CiteULike: dchen's 2008

Spatial cooperativity in soft glassy flows

J Goyon — 2008-07-03T06:17:56-00:00

Nature, Vol. 454, No. 7200., pp. 84-87.

Amorphous glassy materials of diverse nature—concentrated emulsions, granular materials, pastes, molecular glasses—display complex flow properties, intermediate between solid and liquid, which are at the root of their use in many applications1, 2, 3. A general feature of such systems, well documented yet not really understood, is the strongly nonlinear nature of the flow rule relating stresses and strain rates4, 5. Here we use a microfluidic velocimetry technique to characterize the flow of thin layers of concentrated emulsions, confined in gaps of different thicknesses by surfaces of different roughnesses. We find evidence for finite-size effects in the flow behaviour and the absence of an intrinsic local flow rule. In contrast to the classical nonlinearities of the rheological behaviour of amorphous materials, we show that a rather simple non-local flow rule can account for all the velocity profiles. This non-locality of the dynamics is quantified by a length, characteristic of cooperativity within the flow at these scales, that is unobservable in the liquid state (lower emulsion concentrations) and that increases with concentration in the jammed state. Beyond its practical importance for applications involving thin layers (for example, coatings), these non-locality and cooperativity effects have parallels in the behaviour of other glassy, jammed and granular systems, suggesting a possible fundamental universality.

Interfacial instabilities in a microfluidic Hele-Shaw cell

Michinao Hashimoto — 2008-06-21T16:02:24-00:00

Soft Matter, 2008, 4, 1403 - 1413, DOI: 10.1039/b715867

This paper describes surfactant-sensitive, dynamic instabilities that occur to aqueous droplets translating in a continuous flow of hexadecane in a microfluidic Hele-Shaw cell (HSC). A very low interfacial tension (on the order of 0.01 mN m-1) between water and hexadecane allowed for deformation of the droplets along the fields of flow and tip-streaming from moving droplets. In the system of water and hexadecane that we investigated, the use of surfactants in both fluids was necessary to achieve interfacial tension sufficiently low for the instabilities to occur. The droplets entering the HSC stretched orthogonally to the main direction of flow into elongated shapes, with aspect ratios greater than ten to one (width to length). These droplets exhibited two types of instabilities. The first included elongation of droplets, and Rayleigh–Plateau instabilities in the stretched droplets. Arrays of these stretched droplets formed three characteristic patterns that depended on the rates of flow of water and hexadecane. The second was driven by the shear stress exerted on the interface between the two fluids by the top and bottom boundaries of the HSC; this instability is named a shear-driven instability (SDI). Our observations supported that the SDI—an effect similar to tip-streaming—resulted from a redistribution of surfactants at the interface between the two fluids.

Electrorheological fluids: structures and mechanisms

Weijia Wen — 2008-06-20T22:02:08-00:00

Soft Matter, 2008, 4, 200 - 210, DOI: 10.1039/b710948m

Electrorheology denotes the control of a colloid's flow properties through an electric field. We delineate the basic characteristics of electrorheological (ER) fluids, and show that the use of an effective dielectric constant concept can yield quantitative predictions. In particular, the ground state structure, the structural transition that occurs under crossed electric and magnetic fields, the high-field yield stress and its variation with particle size are all in good agreement with the experiments. The recently discovered giant electrorheological effect, owing its origin to molecular dipoles, is described and contrasted with the conventional ER effect that arises from induced polarization effects. Graphical abstract image for this article (ID: b710948m)

Progess in superhydrophobic surface development

Paul Roach — 2008-06-20T20:19:01-00:00

Soft Matter, 2008, 4, 224 - 240, DOI: 10.1039/b712575p

Research into extreme water-repellent surfaces began many decades ago, although it was only relatively recently that the term superhydrophobicity appeared in literature. Here we review the work on the preparation of superhydrophobic surfaces, with focus on the different techniques used and how they have developed over the years, with particular focus on the last two years. We discuss the origins of water-repellent surfaces, examining how size and shape of surface features are used to control surface characteristics, in particular how techniques have progressed to form multi-scaled roughness to mimic the lotus leaf effect. There are notable differences in the terminology used to describe the varying properties of water-repellent surfaces, so we suggest some key definitions.

pH-Responsive microgel dispersions for repairing damaged load-bearing soft tissue

Tony Freemont — 2008-06-20T19:42:24-00:00

Soft Matter, 2008, 4, 919 - 924, DOI: 10.1039/b718441g

An important challenge for colloid scientists is to design injectable dispersions that provide structural support for damaged soft tissue and enable regeneration of tissue over the longer term. In this article we highlight a new area of research that aims to produce pH-responsive microgel dispersions that restore the mechanical properties of damaged, load-bearing, soft tissue. Chronic back pain due to degeneration of the intervertebral disc (IVD) is a major health problem and is the primary potential application for the work discussed. pH-Responsive microgel dispersions contain cross-linked polymer particles that swell when the pH approaches the pKa of the incorporated ionic co-monomer. The work considered here involves microgel particles containing MAA (methacrylic acid). The particles show pronounced pH-triggered swelling. The concentrated microgel dispersions change from a fluid to a gel at pH values greater than ca. 6.2, which is within the physiological pH range. The rheological properties are pH-dependent and can be adjusted using particle composition or concentration. Degenerated IVDs containing injected, gelled, microgel dispersions show improved mechanical properties. The disc height under biomechanically meaningful loads can be restored to values observed in non-degenerated IVDs. We also discuss the steps required to provide a minimally invasive injectable microgel system for restoring both the IVD mechanical properties and regenerating tissue in vivo. The approach

