing the maximum stress value, a crossover from a visco-elastic to a plastic regime is observed.; Comment: 6 pages, 9 figures, accepted in Granular Matter on 01-02-2012

## From cage-jump motion to macroscopic diffusion in supercooled liquids

Pastore, Raffaele; Coniglio, Antonio; Ciamarra, Massimo Pica