CiteULike: weeks's library [361 articles]

Spatial cooperativity in soft glassy flows

J Goyon — 2008-07-03T06:17:56-00:00

Nature, Vol. 454, No. 7200. (2008), 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.

Structural Rearrangements That Govern Flow in Colloidal Glasses

Peter Schall — 2008-03-01T21:29:01-00:00

Science, Vol. 318, No. 5858. (21 December 2007), pp. 1895-1899.

Structural rearrangements are an essential property of atomic and molecular glasses; they are critical in controlling resistance to flow and are central to the evolution of many properties of glasses, such as their heat capacity and dielectric constant. Despite their importance, these rearrangements cannot directly be visualized in atomic glasses. We used a colloidal glass to obtain direct three-dimensional images of thermally induced structural rearrangements in the presence of an applied shear. We identified localized irreversible shear transformation zones and determined their formation energy and topology. A transformation favored successive ones in its vicinity. Using continuum models, we elucidated the interplay between applied strain and thermal fluctuations that governs the formation of these zones in both colloidal and molecular glasses. 10.1126/science.1149308

Layering and Position-Dependent Diffusive Dynamics of Confined Fluids

Jeetain Mittal — 2008-05-02T15:47:32-00:00

Physical Review Letters, Vol. 100, No. 14. (2008)

We study the diffusive dynamics of a hard-sphere fluid confined between parallel smooth hard walls. The position-dependent diffusion coefficient normal to the walls is larger in regions of high local packing density. High density regions also have the largest available volume, consistent with the fast local diffusivity. Indeed, local and global diffusivities as a function of the Widom insertion probability approximately collapse onto a master curve. Parallel and average normal diffusivities are strongly coupled at high densities and deviate from bulk fluid behavior.

Decoupling of exchange and persistence times in atomistic models of glass formers

Lester Hedges — 2008-06-18T13:29:30-00:00

The Journal of Chemical Physics, Vol. 127, No. 21. (2007)

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Application of ferromagnetic nanowires to interfacial microrheology

A Anguelouch — 2008-02-19T17:32:40-00:00

Applied Physics Letters, Vol. 89, No. 11. (2006)

Confocal microscopy of colloidal particles: Towards reliable, optimum coordinates

MC Jenkins — 2008-02-19T17:20:12-00:00

Advances in Colloid and Interface Science, Vol. 136, No. 1-2. (15 January 2008), pp. 65-92.

Over the last decade, the light microscope has become increasingly useful as a quantitative tool for studying colloidal systems. The ability to obtain particle coordinates in bulk samples from micrographs is particularly appealing. In this paper we review and extend methods for optimal image formation of colloidal samples, which is vital for particle coordinates of the highest accuracy, and for extracting the most reliable coordinates from these images. We discuss in depth the accuracy of the coordinates, which is sensitive to the details of the colloidal system and the imaging system. Moreover, this accuracy can vary between particles, particularly in dense systems. We introduce a previously unreported error estimate and use it to develop an iterative method for finding particle coordinates. This individual-particle accuracy assessment also allows comparison between particle locations obtained from different experiments. Though aimed primarily at confocal microscopy studies of colloidal systems, the methods outlined here should transfer readily to many other feature extraction problems, especially where features may overlap one another.

Continuous Particle Separation Through Deterministic Lateral Displacement

Lotien Huang — 2007-11-30T17:48:27-00:00

Science, Vol. 304, No. 5673. (14 May 2004), pp. 987-990.

We report on a microfluidic particle-separation device that makes use of the asymmetric bifurcation of laminar flow around obstacles. A particle chooses its path deterministically on the basis of its size. All particles of a given size follow equivalent migration paths, leading to high resolution. The microspheres of 0.8, 0.9, and 1.0 micrometers that were used to characterize the device were sorted in 40seconds with a resolution of [~]10nanometers, which was better than the time and resolution of conventional flow techniques. Bacterial artificial chromosomes could be separated in 10 minutes with a resolution of [~]12%. 10.1126/science.1094567

Sheared foam as a supercooled liquid?

