CiteULike: dchen's 2007

Slippery diffusion-limited aggregation

Clair Seager — 2008-06-27T08:16:01-00:00

Colloidal particles that interact through strong, short-range, secondary attractions in liquids form irreversible “slippery” bonds that are not shear-rigid. Through event-driven simulations of slippery attractive spheres, we show that space-filling fractal clusters still emerge from the process of “slippery” diffusion-limited aggregation (DLA). Although slippery and classic DLA clusters have the same fractal dimension, df=2.5, their average coordination numbers are quite different: zS=6 whereas zC=2. Local tetrahedral attractive jamming of the particles leads to a structure factor, S(q), that exhibits dense cluster peaks at higher wave numbers, q, and a fractal power-law rise toward lower q.

The interface in demixed colloid–polymer systems: wetting, waves and droplets

Dirk Aarts — 2008-06-20T23:21:17-00:00

Soft Matter, 2007, 3, 19 - 23, DOI: 10.1039/b608479f

Phase transitions in colloid–polymer mixtures have attracted a large amount of attention over the last 20 years (W. C. K. Poon, J. Phys.: Condens. Matter, 2002, 14, R859; R. Tuinier, J. Rieger and C. G. de Kruif, Adv. Colloid Interface Sci., 2003, 103, 1). By comparison, the interfacial tension between the coexisting phases has received little attention. Here, we show that the ultralow interfacial tension in fluid–fluid demixed colloid–polymer systems, which is roughly one million times smaller than in ordinary liquids, manifests itself in a wide variety of interface characteristics and processes. Discussed are the interfacial wetting behaviour close to a hard wall, the thermal capillary waves at the free interface and the process of droplet coalescence and breakup. These subjects can be studied in a single experiment by combining modern soft matter chemistry and laser scanning confocal microscopy. This combination allows a further exploration of a broad range of interface issues.

Colloidal building blocks with potential for magnetically configurable photonic crystals

Pedro Camargo — 2008-06-20T23:16:22-00:00

Soft Matter, 2007, 3, 1215 - 1222, DOI: 10.1039/b706565e

Tunable colloids: control of colloidal phase transitions with tunable interactions

Anand Yethiraj — 2008-06-20T23:09:27-00:00

Soft Matter, 2007, 3, 1099 - 1115, DOI: 10.1039/b704251p

Systems of spherical colloidal particles mimic the thermodynamics of atomic crystals. Control of interparticle interactions in colloids, which has recently begun to be extensively exploited, gives rise to rich phase behaviours as well as crystal structures with nanoscale and micron-scale lattice spacings. This provides model systems in which to study fundamental problems in condensed matter physics, such as the dynamics of crystal nucleation and melting, and the nature of the glass transition, at experimentally accessible lengthscales and timescales. Tunable control of these interactions provides reversible control. This will enable quantitative studies of phase transition kinetics as well as the creation of advanced materials with switchability of function and properties

Colloid–polymer mixtures in the protein limit

Kevin Mutch — 2008-06-20T22:36:36-00:00

Soft Matter, 2007, 3, 155 - 167, DOI: 10.1039/b611137h

This review discusses the structure and phase behaviour of mixtures of colloidal particles and non-adsorbing polymers in the protein limit of large polymers and small colloids. The vast majority of work on colloid–polymer mixtures has been concerned with the colloid limit of large colloidal particles and small polymer chains. In this regime, the diameter of the colloidal particles, , is larger than the characteristic size of the polymer—taken as twice their radius of gyration, Rg. The opposite limit, of size ratios , is called the protein limit due to the common practice of adding polymer to protein solutions in order to aid protein crystallisation. Theoretical predictions for systems in the protein limit are considered briefly and then the main focus is on recent experimental studies of mixtures in the protein limit.

Instabilities in liquid foams

D Weaire — 2008-06-20T22:24:37-00:00

Soft Matter, 2007, 3, 47 - 57, DOI: 10.1039/b608466b

Instabilities play a central role in the physics of foams. Some that change the topology of a dry foam are indicated by the laws promulgated by Plateau in his 1873 book. Their occurrence is less clearly predictable in wet foams. Various other instabilities are related to gravitational loading and gas compressibility. We gather up many examples as a guide to future research and identify problems that remain, including what we call pre-emptive instabilities, which occur before they are expected on the basis of Plateau's laws.

Supercritical water: a fascinating medium for soft matter

Shigeru Deguchi — 2008-06-20T22:19:43-00:00

Soft Matter, 2007, 3, 797 - 803, DOI: 10.1039/b611584e

Properties of water change dramatically at high temperatures and high pressures near and/or above the critical point (Tc = 374 °C, Pc = 22.1 MPa). The dielectric constant, for example, decreases from 78 at 25 °C and 0.1 MPa to 6 at the critical point, the value of which is comparable to that of 1-dodecanol. As fascinating characteristics of soft matter rely on unique properties of ambient liquid water, the change should have significant impacts on soft matter. However, our knowledge of soft matter under such extreme conditions is virtually nonexistent. In this article, properties of colloidal dispersions in water at high temperatures and high pressures are described. Implications of the findings for geological processes in deep-subsurface are also discussed.