Soft matter with hard skin: From skin wrinkles to templating and material characterization

Jan Genzer — 2008-06-20T15:05:08-00:00

Soft Matter, 2006, 2, 310 - 323, DOI: 10.1039/b516741h

The English-language dictionary defines wrinkles as small furrows, ridges, or creases on a normally smooth surface, caused by crumpling, folding, or shrinking. In this paper we review the scientific aspects of wrinkling and the related phenomenon of buckling. Specifically, we discuss how and why wrinkles/buckles form in various materials. We also describe several examples from everyday life, which demonstrate that wrinkling or buckling is indeed a commonplace phenomenon that spans a multitude of length scales. We will emphasize that wrinkling is not always a frustrating feature (e.g., wrinkles in human skin), as it can help to assemble new structures, understand important physical phenomena, and even assist in characterizing chief material properties.

Spider silk as archetypal protein elastomer

Fritz Vollrath — 2008-06-20T14:58:06-00:00

Soft Matter, 2006, 2, 377 - 385, DOI: 10.1039/b600098n

We present an overview of the physical properties of spider silks, and introduce a model designed to study the energy absorbed by the material as it stretches before breaking. Of particular interest are the inter- and intramolecular hydrogen bonds as well as the role of water in modifying the material properties of silk. A solid understanding of this interaction is of paramount importance for any deeper insights into the mechanical properties of any biomaterial. Here we note that the typical biological material has evolved to function in the fully hydrated i.e. elastomeric state. We conclude that silk after its transformation from the hydrated feedstock to the dehydrated fibre state can in fact be analysed in great detail and interpreted as representative of a wide range of elastomeric proteins covering, inter alia, bone, keratins, elastin and collagen.

Stimuli-responsive polymer gels

Suk-Kyun Ahn — 2008-06-20T14:55:03-00:00

Soft Matter, 2008, 4, 1151 - 1157, DOI: 10.1039/b714376a

Stimuli-responsive polymer gels have received considerable attention due to their singular mechanical properties, which make them materials of choice for niche applications. Polymer gels comprising either physical or chemical cross-links can undergo controlled and reversible shape changes in response to an applied field. The stimulus or external field applied may include thermal, electrical, magnetic, pH, UV/visible light, ionic or metallic interactions or combinations thereof. The shape change can manifest itself in two-dimensional actuation, bending motion, or three-dimensional actuation, volume change. This reversible contraction and expansion of polymer gels as well as their mechanical properties are similar to that of biological muscles. This review will describe and critique some of the recent advances in the field of stimuli-responsive polymer gels including the design of new classes of polymeric gels, controlled actuation in response to external stimuli, and ability to tailor material properties for different applications.

Evolution of displacements and strains in sheared amorphous solids

Craig Maloney — 2008-06-17T18:09:23-00:00

Journal of Physics: Condensed Matter, Vol. 20, No. 24. (2008), 244128.

The local deformation of two-dimensional Lennard-Jones glasses under imposed shear strain is studied via computer simulations. Both the mean squared displacement and mean squared strain rise linearly with the length of the strain interval Dg over which they are measured. However, the increase in displacement does not represent single-particle diffusion. There are long-range spatial correlations in displacement associated with slip lines with an amplitude of order the particle size. Strong dependence on system size is also observed. The probability distributions of displacement and strain are very different. For small Dg the distribution of displacement has a plateau followed by an exponential tail. The distribution becomes Gaussian as Dg increases to about 0.03. The strain distributions consist of sharp central peaks associated with elastic regions, and long exponential tails associated with plastic regions. The latter persist to the largest Dg studied.

Underconstrained jammed packings of nonspherical hard particles: Ellipses and ellipsoids

Aleksandar Donev — 2008-06-11T16:29:28-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 5. (2007)

Continuing on recent computational and experimental work on jammed packings of hard ellipsoids [Donev et al., Science 303, 990 (2004)] we consider jamming in packings of smooth strictly convex nonspherical hard particles. We explain why an isocounting conjecture, which states that for large disordered jammed packings the average contact number per particle is twice the number of degrees of freedom per particle (=2df), does not apply to nonspherical particles. We develop first- and second-order conditions for jamming and demonstrate that packings of nonspherical particles can be jammed even though they are underconstrained (hypoconstrained, <2df). We apply an algorithm using these conditions to computer-generated hypoconstrained ellipsoid and ellipse packings and demonstrate that our algorithm does produce jammed packings, even close to the sphere point. We also consider packings that are nearly jammed and draw connections to packings of deformable (but stiff) particles. Finally, we consider the jamming conditions for nearly spherical particles and explain quantitatively the behavior we observe in the vicinity of the sphere point.