SA Langer — 2007-12-31T22:52:55-00:00

EPL (Europhysics Letters), Vol. 49, No. 1. (2000), pp. 68-74.

We conduct numerical simulations on a simple model of a two-dimensional steady-state sheared foam, and define a quantity G that measures stress fluctuations in the constant-area system. This quantity reduces to the temperature in an equilibrium system. We find that the relation between the viscosity and G is the same as that between viscosity and temperature in a very different system, namely a supercooled liquid. This is the first evidence of a common phenomenon linking these two systems.

Magnetic soap films and magnetic soap foams

Florence Elias — 2007-12-21T13:07:36-00:00

Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. 263, No. 1-3. (1 August 2005), pp. 65-75.

We investigate the physical properties of soap films and soap foams, the liquid phase of which contains a stable colloidal suspension of magnetic particles. The physical properties of such systems result from the equilibrium between the capillary forces, the gravity and the local magnetic forces. Therefore, they depend on the strength of an applied magnetic field or force, which makes it possible to act from outside on the system. We present the effect of the external field on the magnetic soap foams and films through two studies. First, we show that the dynamics of drainage in a freely suspended magnetic soap film is modified by an external magnetic field. Second, we present the control, via an external magnetic force, of the structure adopted by a monodisperse foam in a cylindrical tube. Those studies open possible applications of the ferrofluid foam, in the field of technological applications, but also as a model experimental system to study the drainage, the dynamical rheology, and the stability of soap foams.

Crystallization of a Quasi-Two-Dimensional Granular Fluid

PM Reis — 2007-12-10T22:23:17-00:00

Physical Review Letters, Vol. 96, No. 25. (2006), pp. 258001-258001.

We experimentally investigate the crystallization of a uniformly heated quasi-2D granular fluid as a function of the filling fraction. Our experimental results for the Lindemann melting criterion, the radial distribution function, the bond order parameter, and the statistics of topological changes at the particle level are the same as those found in simulations of equilibrium hard disks. This direct mapping suggests that the study of equilibrium systems can be effectively applied to study nonequilibrium steady states such as those found in our driven and dissipative granular system.

Caging Dynamics in a Granular Fluid

PM Reis — 2007-11-30T02:40:21-00:00

Physical Review Letters, Vol. 98, No. 18. (2007)

We report an experimental investigation of the caging motion in a uniformly heated granular fluid for a wide range of filling fractions, . At low the classic diffusive behavior of a fluid is observed. However, as is increased, temporary cages develop and particles become increasingly trapped by their neighbors. We statistically analyze particle trajectories and observe a number of robust features typically associated with dense molecular liquids and colloids. Even though our monodisperse and quasi-2D system is known to not exhibit a glass transition, we still observe many of the precursors usually associated with glassy dynamics. We speculate that this is due to a process of structural arrest provided, in our case, by the presence of crystallization.

Correlation between Dynamic Heterogeneity and Medium-Range Order in Two-Dimensional Glass-Forming Liquids

Takeshi Kawasaki — 2007-12-07T05:03:46-00:00

Physical Review Letters, Vol. 99, No. 21. (2007), pp. 215701-215701.

A glassy state of matter results if crystallization is avoided upon cooling or increasing density. However, the physical factors controlling the ease of vitrification and nature of the glass transition remain elusive. Using numerical simulations of polydisperse hard disks, we find a direct relation between medium-range crystalline ordering and the slow dynamics which characterizes the glass transition. This suggests an intriguing scenario that the strength of frustration controls both the ease of vitrification and nature of the glass transition. Vitrification may be a process of hidden crystalline ordering under frustration, at least in our system.