Role of interparticle forces and interparticle friction on the bulk friction in charged granular media subjected to shearing

SJ Antony — 2008-06-11T22:25:54-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 3. (2007)

We study the consequences of the interplay between electrostatic forces, mechanical contact forces, and frictional properties of grains upon the bulk frictional properties of charged granular media subjected to quasistatic shearing. We show that, the variations in short-range electrostatic forces between the grains (which are often ignored in the existing studies) dominantly affect the bulk friction. Charging enhances the fabric anisotropy of heavily loaded contacts—this enhances the bulk friction, more significantly, in the case of low frictional granular systems.

Influences of the interstitial liquid on segregation patterns of granular slurries in a rotating drum

Tilo Finger — 2008-06-11T22:24:18-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 3. (2007)

Granular mixtures immersed in a liquid (slurries) show segregation dynamics which are quantitatively and qualitatively different from those of dry systems. The principal mechanisms of the segregation dynamics in slurries, as well as the relevant material parameters that must be taken into account in a dynamic description are not sufficiently understood so far. We investigate experimentally the influence of the viscosity of the interstitial liquid on the coarsening of axial segregation patterns in a horizontally rotating mixer. It is found that not only the characteristic time scales but also fundamental structural features of these patterns are influenced by the viscous properties of the liquid component.

Shape and erosion of pebbles

DJ Durian — 2007-03-19T13:52:50-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 2. (2007)

The shapes of flat pebbles may be characterized in terms of the statistical distribution of curvatures measured along their contours. We illustrate this method for clay pebbles eroded in a controlled laboratory apparatus, and also for naturally occurring rip-up clasts formed and eroded in the Mont St.-Michel bay. We find that the curvature distribution allows finer discrimination than traditional measures of aspect ratios. Furthermore, it connects to the microscopic action of erosion processes that are typically faster at protruding regions of high curvature. We discuss in detail how the curvature may be reliably deduced from digital photographs.

Rheology of a granular gas under a plane shear

Kuniyasu Saitoh — 2008-06-11T22:15:13-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 2. (2007)

The rheology of a two-dimensional granular gas under a plane shear is investigated. From the comparison among the discrete element method, the simulation of a set of hydrodynamic equation, and the analytic solution of the steady hydrodynamic equations, it is confirmed that the fluid equations derived from the kinetic theory give us accurate results even in relatively high density cases.

Origin of the reduced attracting force between a rotating dielectric particle and a stationary one

WJ Tian — 2008-06-11T22:06:04-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 2. (2007)

Recently Tao and Lan [Phys. Rev. E. 72, 041508 (2005)] experimentally reported that the rotation of a dielectric particle can reduce significantly the attracting interparticle force between the rotating dielectric particle and a stationary one in argon gas. We develop the Gu-Yu-Hui theory of relaxation [J. Chem. Phys. 116, 24 (2002)] to account for the Tao-Lan observations. Excellent agreement between the theoretical results and the Tao-Lan experimental data shows that the reduction in the attracting interparticle force is due to the effect of charge relaxation. We also show that the relaxation time of touching rotating particles can be accurately determined with the aid of the developed theory, for which, however, the well-known Maxwell-Wagner relaxation time is no longer valid.

Structure and rheology of organoclay suspensions

Jr — 2008-06-11T21:58:58-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 2. (2007)

We have characterized a montmorillonite-based organoclay dispersed in three different nonaqueous solvents using a combination of x-ray scattering, small-angle neutron scattering (SANS), and ultrasmall angle neutron scattering (USANS), together with rheological measurements. Consistent with these measurements, we present a structural model for the incompletely dispersed clay as consisting of randomly oriented tactoids made of partially overlapping clay sheets, with transverse dimensions of several microns. Intersheet correlation peaks are visible in x-ray scattering, and quantitatively fit by our model structure factor. SANS and USANS together show a power law of about −3 over a wide range of wave numbers below the intersheet correlation peak. Our model relates this power law to a power law distribution of the number of locally overlapping layers in a tactoid. The rheology data show that both storage and loss moduli, as well as yield stress, scale with a power law in volume fraction of about three. Equating the gel onset composition with the overlap of randomly oriented tactoids and taking into account the large transverse dimensions of the tactoids, we predict the gel point to be at or below 0.006 volume fraction organoclay. This is consistent with the rheology data.

Dynamics and instabilities of defects in two-dimensional crystals on curved backgrounds

Mark Bowick — 2008-06-11T21:47:51-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 2. (2007)

Point defects are ubiquitous in two-dimensional crystals and play a fundamental role in determining their mechanical and thermodynamical properties. When crystals are formed on a curved background, finite-length grain boundaries (scars) are generally needed to stabilize the crystal. We provide a continuum elasticity analysis of defect dynamics in curved crystals. By exploiting the fact that any point defect can be obtained as an appropriate combination of disclinations, we provide an analytical determination of the elastic spring constants of dislocations within scars and compare them with existing experimental measurements from optical microscopy. We further show that vacancies and interstitials, which are stable defects in flat crystals, are generally unstable in curved geometries. This observation explains why vacancies or interstitials are never found in equilibrium spherical crystals. We finish with some further implications for experiments and future theoretical work.