Monte Carlo study of crystalline order and defects on weakly curved surfaces

A Hexemer — 2008-06-10T16:29:38-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 5. (2007)

We numerically study the ground states of particles interacting via a repulsive Yukawa potential on two rigid substrates shaped as isolated and periodically arranged bumps characterized by a spatially varying Gaussian curvature. Below a critical aspect ratio that describes the substrate deformation, the lattice is frustrated, but defect free. A further increase of the aspect ratio triggers defect unbinding transitions that lower the total potential energy by introducing dislocations either in isolation or within grain boundaries. In the presence of very strong deformations, isolated disclinations are nucleated. We show that the character and spatial distribution of defects observed in the ground state reflect the symmetries and periodicity of the two model surfaces investigated in this study.

Comparison of dynamic light scattering measurements and mode-coupling theory for the tagged particle dynamics of a hard-sphere suspension

W van Megen — 2008-06-10T16:09:19-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 6. (2007)

The mean-squared displacement, velocity autocorrelation function, and the non-Gaussian parameter, obtained by dynamic light scattering on suspensions of particles with hard-sphere interactions, are compared with the results of the idealized version of mode-coupling theory. Both leading order asymptotic and full numerical solutions of the mode-coupling equations are considered. Experiment and the full numerical results of the theory expose similar qualitative changes at the volume fraction of the first order freezing transition. In particular, the emergence of negative algebraic decays in the velocity autocorrelation function of the undercooled suspension suggest the emergence of clusters in which particles are trapped. Consistency of experiment, computer simulation, and theory in this regard suggests that, at particular strengths of the delayed, nonlinear feedback, contained in mode coupling theory, the latter predicts not only structural arrest which, as already established, is symptomatic of a glass transition, but also a more subtle change in dynamics that signals the onset of the first order transition.

Eigenmodes of a hydrodynamically coupled micron-size multiple-particle ring

R Di Leonardo — 2008-02-09T18:56:22-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 6. (2007)

We use a continuous acquisition, high-speed camera with integrated centroid tracking to simultaneously measure the positions of a ring of micron-sized particles held in holographic optical tweezers. Hydrodynamic coupling between the particles gives a set of eigenmodes, each one independently relaxing with a characteristic decay rate (eigenvalue) that can be measured using our positional data. Despite the finite particle size, we find an excellent agreement between the measured eigenvalues and those numerically predicted by Oseen theory applied to the two-dimensional (2D) ring geometry. Particle motions are also analyzed in terms of the alternative eigenmode set obtained by wrapping onto the ring the eigenmodes of a 1D periodic chain. We identify the modes for which the periodic chain is a good approximation to the ring and those for which it is not.

Role of orientation disorder in the formation of fragility of glassy water and glycerol-like liquids

Sergey Lishchuk — 2008-06-10T15:27:23-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 6. (2007)

The role of H bonds in the formation of the fragility and dielectric properties of highly viscous liquids is investigated. The heuristic supposition about the proportionality between the logarithm of the shear viscosity and oscillatory contributions to the mean-square displacement of a molecule is presented. Concrete calculations are carried out for the H-bond subsystem of the two-dimensional model lattice water. The conjecture on the interrelation between the phase transition in the subsystem of H bonds and the glassification point is formulated. It is shown that (i) the glassification temperature is proportional to the H-bonding energy and (ii) the fragilities of glycerol-like liquids differ from each other as a consequence of distinct interaction energies between H bonds. The existence of a close connection between the fragility parameter and dielectric permittivity is established.

Ions in water: From ion clustering to crystal nucleation

José Alejandre — 2008-01-24T12:46:38-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 6. (2007)

The clustering and nucleation of ions in aqueous solutions results from a competition between ion hydration and association. Molecular dynamics simulations of aqueous NaCl solutions are used to investigate ion clustering with a force field adjusted to reproduce experimental properties of the pure NaCl crystal and melt, and of concentrated solutions. The simulation results point to strong sensitivity of the nucleation mechanism to small changes in the force field. We report the numerical evidence for rapid crystal nucleation near saturation or under supercritical conditions.

Spontaneous formation of dual stratification patterns in a large quasi-two-dimensional sand pile

Michiko Shimokawa — 2008-06-10T15:12:31-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 1. (2008)

By pouring a mixture of two types of grains into a large vertical cell with a narrow space, we discovered a dual stratification pattern consisting of two different wavelengths at the upper and lower regions of the resulting sand pile. In the formation of this pattern, we also observed an additional type of kink—a wave that moves toward the top of the sand pile along the slope. The kink, herein called a trapped kink, is essential for the formation of dual stratification patterns. Asymmetric probability distributions are obtained from measurement of the position where the kinks are generated. We proposed a phenomenological model to describe the kink generation process. The results analyzed by this model agree with experimental distributions.

H theorem for contact forces in granular materials

Philip Metzger — 2008-06-10T00:18:03-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 1. (2008)

A maximum entropy theorem is developed and tested for granular contact forces. Although it is idealized, describing two-dimensional packings of round, rigid, frictionless, cohesionless disks with coordination number Z=4, it appears to describe a central part of the physics present in the more general cases. The theorem does not make the strong claims of Edwards' hypothesis, nor does it rely upon Edwards' hypothesis at any point. Instead, it begins solely from the physical assumption that closed loops of grains are unable to impose strong force correlations around the loop. This statement is shown to be a generalization of Boltzmann's assumption of molecular chaos (his stosszahlansatz), allowing for the extra symmetries of granular stress propagation compared to the more limited symmetries of momentum propagation in a thermodynamic system. The theorem that follows from this is similar to Boltzmann's H theorem and is presented as an alternative to Edwards' hypothesis for explaining some granular phenomena. It identifies a very interesting feature of granular packings: if the generalized stosszahlansatz is correct, then the bulk of homogeneous granular packings must satisfy a maximum entropy condition simply by virtue of being stable, without any exploration of phase space required. This leads to an independent derivation of the contact force statistics, and these predictions have been compared to numerical simulation data in the isotropic case. The good agreement implies that the generalized stosszahlansatz is indeed accurate at least for the isotropic state of the idealized case studied here, and that it is the reductionist explanation for contact force statistics in this case.