Confinement, entropy, and single-particle dynamics of equilibrium hard-sphere mixtures

Jeetain Mittal — 2007-12-01T12:11:26-00:00

The Journal of Chemical Physics, Vol. 127, No. 15. (2007)

High-Throughput Synthesis of Anisotropic Colloids via Holographic Lithography

JH Moon — 2007-11-29T01:55:01-00:00

Advanced Materials, Vol. 19 (2007), pp. 2508-2512.

Monodisperse anisotropic colloids are synthesized (see figure) via holographic lithography and using chemically amplified photoresists. The holographic lithography offers a high-throughput and flexible route to produce particles. To engineer colloidal particles, we apply direct modification of the photoresist surface either uniformly or selectively through chemical coupling, physical grafting, and selective metal deposition.

Heterogeneity at the glass transition: what do we know?

Hans Sillescu — 2007-11-28T00:27:28-00:00

Journal of Non-Crystalline Solids, Vol. 307-310 (September 2002), pp. 16-23.

We critically discuss the information that can be obtained from experiments with respect to the existence, the life time, and the length scale of dynamical heterogeneity in glass-forming liquids. The ability to select a dynamically distinguishable subensemble and observe its return to the full equilibrium ensemble is illustrated by examples from multi-dimensional NMR. We also discuss non-resonant hole burning spectroscopy as an example for which two separate time scales are involved.

Computational probes of molecular motion in the Lewis-Wahnström model for ortho-terphenyl

Thomas Lombardo — 2007-05-15T20:53:34-00:00

The Journal of Chemical Physics, Vol. 125, No. 17. (2006), pp. 174507-174507.

We use molecular dynamics simulations to investigate translational and rotational diffusion in a rigid three-site model of the fragile glass former ortho-terphenyl, at 260 KT346 K and ambient pressure. An Einstein formulation of rotational motion is presented, which supplements the commonly used Debye model. The latter is shown to break down at supercooled temperatures as the mechanism of molecular reorientation changes from small random steps to large infrequent orientational jumps. We find that the model system exhibits non-Gaussian behavior in translational and rotational motion, which strengthens upon supercooling. Examination of particle mobility reveals spatially heterogeneous dynamics in translation and rotation, with a strong spatial correlation between translationally and rotationally mobile particles. Application of the Einstein formalism to the analysis of translation-rotation decoupling results in a trend opposite to that seen in conventional approaches based on the Debye formalism, namely, an enhancement in the effective rate of rotational motion relative to translation upon supercooling. ©2006 American Institute of Physics

Translation-rotation paradox for diffusion in fragile glass-forming liquids

Frank Stillinger — 2007-04-01T03:01:19-00:00

Physical Review E, Vol. 50, No. 3. (1994), pp. 2064-2068.

Translational and rotational diffusion rates in low-molecular-weight liquids tend to conform well to the predictions of the classic Stokes-Einstein-Debye model if temperature T is not too low. Specifically; the diffusion constants D trans and D rot are proportional to T /η( T ); where η is the shear viscosity. However; fragile glass formers seem to present a paradox: near the glass transition temperature T g this proportionality continues for D rot ; but D trans can be enhanced by 10 2 . A ‘‘fluidized domain’’ model is proposed to explain these observations. Owing to a suitable combination of domain parameters (mean size; lifetime; concentration; internal viscosity); the observed diffusion rate discrepancy can indeed be rationalized. Rough estimates for these domain parameters are provided for two fragile glass formers (orthoterphenyl and 1;3;5-tri-α-naphthyl benzene) at their respective T g ’s.

Microrheology with modulated optical nanoprobes (MOONs)

Caleb Behrend — 2007-07-26T18:15:07-00:00

Journal of Magnetism and Magnetic Materials, Vol. 293, No. 1. (May 2005), pp. 663-670.

Metal-capping of one hemisphere of a nano or microparticle breaks the particle's optical symmetry, allowing its orientation to be tracked using fluorescence and reflection. Tracking orientation and rotation allows the torques acting on the particles be inferred. In addition to serving as rotational nanoviscometers these probes report on local Brownian, magnetic and biomechanical torques.