Microstructure of equilibrium fluid clusters in colloid-polymer suspensions

V Gopalakrishnan — 2008-06-11T21:45:00-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 2. (2007)

Several studies on colloidal depletion systems have reported the existence of a fluid phase consisting of clusters of particles above a critical polymer concentration that acts as a precursor regime to the gel phase at low colloid volume fractions (0.20). The clusters are found to be stable against further aggregation suggesting that individual particles are localized within a cluster. However the clusters themselves behave as distinct entities in an equilibrium fluid phase. In this study, we probe the internal microstructure of the cluster entities by ultrasmall angle x-ray scattering (USAXS) techniques. These studies reveal that over the accessible length scales, the microstructure of the particle clusters are similar to that observed in dense space-spanning depletion gels. The origin of these clusters is unclear but the scattering patterns as they settle with time reveal that the percolation of the clusters to form space-spanning gels does not influence their internal microstructure. These observations lend support to the hypothesis that the formation of space-spanning depletion gels at a given volume fraction is driven by the percolation of the particle clusters. Settling experiments at =0.08 also provide rough estimates of the cluster sizes that appear consistent with the observations from the USAXS experiments.

Ordering of two-dimensional crystals confined in strips of finite width

A Ricci — 2008-06-11T21:38:50-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 1. (2007)

Monte Carlo simulations are used to study the effect of confinement on a crystal of point particles interacting with an inverse power law potential r−12 in d=2 dimensions. This system can describe colloidal particles at the air-water interface, a model system for experimental study of two-dimensional melting. It is shown that the state of the system (a strip of width D) depends very sensitively on the precise boundary conditions at the two “walls” providing the confinement. If one uses a corrugated boundary commensurate with the order of the bulk triangular crystalline structure, both orientational order and positional order is enhanced, and such surface-induced order persists near the boundaries also at temperatures where the system in the bulk is in its fluid state. However, using smooth repulsive boundaries as walls providing the confinement, only the orientational order is enhanced, but positional (quasi-)long range order is destroyed: The mean-square displacement of two particles n lattice parameters apart in the y direction along the walls then crosses over from the logarithmic increase (characteristic for d=2) to a linear increase with n (characteristic for d=1). The strip then exhibits a vanishing shear modulus. These results are interpreted in terms of a phenomenological harmonic theory. Also the effect of incommensurability of the strip width D with the triangular lattice structure is discussed, and a comparison with surface effects on phase transitions in simple Ising and XY models is made.

Slippery diffusion-limited aggregation

Clair Seager — 2008-06-11T21:34:00-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 1. (2007)

Colloidal particles that interact through strong, short-range, secondary attractions in liquids form irreversible “slippery” bonds that are not shear-rigid. Through event-driven simulations of slippery attractive spheres, we show that space-filling fractal clusters still emerge from the process of “slippery” diffusion-limited aggregation (DLA). Although slippery and classic DLA clusters have the same fractal dimension, df=2.5, their average coordination numbers are quite different: zS=6 whereas zC=2. Local tetrahedral attractive jamming of the particles leads to a structure factor, S(q), that exhibits dense cluster peaks at higher wave numbers, q, and a fractal power-law rise toward lower q.

Colloidal transport through optical tweezer arrays

Yael Roichman — 2008-06-11T21:24:05-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 1. (2007)

Viscously damped particles driven past an evenly spaced array of potential energy wells or barriers may become kinetically locked in to the array, or else may escape from the array. The transition between locked-in and free-running states has been predicted to depend sensitively on the ratio between the particles' size and the separation between wells. This prediction is confirmed by measurements on monodisperse colloidal spheres driven through arrays of holographic optical traps.

Transient filamentous network structure of a colloidal suspension excited by stepwise electric fields

Yu Tian — 2008-06-11T21:18:17-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 1. (2007)

Jamming and force networks observed in electrorheological (ER) fluids bear many similarities to those observed in various granular and colloidal systems. We have measured the time evolution (transient stresses) of filamentous networks of colloidal particles in suspensions subjected to continuous tensile strain concomitant with the switching on and off of electric fields. The density of particle chains was found to increase exponentially with the applied tensile strain via a rapid formation of single chains followed by a slower coarsening (aggregation) of the chains. The two processes can be ascribed to the field-induced short-range and long-range interparticle forces, respectively, along with the tensile viscous force.

Effect of water-wall interaction potential on the properties of nanoconfined water

Pradeep Kumar — 2008-06-11T20:12:46-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 1. (2007)

Much of the understanding of bulk liquids has progressed through study of the limiting case in which molecules interact via purely repulsive forces, such as a hard-core or “repulsive ramp” potential. In the same spirit, we report progress on the understanding of confined water by examining the behavior of waterlike molecules interacting with planar walls via purely repulsive forces and compare our results with those obtained for Lennard-Jones (LJ) interactions between the molecules and the walls. Specifically, we perform molecular dynamics simulations of 512 waterlike molecules interacting via the TIP5P potential and confined between two smooth planar walls that are separated by 1.1 nm. At this separation, there are either two or three molecular layers of water, depending on density. We study two different forms of repulsive confinement, when the water-wall interaction potential is either (i) 1/r9 or (ii) a WCA-like repulsive potential. We find that the thermodynamic, dynamic, and structural properties of the liquid in purely repulsive confinements qualitatively match those for a system with a pure LJ attraction to the wall. In previous studies that include attractions, freezing into monolayer or trilayer ice was seen for this wall separation. Using the same separation as these previous studies, we find that the crystal state is not stable with 1/r9 repulsive walls but is stable with WCA-like repulsive confinement. However, by carefully adjusting the separation of the plates with 1/r9 repulsive interactions so that the effective space available to the molecules is the same as that for LJ confinement, we find that the same crystal phases are stable. This result emphasizes the importance of comparing systems only using the same effective confinement, which may differ from the geometric separation of the confining surfaces.