Shear-induced fractures and three-dimensional motions in an organogel

Pauline Grondin — 2008-06-10T00:14:08-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 1. (2008)

The flow behavior of a viscoelastic organogel is investigated using ultrasonic velocimetry combined with rheometry. Our gel presents a decreasing flow curve, i.e., the measured stress decreases as a function of the applied shear rate. Strikingly, we note that the local flow curve calculated from the velocity profiles also exhibits a decreasing part. We attribute this regime to the presence of a fracturing process and three-dimensional motions in the bulk of the sample.

Interplay between hydrodynamic and Brownian fluctuations in sedimenting colloidal suspensions

JT Padding — 2008-01-23T07:40:30-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 1. (2008)

We apply a hybrid molecular dynamics and mesoscopic simulation technique to study the steady-state sedimentation of hard sphere particles for Peclet number (Pe) ranging from 0.08 to 12. Hydrodynamic backflow causes a reduction of the average sedimentation velocity relative to the Stokes velocity. We find that this effect is independent of Pe number. Velocity fluctuations show the expected effects of thermal fluctuations at short correlation times. At longer times, nonequilibrium hydrodynamic fluctuations are visible, and their character appears to be independent of the thermal fluctuations. The hydrodynamic fluctuations dominate the diffusive behavior even for modest Pe number, while conversely the short-time fluctuations are dominated by thermal effects for surprisingly large Pe numbers. Inspired by recent experiments, we also study finite sedimentation in a horizontal planar slit. In our simulations distinct lateral patterns emerge, in agreement with observations in the experiments.

Profile blunting and flow blockage in a yield-stress fluid: A molecular dynamics study

F Varnik — 2008-06-09T22:44:55-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 1. (2008)

The flow of a simple glass forming system (a 80:20 binary Lennard-Jones mixture) through a planar channel is studied via molecular dynamics simulations. The flow is driven by an external body force similar to gravity. Previous studies show that the model exhibits both a static [F. Varnik et al., J. Chem. Phys. 120, 2788 (2004)] and a dynamic [F. Varnik and O. Henrich, Phys. Rev. B 73, 174209 (2006)] yield stress in the glassy phase. These observations are corroborated by the present work, where we investigate how the presence of a yield stress may affect the system behavior in a Poiseuille-type flow geometry. In particular, we observe a blunted velocity profile across the channel: A relatively wide region in the channel center flows with a constant velocity (zero shear rate) followed by a nonlinear change of the shear rate as the walls are approached. The observed velocity gradients are compared to those obtained from the knowledge of the shear stress across the channel and the flow curves (stress versus shear rate), the latter being determined in our previous simulations of homogeneous shear flow. Furthermore, using the value of the (dynamic) yield stress known from previous simulations, we estimate the threshold body force for a complete arrest of the flow. Indeed, a blockage is observed as the imposed force falls below this threshold value. Small but finite shear rates are observed at stresses above the dynamic but below the static yield stress. We discuss the possible role of the stick-slip-like motion for this observation.

Scaling and dynamics of sphere and disk impact into granular media

Daniel Goldman — 2008-04-14T00:46:54-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 2. (2008)

Direct measurements of the acceleration of spheres and disks impacting granular media reveal simple power law scalings along with complex dynamics which bear the signatures of both fluid and solid behavior. The penetration depth scales linearly with impact velocity while the collision duration is constant for sufficiently large impact velocity. Both quantities exhibit power law dependence on sphere diameter and density, and gravitational acceleration. The acceleration during impact is characterized by two jumps: a rapid, velocity-dependent increase upon initial contact and a similarly sharp depth-dependent decrease as the impacting object comes to rest. Examination of the measured forces on the sphere in the vicinity of these features leads to an experimentally based granular force model for collision. We discuss our findings in the context of recently proposed phenomenological models that capture qualitative dynamical features of impact but fail both quantitatively and in their inability to capture significant acceleration fluctuations that occur during penetration and which depend on the impacted material.

When a crack is oriented by a magnetic field

L Pauchard — 2008-06-09T21:59:24-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 2. (2008)

Upon drying, colloidal suspensions undergo a phase transformation from a “liquid” to a “gel” state. With further solvent evaporation, tensile stresses develop in the gel, which ultimately leads to fractures. These generally manifest themselves in regular cracking patterns which reflect the physical conditions of the drying process. Here we show experimentally and theoretically how, in the case of a drying droplet of magnetic colloid (ferrofluid), an externally applied magnetic field modifies the stress in the gel and therefore the crack patterns. We find that the analysis of the shape of the cracks allows one to estimate the value of the gel Young's modulus just before the crack nucleation.