Microrheology of complex fluids

T Waigh — 2005-02-12T20:30:32-00:00

Reports on Progress in Physics, Vol. 68, No. 3. (March 2005), pp. 685-742.

Microrheology from Rotational Diffusion of Colloidal Particles

Efren Reyes — 2007-05-18T11:12:10-00:00

Physical Review Letters, Vol. 94, No. 10. (2005)

The microrheology of viscoelastic fluids is obtained from rotational diffusion of optically anisotropic spherical colloidal probes, measured by depolarized dynamic light scattering. The storage and loss moduli obtained from the rotational mean squared displacement is in excellent agreement with those obtained from translational diffusion and by mechanical measurements. We also show that this method is applicable to samples with strong light scattering components. This extends the capabilities of the microrheological methods based on the diffusional motion of colloidal probes.

Optical Microrheology Using Rotating Laser-Trapped Particles

Alexis Bishop — 2007-06-18T23:26:55-00:00

Physical Review Letters, Vol. 92, No. 19. (2004)

We demonstrate an optical system that can apply and accurately measure the torque exerted by the trapping beam on a rotating birefringent probe particle. This allows the viscosity and surface effects within liquid media to be measured quantitatively on a micron-size scale using a trapped rotating spherical probe particle. We use the system to measure the viscosity inside a prototype cellular structure.

Translational and rotational dynamics of colloidal rods by direct visualization with confocal microscopy

Deshpremy Mukhija — 2007-09-21T19:04:19-00:00

Journal of Colloid and Interface Science, Vol. 314, No. 1. (1 October 2007), pp. 98-106.

We report an experimental method to characterize the dynamics of colloidal rods by measuring their rotation and translation in three dimensions with confocal microscopy. The method relies on solvent viscosification to retard dynamics to time scales that are compatible with 3D confocal optical microscopy. Because the method yields a full three-dimensional characterization of rod displacement and orientation, it is applicable to situations in which complex, anisotropic dynamics emerge. Examples include behavior in liquid crystal phases with both orientational and positional order, suspensions subjected to applied fields such as shear flow or sedimentation, and the emerging area of anisotropic particle dynamics. We demonstrate the performance of the method by quantifying the Brownian motion of fluorescent poly(methyl methacrylate) rods (aspect ratio, L/D=3.1 and 7.0) grafted with poly(dimethylsiloxane) stabilizer. The rods are dispersed at dilute concentration in a solvent mixture of viscosity 2.0 Pa[thin space]s. Rod translational and rotational diffusivities are extracted from the measured translational mean square displacement of the centroid positions and of the rod unit vector u(t), respectively. Rod orientational dynamics are characterized relative to both their azimuthal and polar angles. Probability distributions for the translation and rotation in the frame of rod are computed from the measurements. Experimental values obtained agree well with theory available for the dynamics of isolated rods.

Shear Thickening of Cornstarch Suspensions as a Reentrant Jamming Transition

Abdoulaye Fall — 2008-03-18T01:32:20-00:00

Physical Review Letters, Vol. 100, No. 1. (2008)

We study the rheology of cornstarch suspensions, a non-Brownian particle system that exhibits shear thickening. From magnetic resonance imaging velocimetry and classical rheology it follows that as a function of the applied stress the suspension is first solid (yield stress), then liquid, and then solid again when it shear thickens. For the onset of thickening we find that the smaller the gap of the shear cell, the lower the shear rate at which thickening occurs. Shear thickening can then be interpreted as the consequence of dilatancy: the system under flow wants to dilate but instead undergoes a jamming transition because it is confined, as confirmed by measurement of the dilation of the suspension as a function of the shear rate.

Quantitative Imaging of Aggregated Emulsions

R Penfold — 2008-06-27T15:07:13-00:00

Langmuir, Vol. 22, No. 5. (28 February 2006), pp. 2005-2015.