Aging after shear rejuvenation in a soft glassy colloidal suspension: Evidence for two different regimes

F Ianni — 2008-06-11T19:44:12-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 1. (2007)

The aging dynamics after shear rejuvenation in a glassy clay suspension have been investigated through dynamic light scattering (DLS). Two different aging regimes are observed: one is attained if the sample is rejuvenated before its gelation and one after the rejuvenation of the gelled sample. In the first regime, the application of shear fully rejuvenates the sample, as the system dynamics soon after shear cessation follow the same aging evolution characteristic of standard aging. In the second regime, aging proceeds very fast after shear rejuvenation, and classical DLS cannot be used. An original protocol to measure an ensemble-averaged intensity-correlation function is proposed and its consistency with classical DLS is verified. The fast aging dynamics of rejuvenated gelled samples exhibit a power-law dependence of the slow relaxation time on the waiting time.

Critical packing in granular shear bands

S Fazekas — 2008-06-11T19:37:09-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 1. (2007)

In a realistic three-dimensional setup, we simulate the slow deformation of idealized granular media composed of spheres undergoing an axisymmetric triaxial shear test. We follow the self-organization of the spontaneous strain localization process leading to a shear band and demonstrate the existence of a critical packing density inside this failure zone. The asymptotic criticality arising from the dynamic equilibrium of dilation and compaction is found to be restricted to the shear band, while the density outside of it keeps the memory of the initial packing. The critical density of the shear band depends on friction (and grain geometry) and in the limit of infinite friction it defines a specific packing state, namely the dynamic random loose packing.

Types of gas fluidization of cohesive granular materials

Jose Valverde — 2008-06-11T19:21:58-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 3. (2007)

Some years ago it was shown that gas-fluidized powders may transit from solidlike to fluidlike fluidization prior to bubbling, shedding light on a long-standing controversy on the nature of “homogeneous” fluidization. In this paper it is shown that some gas-fluidized powders may also transit from the fluidlike regime to elutriation, with full suppression of the bubbling regime. We provide a diagram that can be used to predict these types of fluidization exhibited by cohesive powders based on simple phenomenological equations in which particle aggregation due to attractive forces is a key ingredient.

Structure and stability of bent core liquid crystal fibers

C Bailey — 2008-06-11T19:04:44-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 3. (2007)

Recently it was found that fluid smectic phases of bent core liquid crystals formed freestanding fibers of extremely high slenderness ratios. Studies of these fibers showed that their structure was composed of concentric cylindrical smectic layers. For this configuration to be stable there must be an energy term that desires bending of the smectic layers. We show that an energy term that deals with the divergence of the dipolar direction can encourage layer bending if the layer chirality value is allowed to vary. The energy term associated with holding the layer chirality is closely related to layer compressions and electrical self-interactions. For our model, we assumed a simple smectic-C geometry with constant molecular tilt and cone angle defined by the director with respect to the layer normal, but allowed a constant variation of the polar direction about the director. Applying this simplified model to a free energy which accounts for director distortions, divergence of the polar direction, biaxial layer strain, surface tension, and electrical self-interactions, we were able to show consistency between the stable fiber radius and other properties predicted in our model to results from experimental studies.

Shear-induced crystallization in jammed systems

Nathan Duff — 2008-06-11T18:55:11-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 3. (2007)

Simulations are used to address the effects of oscillating shear strain on jammed systems composed of spherical particles. The simulations show that shear oscillations with amplitudes of more than a few percent lead to substantial crystallization of the system. To ensure that the conclusions are independent of the simulation methodology, a range of simulations are carried out that use both molecular dynamics and athermal dynamics methods, soft and hard potentials, potentials with and without attractive forces, and systems with and without surrounding walls. The extent of crystallization is monitored primarily by the Q6 order parameter, but also in some simulations by the potential energy and the radial distribution function, and by direct visual inspection. A mechanism is proposed for shear-induced crystallization of jammed systems, based on fold catastrophes of the free-energy landscape.