Magnetically tunable optical absorbance in a colloidal system

M Adrian — 2008-06-09T21:56:04-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 2. (2008)

We study the optical absorbance from a magnetically arranged colloidal structure, and investigate the possibility of creating a magnetically controlled optical sensor using this system. The colloids form chains when exposed to an external magnetic field, which tend to collapse and form a more random particle arrangement when the field is removed. We show that a small magnetic field is able to change the sensor's reflection coefficient by more than 30%, and investigate in detail the relaxation mechanism when the field is turned off.

Shear-induced crystallization of an amorphous system

Anatolii Mokshin — 2008-06-09T20:41:12-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 2. (2008)

The influence of a stationary shear flow on the crystallization in a glassy system is studied by means of molecular dynamics simulations and subsequent cluster analysis. The results reveal two opposite effects of the shear flow on the processes of topological ordering in the system. Shear promotes the formation of separated crystallites and suppresses the appearance of the large clusters. The shear-induced ordering proceeds in two stages, where the first stage is related mainly to the growth of crystallites and the second stage is due to an adjustment of the created clusters and a progressive alignment of their lattice directions. The influence of strain and shear rate on the crystallization is also investigated. In particular, we find two plausible phenomenological relations between the shear rate and the characteristic time scale needed for ordering of the amorphous system under shear.

Bubble-raft model for a paraboloidal crystal

Mark Bowick — 2008-06-09T20:35:14-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 2. (2008)

We investigate crystalline order on a two-dimensional paraboloid of revolution by assembling a single layer of millimeter-sized soap bubbles on the surface of a rotating liquid, thus extending the classic work of Bragg and Nye on planar soap bubble rafts. Topological constraints require crystalline configurations to contain a certain minimum number of topological defects such as disclinations or grain boundary scars whose structure is analyzed as a function of the aspect ratio of the paraboloid. We find the defect structure to agree with theoretical predictions and propose a mechanism for scar nucleation in the presence of large Gaussian curvature.

Microscopic origin of granular ratcheting

S Mcnamara — 2008-06-09T19:58:04-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 3. (2008)

Numerical simulations of assemblies of grains under cyclic loading exhibit “granular ratcheting:” a small net deformation occurs with each cycle, leading to a linear accumulation of deformation with cycle number. We show that this is due to a curious property of the most frequently used models of the particle-particle interaction: namely, that the potential energy stored in contacts is path dependent. There exist closed paths that change the stored energy, even if the particles remain in contact and do not slide. An alternative method for calculating the tangential force removes granular ratcheting.

Influence of adhesion and friction on the geometry of packings of spherical particles

CL Martin — 2008-06-09T19:55:59-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 3. (2008)

We study the effect of both adhesion and friction on the geometry of monosized packings of spheres by means of discrete element simulations. We use elastic properties that are characteristic of materials typically used for particulate processing (Young's modulus in the range 20–200 GPa). The geometrical features, both global and local, of the packings are studied using a variety of approaches in order to investigate their ability to quantify the effect of adhesion and/or friction. We show that both adhesion and friction interaction decrease the packing fraction. The very localized ordering that adhesion triggers is particularly investigated by use of the radial distribution function, the ordering parameter Q6, and four triclinic cells that allow a description of the microstructure at the local level. We show that the probability of occurrence of these triclinic cells is approximately proportional to their degree of freedom when neither adhesion nor friction plays a role. We find that the introduction of adhesive interactions increases the probability of occurrence of those cells that have the lowest degree of freedom.

Simulation of density segregation in vibrated beds

C Zeilstra — 2008-06-09T19:22:35-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 3. (2008)

We have investigated by numerical simulation the density segregation of fine equal-sized bronze and glass particles subject to vertical vibrations. The model was found to be capable of predicting the two main segregation forms (“bronze on top” and “sandwich”) in roughly the same regions of the phase diagram as was found experimentally by Burtally et al. We investigated the effects of pressure air forcing, friction and restitution of kinetic energy in collisions, and box size on the segregation behavior. We find that next to the interstitial air friction also has a large influence on the formation of the sandwich structure.

Breakdown of the Yukawa model in de-ionized colloidal suspensions

Aldemar Torres — 2008-06-09T19:14:34-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 3. (2008)

We study effective colloidal interactions in de-ionized colloidal mixtures through sedimentation-diffusion equilibrium. We derive a coarse-grained effective model (EM) and compare its density profiles with those of the computationally much more expensive primitive model (PM) of colloids and counterions in gravity. The EM, which contains not only standard pairwise screened-Coulomb interactions, but also explicit many-body effects by means of a so-called volume term, can quantitatively account for all observed sedimentation phenomena such as lifting of colloids to high altitudes, segregation into layers in mixtures, and floating of heavy colloids on top of lighter ones. Without the volume term there is no quantitative agreement between the PM and EM, even in the present high-temperature limit of interest, showing that de-ionized colloidal suspensions cannot be described by a pairwise Yukawa model.

Shear-banding phenomena and dynamical behavior in a Laponite suspension

F Ianni — 2008-06-09T19:01:09-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 3. (2008)

Shear localization in an aqueous clay suspension of Laponite is investigated through dynamic light scattering, which provides access both to the dynamics of the system (homodyne mode) and to the local velocity profile (heterodyne mode). When shear bands form, a relaxation of the dynamics typical of a gel phase is observed in both bands soon after the flow stops. Periodic oscillations of the flow behavior, typical of a stick-slip phenomenon, are also observed when shear localization occurs. Both results are discussed in the light of various theoretical models for soft glassy gels.