Abstract: Noise reduction, restoration, and segmentation methods are developed for the quantitative structural analysis in three dimensions of aggregated oil-in-water emulsion systems imaged by fluorescence confocal laser scanning microscopy. Mindful of typical industrial formulations, the methods are demonstrated for concentrated (30% volume fraction) and polydisperse emulsions. Following a regularized deconvolution step using an analytic optical transfer function and appropriate binary thresholding, novel application of the Euclidean distance map provides effective discrimination of closely clustered emulsion droplets with size variation over at least 1 order of magnitude. The a priori assumption of spherical nonintersecting objects provides crucial information to combat the ill-posed inverse problem presented by locating individual particles. Position coordinates and size estimates are recovered with sufficient precision to permit quantitative study of static geometrical features. In particular, aggregate morphology is characterized by a novel void distribution measure based on the generalized Apollonius problem. This is also compared with conventional Voronoi/Delauney analysis.

Flow of Wet Granular Materials

N Huang — 2008-06-22T00:55:36-00:00

Physical Review Letters, Vol. 94, No. 2. (2005)

The transition from frictional to lubricated flows of a dense suspension of non-Brownian particles is studied. The pertinent parameter characterizing this transition is the Leighton number Le = , the ratio of lubrication to frictional forces. Le defines a critical shear rate below which no steady flow without localization exists. In the frictional regime the shear flow is localized. The lubricated regime is not simply viscous: the ratio of shear to normal stresses remains constant and the velocity profile has a universal form in both frictional and lubricated regimes. Finally, a discrepancy between local and global measurements of viscosity is identified, which suggests inhomogeneity of the material under flow.

Force, relative-displacement, and work networks in granular materials subjected to quasistatic deformation

NP Kruyt — 2008-04-26T22:31:19-00:00

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

To describe the heterogeneous nature of stress transmission in granular materials, the concept of the “strong” network consisting of contacts with large normal forces has been proposed by Radjaï et al. [Phys. Rev. Lett. 80, 61 (1998)]. The shear stress is mainly determined by this strong network. The dual viewpoint is adopted here, by not only considering the forces at contacts, but also the deformation. It is shown that the strain increments are determined by the tangential component of the relative displacements at the contacts. A “mobile” network consisting of contacts with large tangential relative displacements is defined that primarily accounts for the strain increments. The investigation of the relation between the strong and the mobile networks shows that these networks are largely unrelated. An alternative network is defined that consists of contacts at which the contribution to the work input is large. It is found that this work input occurs primarily through the tangential forces and tangential relative displacements.

How Does the Relaxation of a Supercooled Liquid Depend on Its Microscopic Dynamics?

Tobias Gleim — 2007-11-07T00:33:45-00:00

Physical Review Letters, Vol. 81, No. 20. (16 November 1998), 4404.

Using molecular dynamics computer simulations we investigate how the relaxation dynamics of a simple supercooled liquid with Newtonian dynamics differs from the one with a stochastic dynamics. We find that; apart from the early β-relaxation regime; the two dynamics give rise to the same relaxation behavior. The increase of the relaxation times of the system upon cooling; the details of the α relaxation; as well as the wave-vector dependence of the Edwards-Anderson parameters; are independent of the microscopic dynamics.

Energy Landscape, Antiplasticization, and Polydispersity Induced Crossover of Heterogeneity in Supercooled Polydisperse Liquids

Sneha Abraham — 2008-05-05T20:40:53-00:00

Physical Review Letters, Vol. 100, No. 16. (2008)

Polydispersity is found to have a significant effect on the potential energy landscape; the average inherent structure energy decreases with polydispersity. Increasing polydispersity at a fixed volume fraction decreases the glass transition temperature and the fragility of glass formation analogous to the antiplasticization seen in some polymeric melts. An interesting temperature dependent crossover of heterogeneity with polydispersity is observed at low temperature due to the faster buildup of dynamic heterogeneity at lower polydispersity.