Collision process between an incident bead and a three-dimensional granular packing

Djaoued Beladjine — 2008-06-11T18:16:50-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 6. (2007)

We report on experimental studies of the collision process between an incident bead and a three-dimensional granular packing (made of particles identical to the impacting one). The understanding of such a process and the resulting ejection of particles is, in particular, crucial to describe eolian sand transport. We present here an extensive experimental analysis of the collision and ejection process. The analysis is two dimensional in the sense that we determined only the vertical component Vz of the ejection velocity of the splashed particles and the horizontal component Vx lying in the incident plane. We extracted in particular the distribution of the ejection velocities for a wide range of impact angles i and incident velocity Vi. We show that the mean quadratic horizontal velocity of the splashed particles is almost insensitive to changes in the impact angle and velocity, while the mean quadratic vertical velocity slightly increases with increasing impact velocity (as Vi1/2" align="middle">). Moreover, the mean number of splashed particles per collision is found to be dependent on both the impact angle and velocity, and to scale with the impact speed as Vi3/2" align="middle">. A consequence of these outcomes is that the sum of the kinetic energy of the splashed particles is directly proportional to the kinetic energy of the incident particle. Finally, we provide the bivariate probability distribution function P(Vx,Vz) of the ejection velocities and show that it can be approximated by the product of a log-normal distribution and a circular normal one.

Crystallization of colloidal hard spheres under gravity

Matthieu Marechal — 2008-06-11T16:52:31-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 6. (2007)

Using grand canonical Monte Carlo simulations, we study the crystallization of colloidal hard spheres under gravity. More specifically, we investigate the nature of the freezing transition as a function of gravity and chemical potential of the hard spheres. We find a discontinuous freezing transition where several fluid layers close to the bottom of the sample freeze simultaneously, i.e., at the same chemical potential. We also find that the number of layers that freezes at the same chemical potential decreases for higher gravitational field strength. Upon increasing the chemical potential further, the crystalline film thickness increases continuously.

Magnetic properties of polydisperse ferrofluids: A critical comparison between experiment, theory, and computer simulation

Alexey Ivanov — 2008-06-11T16:50:04-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 6. (2007)

Experimental magnetization curves for a polydisperse ferrofluid at various concentrations are examined using analytical theories and computer simulations with the aim of establishing a robust method for obtaining the magnetic-core diameter distribution function p(x). Theoretical expressions are fitted to the experimental data to yield the parameters of p(x). It is shown that the majority of available theories yield results that depend strongly on the ferrofluid concentration, even though the magnetic composition should be fixed. The sole exception is the second-order modified mean-field (MMF2) theory of Ivanov and Kuznetsova [Phys. Rev. E 64, 041405 (2001)] which yields consistent results over the full experimental range of ferrofluid concentration. To check for consistency, extensive molecular dynamics and Monte Carlo simulations are performed on systems with discretized versions of p(x) corresponding as closely as possible to that of the real ferrofluid. Essentially perfect agreement between experiment, theory, and computer simulation is demonstrated. In addition, the MMF2 theory provides excellent predictions for the initial susceptibility measured in simulations.

Wall slip, shear banding, and instability in the flow of a triblock copolymer micellar solution

Sébastien Manneville — 2008-06-11T16:47:04-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 6. (2007)

The shear flow of a triblock copolymer micellar solution (PEO-PPO-PEO Pluronic P84 in brine) is investigated using simultaneous rheological and velocity profile measurements in the concentric cylinder geometry. We focus on two different temperatures below and above the transition temperature Tc which was previously associated with the apparition of a stress plateau in the flow curve. (i) At T=37.0 °C<Tc, the bulk flow remains homogeneous and Newtonian-like, although significant wall slip is measured at the rotor that can be linked to an inflexion point in the flow curve. (ii) At T=39.4 °C>Tc, the stress plateau is shown to correspond to stationary shear-banded states characterized by two high shear rate bands close to the walls and a very weakly sheared central band, together with large slip velocities at the rotor. In both cases, the high shear branch of the flow curve is characterized by flow instability. Interpretations of wall slip, three-band structure, and instability are proposed in light of recent theoretical models and experiments.

Swirling motion in a system of vibrated elongated particles

Igor Aranson — 2008-06-11T16:41:58-00:00

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

Large-scale collective motion emerging in a monolayer of vertically vibrated elongated particles is studied. The motion is characterized by recurring swirls, with the characteristic scale exceeding several times the size of an individual particle. Our experiments identified a small horizontal component of the oscillatory acceleration of the vibrating plate in combination with orientation-dependent bottom friction (with respect to horizontal acceleration) as a source for the swirl formation. We developed a continuum model operating with the velocity field and local alignment tensor, which is in qualitative agreement with the experiment.

Velocity distribution and the effect of wall roughness in granular Poiseuille flow

KC Vijayakumar — 2008-06-11T16:09:47-00:00

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

From event-driven simulations of a gravity-driven channel flow of inelastic hard disks, we show that the velocity distribution function remains close to a Gaussian for a wide range densities (even when the Knudsen number is of order 1) if the walls are smooth and the particle collisions are nearly elastic. For dense flows, a transition from a Gaussian to a power-law distribution for the high-velocity tails occurs with increasing dissipation in the center of the channel, irrespective of wall roughness. For a rough wall, the near-wall distribution functions are distinctly different from those in the bulk, even in the quasielastic limit.