Frustration-induced magic number clusters of colloidal magnetic particles

Larysa Baraban — 2008-06-09T18:57:10-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 3. (2008)

We report the formation of stable two-dimensional clusters consisting of long-range-interacting colloidal particles with predefined magnetic moments. The symmetry and arrangement of the particles within the cluster are imposed by the magnetic frustration. By satisfying the criteria of stability, a series of magic number clusters is formed. The magic clusters are close packed and have compensating magnetic moments and chirality. Thus, the system can be regarded as a classical mesoscopic model for spin arrangements in two-dimensional triangular antiferromagnets, although the exact nature of the interactions between the macroscopic magnetic moments is different.

Band formation on shearing in phase-separated polymer solutions

Hans Tromp — 2008-06-09T18:31:27-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 3. (2008)

In a phase-separated mixture of two Newtonian polymer solutions sheared in a cone-plate geometry adapted for microscopic observation at a zero-velocity plane, shear-induced coalescence of droplets of the broken-up phase, followed by band formation, was observed. Initially wormlike structures developed into doughnut-shaped bands, disconnected from the walls of the geometry. The shear rate and composition inside the bands differed from that in the outside solution. The shear-shear rate instability preceding the band formation could be qualitatively described by a van der Waals–loop-shaped shear rate dependence of droplet deformation.

Granular flow in rotating cylinders with noncircular cross sections

DVN Prasad — 2008-06-08T23:50:12-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

An experimental and theoretical study is carried out of the flow of granular material in cylinders with different cross-sectional shapes rotated about their axes. The flow of particles in such geometries is confined to a shallow layer at the free surface. The length and thickness of the layer shrink and expand periodically with rotation of the cylinder, resulting in chaotic advection and improved mixing of passive tracers. Experimental results obtained by flow visualization are reported for quasi-two-dimensional mixers half filled with glass beads. A depth-averaged flow model to predict the time-varying layer thickness profile is presented, along with a perturbation solution in terms of a small parameter k, which is the ratio of the maximum layer thickness to the half length of the layer (L), at the cross-section orientation when the length is minimum. To the lowest order [O(k0)], the model predicts that the layer profiles scaled with L() at different mixer orientation angles () are identical and the same as that for a circle. The measured layer thickness profiles averaged over different orientations of noncircular mixers match reasonably well with the theory, but the standard deviations are larger for the noncircular cylinders compared to the circle. The O(k) perturbation solution and the full theory both predict that the scaled layer thickness varies periodically; the deviations are proportional to the rate of change of the length with orientation. The perturbation solution gives results close to those from the numerical solution except at cylinder orientations when the length of the flowing layer changes sharply. The measured variation of the scaled midlayer thickness with orientation for all geometries is well predicted by the theory.

Anomalous behavior of a single particle falling through a funnel

Yuan Fang — 2008-06-08T23:48:50-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

We show several surprising phenomena that occur in an extremely simple system of a single frictionless, inelastic, spherical particle falling under gravity through a symmetric funnel. One might naively expect that particles would fall through funnels with steeper sides more quickly, exert a smaller total impulse on the funnel walls, and lose less energy. However, we show that there are special ranges of angles of the funnel walls for which exactly the opposite occurs. Typically, the particle will experience a sequence of collisions that is highly sensitive to the location at which it enters the funnel and nearby particle trajectories become widely dispersed. However, in the special angular ranges this is not the case and the particle can experience sequences of collisions that have a highly coherent structure. We provide a theoretical analysis that can predict and explain this surprising behavior.

Effects of friction and disorder on the quasistatic response of granular solids to a localized force

C Goldenberg — 2008-06-08T23:44:30-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

The response to a localized force provides a sensitive test for models of stress transmission in granular solids. Elasto-plastic models, traditionally used by engineers, have been challenged by theories and experiments that suggest a wavelike (hyperbolic) propagation of the stress, as opposed to the elliptic equations of static elasticity. Simulations of two-dimensional granular systems subject to a localized external force have been employed to examine the nature of stress transmission in these systems as a function of the magnitude of this force, the frictional parameters, and degree of disorder. The results indicate that in large systems (as considered by engineers) the response is close to that predicted by isotropic elasticity, whereas for small systems (or strongly forced ones) it is strongly anisotropic. In the latter case the applied force induces changes in the contact network accompanied by frictional sliding and gives rise to hyperboliclike stress propagation. The larger the static friction, the more extended the range of forces for which the response is elastic, and the smaller the anisotropy. Increase in the degree of polydispersity (in the studied range, up to 25%) decreases the range of elastic response. This paper is an extension of a previously published Letter [C. Goldenberg and I. Goldhirsch, Nature (London) 435, 188 (2005)].

Start and stop of an avalanche in a granular medium subjected to an inner water flow

P Philippe — 2008-06-08T23:35:29-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

We report experimental results on the maximum angle of stability, i.e., the so-called avalanche angle, of a granular medium subjected to an inner water flow controlled by a constant pressure drop. A unique avalanche threshold is derived by two alternative theoretical developments, namely a continuum and a discrete approach, and is successfully confronted to many measurements in a large experimental range. A qualitative analysis of the instability triggering reveals different dynamical behaviors depending on whether the water flow is downward or upward in the granular layer, namely stabilizing versus destabilizing regime. Contrary to the purely hydrostatic situation, the free surface following an avalanche departs from a linear shape because the dynamical pressure gradient is no longer constant in the medium. A simple model is proposed that can satisfactorily predict the postavalanche height profile as well as its subsequent evolution for higher inclinations.