Shear-Induced Changes in Two-Dimensional Foam

Abd — 2008-04-26T22:15:30-00:00

Physical Review Letters, Vol. 82, No. 12. (22 March 1999), 2610.

The effects of shear strain on a two-dimensional foam, comprising a monolayer of bubbles bridging from a soap solution to a cover glass, have been studied. For initially disordered foam, m2, the second central moment of the distribution of bubble coordination numbers, fell under strain, indicating ordering. The reduction in m2 is proportional to its initial value. Further, the accompanying bubblelevel changes occur in clusters, some of which are large. Comparisons are drawn with recent studies.

Two-Dimensional Foam Rheology with Viscous Drag

E Janiaud — 2008-04-26T22:13:18-00:00

Physical Review Letters, Vol. 97, No. 3. (2006)

We formulate and apply a continuum model that incorporates elasticity, yield stress, plasticity, and viscous drag. It is motivated by the two-dimensional foam rheology experiments of Debregeas et al. [Phys. Rev. Lett. 87, 178305 (2001)] and Wang et al. [Phys. Rev. E 73, 031401 (2006)], and is successful in exhibiting their principal features, which are an exponentially decaying velocity profile and strain localization. Transient effects are also identified.

Shear-Induced Stress Relaxation in a Two-Dimensional Wet Foam

John Lauridsen — 2008-04-26T22:16:37-00:00

Physical Review Letters, Vol. 89, No. 9. (2002), 098303.

We report on experimental measurements of the flow behavior of a wet, two-dimensional foam under conditions of slow, steady shear. The initial response of the foam is elastic. Above the yield strain, the foam begins to flow. The flow consists of irregular intervals of elastic stretch followed by sudden reductions of the stress, i.e., stress drops. We report on the distribution of the stress drops as a function of the applied shear rate. We also comment on our results in the context of various two-dimensional models of foams.

Local jamming via penetration of a granular medium

MB Stone — 2008-03-19T18:41:36-00:00

Physical Review E, Vol. 70, No. 4. (29 October 2004), 041301.

We present a series of measurements examining the penetration force required to push a flat plate vertically through a dense granular medium; focusing in particular on the effects of the bottom boundary of the vessel containing the medium. Our data demonstrate that the penetration force near the bottom is strongly affected by the surface properties of the bottom boundary; even many grain diameters above the bottom. Furthermore; the data indicate an intrinsic length scale for the interaction of the penetrating plate with the vessel bottom via the medium. This length scale; which corresponds to the extent of local jamming induced by the penetrating plate; has a square root dependence both upon the plate radius and the ambient granular stress near the bottom boundary; but it is independent of penetration velocity and grain diameter.

Effect of polydispersity on the crystallization kinetics of suspensions of colloidal hard spheres when approaching the glass transition

HJ Schope — 2008-02-24T19:23:18-00:00

J. Chem. Phys., Vol. 127 (2007), 084505.

We present a comprehensive study of the solidification scenario in suspensions of colloidal hard spheres for three polydispersities between 4.8% and 5.8%, over a range of volume fractions from near freezing to near the glass transition. From these results, we identify four stages in the crystallization process: (i) an induction stage where large numbers of precursor structures are observed, (ii) a conversion stage as precursors are converted to close packed structures, (iii) a nucleation stage, and (iv) a ripening stage. It is found that the behavior is qualitatively different for volume fractions below or above the melting volume fraction. The main effect of increasing polydispersity is to increase the duration of the induction stage, due to the requirement for local fractionation of particles of larger or smaller than average size. Near the glass transition, the nucleation process is entirely frustrated, and the sample is locked into a compressed crystal precursor structure. Interestingly, neither polydispersity nor volume fraction significantly influences the precursor stage, suggesting that the crystal precursors are present in all solidifying samples. We speculate that these precursors are related to the dynamical heterogeneities observed in a number of dynamical studies.