Coefficient of restitution for viscoelastic disks

Thomas Schwager — 2008-06-11T15:16:12-00:00

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

The dissipative collision of two identical viscoelastic disks is studied. By using a known law for the elastic part of the interaction force and the viscoelastic damping model an analytical solution for the coefficient of restitution is given. The coefficient of restitution depends significantly on the impact velocity. It approaches 1 for small velocities and decreases for increasing velocities.

Forcing independent velocity distributions in an experimental granular fluid

PM Reis — 2008-06-11T14:56:39-00:00

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

We present experimental results on the velocity statistics of a granular fluid with an effective stochastic thermostat, in a quasi-two-dimensional configuration. We find the base state, as measured by the single particle velocity distribution P(c) in the central high-probability regions, to be well described by P(c)=fMB[1+a2S2(c2)]: It deviates from a Maxwell-Boltzmann fMB by a second order Sonine polynomial S2(c2) with a single adjustable parameter a2. We find a2 to be a function of the filling fraction and independent of the driving over a wide range of frequencies and accelerations. Moreover, there is a consistent overpopulation in the distribution's tails, which scale as Pexp(−A×c3/2). To our knowledge, this is the first time that Sonine deviations have been measured in an experimental system.

Effective potential between two spheres in a suspension of adhesive rods

Chengyu Zhang — 2008-06-11T14:49:19-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 4. (2007)

Analytic treatment and Monte Carlo simulations are used to study the effective potential between two spheres in a suspension of rods whose ends can adhere to the surface of the spheres. When only one end of each rod is adhesive the effective potential changes from being attractive to repulsive with enhancing the adherence, but when both ends of each rod are adhesive the effective potential is not a monotonic function of the distance between the two spheres for strong adherence. As the adhesive strength is fixed, the range of the effective potential will increase with increasing the length of the rods. When the adhesive range is much smaller than the diameter of the spheres, its influence on single end adhesion is approximately linear and on two end adhesion is about quadratic. Our results are qualitatively consistent with a recent experimental work.

Segregation of fractal aggregates grown from two seeds

Deepak Bankar — 2008-06-11T14:43:47-00:00

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

We study the generalized diffusion-limited aggregates, grown from two proximal nucleation seeds placed at distance d lattice units and investigate the probability p(d) that these aggregates get connected. We vary the sticking probability to get a range of aggregate geometry from fractal to compact one. For fractal aggregates, p(d) decays rapidly with d, while for compact ones, the decay is so slow that p(d)1 for all practical distances. We experimentally demonstrate similar behavior for viscous fingering patterns with two injection points and electrochemical deposits grown on two cathodes. Our observations along with previous results on competitive growth suggest a common underlying principle.

Stability diagram for dense suspensions of model colloidal Al[sub 2]O[sub 3] particles in shear flow

Martin Hecht — 2008-06-11T14:33:11-00:00

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

In Al2O3 suspensions, depending on the experimental conditions, very different microstructures can be found, comprising fluidlike suspensions, a repulsive structure, and a clustered microstructure. For technical processing in ceramics, the knowledge of the microstructure is of importance, since it essentially determines the stability of a workpiece to be produced. To enlighten this topic, we investigate these suspensions under shear by means of simulations. We observe cluster formation on two different length scales: the distance of nearest neighbors and on the length scale of the system size. We find that the clustering behavior does not depend on the length scale of observation. If interparticle interactions are not attractive the particles form layers in the shear flow. The results are summarized in a stability diagram.

Evidence for three-dimensional unstable flows in shear-banding wormlike micelles

Lydiane Bécu — 2008-06-11T00:06:30-00:00

We report on an experimental study of the shear-banding phenomenon in the concentrated wormlike micellar system CTAB at 20 wt. % in D2O. Time-resolved velocity profiles are recorded using ultrasonic velocimetry simultaneously to global rheological data. Our results confirm the studies performed previously by Fischer and Callaghan [Phys. Rev. E 64, 011501 (2001)]. Time averaged velocity profiles display an unsheared “nematic gel.” In the range of applied shear rate, the flow field exhibits very fast temporal fluctuations. Suspicions for the presence of three-dimensional flow are evidenced and possible causes for a three-dimensional instability are discussed together with the coupling of wall slip to bulk dynamic.

Do short-time fluctuations predict the long-time dynamic heterogeneity in a supercooled liquid?

Rodriguez Fris — 2008-06-10T23:58:43-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 1. (2007)

Recent investigations have demonstrated that the short-time fluctuations in a supercooled liquid can be used as predictors of the long-time dynamic propensity (that is, the regions of the sample with enhanced tendency to be mobile within time scales on the order of the -relaxation time). This could mean that the long-time dynamics (the actual mobility of the particles at such long times) would be implicit in the short-time dynamics or else, that the long-time dynamic propensity [as defined in A. Widmer-Cooper and P. Harrowell, Phys. Rev. Lett. 96, 185701 (2006)], while providing a measure of the degree of jamming of the local structure, would only be sensitive to the short time behavior. The first scenario is in clear disagreement with our recent finding that the influence of the local structure on dynamics (as determined by the propensity for motion) is only local in time, fading out at times close to the metabasin lifetime, much before the -relaxation time. Thus, in this work we show that the short-time fluctuations in supercooled liquids do in fact represent precursors to the dynamics at intermediate times commensurate with the metabasin lifetime (being thus able to predict the regions of the sample that will present high propensity for motion at such stage) but that the dynamical behavior at later times of the relaxation is unpredictable, in agreement with a metabasin random walk scenario.