Evolving loop structure in gradually tilted two-dimensional granular packings

Ashley Smart — 2008-06-08T23:25:21-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

Granular packings, especially near the jamming transition, form fragile networks where small perturbations can lead to destabilization and large scale rearrangements. A key stabilizing element in two dimensions is the contact loop, yet surprisingly little is known about contact loop statistics in realistic granular networks. In this paper, we use particle dynamics to study the evolution of contact loop structure in a gradually tilted two-dimensional granular bed. We find that the resulting contact loop distributions (1) are sensitive to material properties, (2) deviate from the expected structure of a randomly wired lattice, and (3) are uniquely dependent on tilting angle. Also, we introduce a quantitative measure of loop stability and show that increased tilting results in a gradual destabilization of individual loops. We briefly discuss the considerations for extending our approach to three dimensions.

Shear-induced fluid-tracer diffusion in a semidilute suspension of spheres

Takuji Ishikawa — 2008-06-08T23:08:06-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

We calculated tracer diffusion in a sheared suspension of non-Brownian rigid spheres and propose a numerical method based on a boundary element method and Stokesian dynamics method. We present details of the numerical method and examine the accuracy of the method. The limitation of semidiluteness is due to the accuracy of the tracer velocity calculation. The results show that the diffusivity of fluid tracers is greater than that of suspended spheres in the semidilute regime. The diffusivity of the velocity gradient direction is about threefold greater than that in the vorticity direction. Simple scaling demonstrates that the diffusivity of fluid tracers increases with the square of the volume fraction of spheres in the semidilute regime, which is confirmed numerically.

Stripe formation in an immiscible polymer blend under electric and shear-flow fields

Yang Na — 2008-06-08T22:59:03-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

We found a stripe formation in an emulsion of a liquid crystalline polymer (LCP) and a machine oil (OIL) in electric and shear fields. Through the simultaneous measurement with a confocal scanning laser microscope and a rheometer, it was clearly shown that the formation of stripes, which are periodically arrayed, leads to the increase of the shear stress. The droplets, which are one component of the emulsion, start to be connected at low electric fields and then change into the stripes with the increase of electric field. Finally, a three-dimensional network is formed at high electric fields. The period and fluctuation of the stripe structure were also investigated in detail.

Melting of two-dimensional tunable-diameter colloidal crystals

Y Han — 2008-06-08T22:54:09-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

Melting of two-dimensional colloidal crystals is studied by video microscopy. The samples were composed of microgel spheres whose diameters could be temperature tuned, and whose pair potentials were measured to be short ranged and repulsive. We observed two-step melting from the crystal to a hexatic phase and from the hexatic to the liquid phase as a function of the temperature-tunable volume fraction. The translational and orientational susceptibilities enabled us to definitively determine the phase transition points, avoiding ambiguities inherent in other analyses and resolving a “dislocation precursor stage” in the solid phase that some of the traditional analyses may incorrectly associate with the hexatic phase. A prefreezing stage of the liquid with ordered patches was also found.

Superparamagnetic colloids confined in narrow corrugated substrates

Herrera Velarde — 2008-06-08T22:46:17-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

We report a Brownian dynamics simulation study of the structure and dynamics of superparamagnetic colloids subject to external substrate potentials and confined in narrow channels. Our study is motivated by the importance of phenomena like commensurable-incommensurable phase transitions, anomalous diffusion, and stochastic activation processes that are closely related to the system under investigation. We focus mainly on the role of the substrate in the order-disorder mechanisms that lead to a rich variety of commensurate and incommensurate phases, as well as its effect on the single-file diffusion in interacting systems and the depinning transition in one dimension.

Slip boundary conditions for shear flow of polymer melts past atomically flat surfaces

Anoosheh Niavarani — 2008-06-08T21:49:42-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

Molecular dynamics simulations are carried out to investigate the dynamic behavior of the slip length in thin polymer films confined between atomically smooth thermal surfaces. For weak wall-fluid interactions, the shear rate dependence of the slip length acquires a distinct local minimum followed by a rapid growth at higher shear rates. With increasing fluid density, the position of the local minimum is shifted to lower shear rates. We found that the ratio of the shear viscosity to the slip length, which defines the friction coefficient at the liquid/solid interface, undergoes a transition from a nearly constant value to power law decay as a function of the slip velocity. In a wide range of shear rates and fluid densities, the friction coefficient is determined by the product of the value of the surface-induced peak in the structure factor and the contact density of the first fluid layer near the solid wall.

Autocalibrated colloidal interaction measurements with extended optical traps

Marco Polin — 2008-06-08T20:06:58-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 5. (2008)

We describe an efficient technique for measuring the effective interaction potential for pairs of colloidal particles. The particles to be tested are confined in an extended optical trap, also known as a line tweezer, that is projected with the holographic optical trapping technique. Their diffusion along the line reflects not only their intrinsic interactions with each other, but also the influence of the line's potential energy landscape and interparticle interactions mediated by scattered light. We demonstrate that measurements of the particles' trajectories at just two laser powers can be used to correct explicitly for optically induced forces and that statistically optimal analysis for optically induced forces yields autocalibrated measurements of the particles' intrinsic interactions with remarkably few statistically independent measurements of the particles' separation.