Evidence for Out-of-Equilibrium Crystal Nucleation in Suspensions of Oppositely Charged Colloids

Eduardo Sanz — 2007-12-19T15:45:09-00:00

Physical Review Letters, Vol. 99, No. 5. (2007)

We report a numerical study of the rate of crystal nucleation in a binary suspension of oppositely charged colloids. Two different crystal structures compete in the thermodynamic conditions under study. We find that the crystal phase that nucleates is metastable and, more surprisingly, its nucleation free-energy barrier is not the lowest one. This implies that, during nucleation, there is insufficient time for subcritical nuclei to relax to their lowest free-energy structure. Such behavior is in direct contradiction with the common assumption that the phase that crystallizes most readily is the one with the lowest free-energy barrier for nucleation. The phenomenon that we describe should be relevant for crystallization experiments where competing solid structures are not connected by an easy transformation.

Dynamics of a rigid rod in a glassy medium

AJ Moreno — 2007-11-28T01:19:10-00:00

Europhys. Lett., Vol. 67 (2004), pp. 820-826.

We present simulations of the motion of a single rigid rod in a disordered static 2d array of disk-like obstacles. The rotational, DR, and center-of-mass translational, DCM, diffusion constants are calculated for a wide range of rod length L and density of obstacles ρ. It is found that DCM follows the behavior predicted by kinetic theory for a hard disk with an effective radius R(L). A dynamic crossover is observed in DR for L comparable to the typical distance between neighboring obstacles dnn. Using arguments from kinetic theory and reptation, we rationalize the scaling laws, dynamic exponents, and prefactors observed for DR. In analogy with the enhanced translational diffusion observed in deeply supercooled liquids, the Stokes-Einstein-Debye relation is violated for L > 0.6dnn.

Fractional Stokes-Einstein and Debye-Stokes-Einstein Relations in a Network-Forming Liquid

Stephen Becker — 2007-06-30T17:34:58-00:00

Physical Review Letters, Vol. 97, No. 5. (2006), pp. 055901-055901.

We study the breakdown of the Stokes-Einstein (SE) and Debye-Stokes-Einstein (DSE) relations for translational and rotational motion in a prototypical model of a network-forming liquid, the ST2 model of water. We find that the emergence of fractional SE and DSE relations at low temperature is ubiquitous in this system, with exponents that vary little over a range of distinct physical regimes. We also show that the same fractional SE relation is obeyed by both mobile and immobile dynamical heterogeneities of the liquid.

Time-correlation functions and molecular motion

G Williams — 2007-11-26T19:49:07-00:00

Chem. Soc. Rev., Vol. 7 (1978), pp. 89-131.

The time-independent thermodynamic properties of molecular liquids and solids and their rationalization in terms of time-independent statistical mechanics receive considerable attention in undergraduate courses in chemistry and physics. The dynamical properties of such systems involve translational, vibrational, and reorientational modes of molecular motion and whereas the vibrational motions, which give rise to infrared, Raman, and neutron-scattering spectra, are well covered in undergraduate courses, considerably less attention is given to translational and reorientational modes of motion. Many of the important physical properties of liquids and solids relate to the latter modes of motion, and in recent years there has been a considerable interest in their study using many experimental techniques. Table 1 lists a selection of the techniques, and will be discussed below. In parallel with experimental studies, a sound theoretical framework has emerged, based on time-correlation functions, which allows translational and reorientational motions to be described formally and in physical terms.

Does confining the hard-sphere fluid between hard walls change its average properties?

Jeetain Mittal — 2007-10-30T01:06:27-00:00

The Journal of Chemical Physics, Vol. 126, No. 24. (2007), pp. 244708-244708.