Physical age of soft-jammed systems

G Ovarlez — 2008-06-10T23:51:18-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 1. (2007)

We study experimentally the liquid-solid transition in a soft-jammed system and focus on its aging in the solid regime. We investigate the impact of temperature, density, and load changes on the material behavior. We show that all elastic modulus versus time curves fall on a single master curve when rescaled by an appropriate factor function of the density, the temperature, the load, and the time elapsed since preshear. This allows us to distinguish the effect of temperature and density on the mechanical properties and their effect on aging. Since the time evolutions of the elastic modulus under various conditions are similar within a factor, we suggest that the rescaled time reflects the physical age of the material; i.e., it describes the degree of progress of the structural organization relative to a state of reference of the system in the solid regime and constitutes a means for characterizing the effective state of such systems.

Spatial force correlations in granular shear flow. I. Numerical evidence

Gregg Lois — 2008-06-10T23:40:05-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 2. (2007)

We investigate the emergence of correlations in granular shear flow. By increasing the density of a simulated granular flow, we observe a transition from a dilute regime, where interactions are dominated by binary collisions, to a dense regime characterized by large force networks and collective motions. With increasing density, interacting grains tend to form networks of simultaneous contacts due to the dissipative nature of collisions. We quantify the size of these networks by measuring two-point force correlations and find dramatic changes in the statistics of contact forces as the size of the networks increases.

Vertical ordering of rods under vertical vibration

M Ramaioli — 2008-06-10T23:33:20-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 2. (2007)

Granular media composed of elongated particles rearrange and order vertically upon vertical vibration. We perform pseudo-two-dimensional discrete element model simulations and show that this phenomenon also takes place with no help from vertical walls. We quantitatively analyze the sizes of voids forming during vibrations and consider a void-filling mechanism to explain the observed vertical ordering. Void filling can explain why short rods are less prone to align vertically than long ones. We cannot, however, explain, invoking just void filling, the existence of an optimum acceleration to promote vertical ordering and its dependence on particle length. We finally introduce an interpretation of the phenomenon, by considering the energetic barriers that particles have to overcome to exit a horizontal or a vertical lattice. By comparing these energetic thresholds with the peak mean particle fluctuant kinetic energy, we identify three different regimes. In the intermediate regime a vertical lattice is stable, while a horizontal is not. This interpretation succeeds in reconciling both dependencies on vibration acceleration and on particle length.

Topological persistence and dynamical heterogeneities near jamming

AR Abate — 2008-01-06T13:11:39-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 2. (2007)

We introduce topological methods for quantifying spatially heterogeneous dynamics, and use these tools to analyze particle-tracking data for a quasi-two-dimensional granular system of air-fluidized beads on approach to jamming. In particular, we define two overlap order parameters, which quantify the correlation between particle configurations at different times, based on a Voronoi construction and the persistence in the resulting cells and nearest neighbors. Temporal fluctuations in the decay of the persistent area and bond order parameters define two alternative dynamic four-point susceptibilities A() and B(), well suited for characterizing spatially heterogeneous dynamics. These are analogous to the standard four-point dynamic susceptibility 4(l,), but where the space dependence is fixed uniquely by topology rather than by discretionary choice of cutoff function. While these three susceptibilities yield characteristic time scales that are somewhat different, they give domain sizes for the dynamical heterogeneities that are in good agreement and that diverge on approach to jamming.

Rheological properties of aging thermosensitive suspensions

Eko Purnomo — 2008-06-10T22:05:15-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 2. (2007)

Aging observed in soft glassy materials inherently affects the rheological properties of these systems and has been described by the soft glassy rheology (SGR) model [S. M. Fielding et al., J. Rheol. 44, 323 (2000)]. In this paper, we report the measured linear rheological behavior of thermosensitive microgel suspensions and compare it quantitatively with the predictions of the SGR model. The dynamic moduli [G(,t) and G(,t)] obtained from oscillatory measurements are in good agreement with the model. The model also predicts quantitatively the creep compliance J(t−tw,tw), obtained from step stress experiments, for the short time regime [(t−tw)<tw]. The relative effective temperature /g obtained from both the oscillatory and the step stress experiments is indeed less than 1 (/g<1) in agreement with the definition of aging. Moreover, the elasticity of the compressed particles (Gp) increases with increased compression, i.e., the degree of hindrance and consequently also the bulk elasticity (G and 1/J) increases with the degree of compression.

Lubrication effects on the flow of wet granular materials

Qing Xu — 2008-01-23T19:06:56-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 3. (2007)

We investigate the dynamics of a partially saturated grain-liquid mixture with a rotating drum apparatus. The drum is partially filled with the mixture and then rotated about its horizontal axis. We focus on the continuous avalanching regime and measure the impact of the volume fraction and viscosity of the liquid on the dynamic surface angle. The inclination angle of the surface is observed to increase sharply to a peak and then decrease as a function of liquid volume fraction. The height of the peak is observed to increase with rotation rate. For higher liquid volume fractions, the inclination angle of the surface can decrease with viscosity before increasing. The viscosity where the minimum occurs decreases with the rotation rate of the drum. Limited measurements of the flow depth were made, and these were observed to show only fractional changes with volume fraction and rotation speeds. We show that the qualitative features of our observations can be understood by analyzing the effect of lubrication forces on the time scale over which particles come in contact.