Particle dynamics within a wetting layer in a colloid-polymer mixture

P Voudouris — 2008-06-08T20:04:09-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 5. (2008)

The near-wall dynamics at the top and bottom phases of a phase-separated colloid-polymer mixture were measured by evanescent-wave dynamic light scattering. The short-time dynamics near the wall were found to be liquidlike in both phases, confirming the presence of a liquid wetting layer. The short-time diffusion within the wetting layer was slower than in the bulk liquid phase. Similarly, the near-wall dynamics in both phases of the colloid-polymer mixture were also slower compared to the near-wall colloidal dynamics in a pure concentrated suspension at the same volume fraction. These effects highlight the role of interparticle attractions and specific wall-induced hydrodynamic interactions in slowing down the colloidal motion in confinement.

Effective temperature dynamics in an athermal amorphous plasticity theory

Eran Bouchbinder — 2008-06-08T18:58:53-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 5. (2008)

Recent developments in the theory of amorphous plasticity point to the central role played by the concept of an effective disorder temperature Teff. An athermal dynamics for Teff is proposed in the framework of a deformation theory and discussed in light of recent steady-state simulations by Haxton and Liu [T. K. Haxton and A. J. Liu, Phys. Rev. Lett. 99, 195701 (2007)]. The structure of the resulting theory, its parameters and transient dynamics are discussed and compared to available data.

Phase behavior of wormlike rods

Giorgio Cinacchi — 2008-06-08T18:43:59-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 5. (2008)

By employing molecular dynamics computer simulations, the phase behavior of systems of rodlike particles with varying degree of internal flexibility has been traced from the perfectly rigid rod limit till very flexible particles, and from the high density region till the isotropic phase. From the perfectly rigid rod limit and enhancing the internal flexibility, the range of the smectic-A phase is squeezed out by the concomitant action of the scarcely affected crystalline phase at higher density and the nematic phase at lower density, until it disappears. These results confirm the supposition, drawn from previous theoretical, simulational and experimental studies, that the smectic-A phase is destabilized by introducing and enhancing the degree of particle internal flexibility. However, no significant changes in the order of nematic–to–smectic-A phase transition, which appears always first order, nor in the value of the layer spacing, are observed upon varying the degree of particle internal flexibility. Moreover, no evidence of a columnar phase, which was thought of as a possible superseder of the smectic-A phase in flexible rods, has been obtained.

Crystal nucleation enhanced at the diffuse interface of immiscible polymer blends

Yu Ma — 2008-06-08T18:30:03-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 6. (2008)

We report dynamic Monte Carlo simulations of immiscible binary polymer blends, which exhibit weakly enhanced crystal nucleation near interfaces between two phase-separated polymers. We found that this enhancement is not accompanied by any preferred crystal orientation, implying its origin is mainly of enthalpic rather than entropic nature. Mean-field theory of polymer blends predicts that for immiscible polymers the melting point of the crystallizable component increases upon dilution in the other component, while it normally decreases for miscible blends. A local dilution is forced to occur at the diffuse interface of immiscible polymers; therefore the melting point of crystallizable polymers rises, which, in turn, enhances the thermodynamic driving force for crystal nucleation near the interface.

Exotic crystal superstructures of colloidal crystals in confinement

Ana Fontecha — 2008-06-08T18:22:55-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 6. (2008)

Colloidal model systems have been used for over three decades for investigating liquids, crystals, and glasses. Colloidal crystal superstructures have been observed in binary systems of repulsive spheres as well as oppositely charged sphere systems showing structures well known from atomic solids. In this work we study the structural transition of colloidal crystals under confinement. In addition to the known sequence of crystalline structures, crystal superstructures with dodecagonal and hexagonal symmetry are observed in one component systems. These structures have no atomic counterpart.

Passive Oscillations of Two Tandem Flexible Filaments in a Flowing Soap Film

Lai Jia — 2008-06-08T18:18:59-00:00

Physical Review Letters, Vol. 100, No. 22. (2008)

The passive oscillations of flexible filaments in a flowing soap film were investigated to learn the serial interaction between them. When arranged in tandem, the downstream filament flaps at the same frequency as that of the upstream one, but with a larger amplitude, whereas the upstream one is almost unaffected compared to the single filament case. The data analysis shows the downstream filament indeed extracts energy from the vortex street and receives greater force than the upstream one or a single filament in a uniform flow.

Proteins: Coexistence of Stability and Flexibility

Shlomi Reuveni — 2008-06-06T18:12:26-00:00

Physical Review Letters, Vol. 100, No. 20. (2008)

We introduce an equation for protein native topology based on recent analysis of data from the Protein Data Bank and on a generalization of the Landau-Peierls instability criterion for fractals. The equation relates the protein fractal dimension df, the spectral dimension ds, and the number of amino acids N. Deviations from the equation may render a protein unfolded. The fractal nature of proteins is shown to bridge their seemingly conflicting properties of stability and flexibility. Over 500 proteins have been analyzed (df, ds, and N) and found to obey this equation of state.