Forces between single pairs of charged colloids in aqueous salt solutions

C Gutsche — 2007-10-02T11:29:36-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 3. (2007)

Forces between single pairs of negatively charged micrometer-sized colloids in aqueous solutions of monovalent, divalent, or trivalent counter-ions at varying concentrations have been measured by employing optical tweezers. The experimental data have been analyzed by using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and a numerical solution of the Poisson-Boltzmann (PB) equation. With monovalent counterions, the data are well described by the DLVO and PB theories, suggesting that the DLVO theory is adequate to describe the colloidal forces at these conditions. At higher counter-ion valence, the approximations within the two theories become evident.

Connecting microscopic simulations with kinetically constrained models of glasses

Matthew Downton — 2008-06-10T21:37:45-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 3. (2007)

Kinetically constrained spin models are known to exhibit dynamical behavior mimicking that of glass forming systems. They are often understood as coarse-grained models of glass formers, in terms of some “mobility” field. The identity of this “mobility” field has remained elusive due to the lack of coarse-graining procedures to obtain these models from a more microscopic point of view. Here we exhibit a scheme to map the dynamics of a two-dimensional soft disk glass former onto a kinetically constrained spin model, providing an attempt at bridging these two approaches.

Nonlinear microrheology of wormlike micelle solutions using ferromagnetic nanowire probes

Nathan Cappallo — 2008-06-10T21:28:50-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 3. (2007)

Ferromagnetic nanowires were employed to investigate the microrheology of wormlike micelle solutions composed of equimolar cetylpyridinium chloride–sodium salicylate. For a wire rotated about a short axis, the drag at low rotation rate R is independent of R and strongly temperature dependent, consistent with the macroscopic shear viscosity. Above a critical rotation rate c, the drag is independent of temperature and decreases as a power law with increasing rate. The onset of nonlinear drag is characterized by a peak associated with contributions from extensional flow. Above c, the fluid generates an additional torque that tilts the wire out of regions of high shear flow and that is interpreted as a consequence of a shear-induced transition to nematic order among the micelles. Rotation of the wire in response to this torque reveals directly the anisotropy of the drag in the nonlinear state.

Homogeneous and heterogeneous nucleation of Lennard-Jones liquids

Hui Wang — 2008-06-10T21:23:56-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 3. (2007)

The homogeneous and heterogeneous nucleation of a Lennard-Jones liquid is investigated using the umbrella sampling method. The free energy cost of forming a nucleating droplet is determined as a function of the quench depth, and the saddle point nature of the droplets is verified using an intervention technique. The structure and symmetry of the nucleating droplets are found for a range of temperatures. We find that for deep quenches the nucleating droplets become more anisotropic and diffuse with no well-defined core or surface. The environment of the nucleating droplets forms randomly stacked hexagonal planes. This behavior is consistent with a spinodal nucleation interpretation. We also find that the free energy barrier for heterogeneous nucleation is a minimum when the lattice spacing of the impurity equals the lattice spacing of the equilibrium crystalline phase. If the lattice spacing of the impurity is different, the crystal grows into the bulk instead of wetting the impurity.

Simulated three-component granular segregation in a rotating drum

DC Rapaport — 2008-03-07T17:08:22-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 4. (2007)

Discrete particle simulations are used to model segregation in granular mixtures of three different particle species in a horizontal rotating drum. Axial band formation is observed, with medium-size particles tending to be located between alternating bands of big and small particles. Partial radial segregation also appears; it precedes the axial segregation and is characterized by an inner core region richer in small particles. Axial bands are seen to merge during the long simulation runs, leading to a coarsening of the band pattern; the relocation of particles involved in one such merging event is examined. Overall, the behavior is similar to experiment and represents a generalization of what occurs in the simpler two-component mixture.

Effects of fluid viscosity on band segregation dynamics in bidisperse granular slurries

Stanley Fiedor — 2008-06-10T20:53:58-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 4. (2007)

Parallel experiments in long, axially rotated cylinders are used to study the influence of interstitial fluid viscosity on particle segregation in bidisperse granular slurries. A uniformly mixed initial state segregates into surface bands, which alternate between regions of large particles and regions composed of a mixture of small and large particles. As the tumbler rotates, the relative area of the mixed particle bands increases and saturates, while the number of bands reaches a peak and then decreases logarithmically in time for all viscosities studied. With increasing interstitial fluid viscosity, the asymptotic mixed band area increases proportionally, the time for bands to appear at the surface decreases, and the peak number of bands goes through a maximum at a viscosity of ~3 cP. Extrapolation to the low-viscosity limit matches the data for dry granular systems; at the high-end viscosity there is a value beyond which no axial banding occurs. A heuristic mechanism based on the coexistence of pure and mixed particle phases and their dependence on viscosity is presented to rationalize key aspects of the results.