We use grand canonical transition-matrix Monte Carlo and discontinuous molecular dynamics simulations to generate precise thermodynamic and kinetic data for the equilibrium hard-sphere fluid confined between smooth hard walls. These simulations show that the pronounced inhomogeneous structuring of the fluid normal to the confining walls, often the primary focus of density functional theory studies, has a negligible effect on many of its average properties over a surprisingly broad range of conditions. We present one consequence of this insensitivity to confinement: a simple analytical equation relating the average density of the confined fluid to that of the bulk fluid with equal activity. Nontrivial implications of confinement for average fluid properties do emerge in this system, but only when the fluid is both (i) dense and (ii) confined to a gap smaller than approximately three particle diameters. For this limited set of conditions, we find that “in-phase” oscillatory deviations in excess entropy and self-diffusivity (relative to the behavior of the bulk fluid at the same average density) occur as a function of gap size. These paired thermodynamic/kinetic deviations from bulk behavior appear to reflect the geometric packing frustration that arises when the confined space cannot naturally accommodate an integer number of particle layers. ©2007 American Institute of Physics

Relationships between Self-Diffusivity, Packing Fraction, and Excess Entropy in Simple Bulk and Confined Fluids

J Mittal — 2007-10-30T01:04:55-00:00

J. Phys. Chem. B, Vol. 111, No. 34. (30 August 2007), pp. 10054-10063.

Abstract: Static measures such as density and entropy, which are intimately connected to structure, have featured prominently in modern thinking about the dynamics of the liquid state. Here, we explore the connections between self-diffusivity, density, and excess entropy for two of the most widely used model "simple" liquids, the equilibrium Lennard-Jones and square-well fluids, in both bulk and confined environments. We find that the self-diffusivity data of the Lennard-Jones fluid can be approximately collapsed onto a single curve (i) versus effective packing fraction and (ii) in appropriately reduced form versus excess entropy, as suggested by two well-known scaling laws. Similar data collapse does not occur for the square-well fluid, a fact that can be understood on the basis of the nontrivial effects that temperature has on its static structure. Nonetheless, we show that the implications of confinement for the self-diffusivity of both of these model fluids, over a broad range of equilibrium conditions, can be predicted on the basis of knowledge of the bulk fluid behavior and either the effective packing fraction or the excess entropy of the confined fluid. Excess entropy is perhaps the most preferable route due to its superior predictive ability and because it is a standard, unambiguous thermodynamic quantity that can be readily predicted via classical density functional theories of inhomogeneous fluids.

Random sequential filling of intervals on a line

B Widom — 2007-10-29T23:26:08-00:00

The Journal of Chemical Physics, Vol. 58, No. 9. (1973), pp. 4043-4044.

Clusters and Fluctuations at Mean-Field Critical Points and Spinodals

W Klein — 2007-10-29T21:00:46-00:00

Physical Review Letters, Vol. 85, No. 6. (2000), pp. 1270-1273.

We show that the structure of the fluctuations close to spinodals and mean-field critical points is qualitatively different from the structure close to non-mean-field critical points. This difference has important implications for many areas including the formation of glasses in supercooled liquids. In particular; the divergence of the measured static structure function in near-mean-field systems close to the glass transition is suppressed relative to the mean-field prediction in systems for which a spatial symmetry is broken.

Entropic barriers, activated hopping, and the glass transition in colloidal suspensions

Kenneth Schweizer — 2007-10-29T19:56:08-00:00

The Journal of Chemical Physics, Vol. 119, No. 2. (2003), pp. 1181-1196.

A microscopic kinetic description of single-particle transient localization and activated transport in glassy fluids is developed which combines elements of idealized mode-coupling theory, density functional theory, and activated rate theory. Thermal fluctuations are included via a random force which destroys the idealized glass transition and restores ergodicity through activated barrier hopping. The approach is predictive, containing no adjustable parameters or postulated underlying dynamic or thermodynamic divergences. Detailed application to hard-sphere colloidal suspensions reveals good agreement with experiment for the location of the kinetic glass transition volume fraction, the dynamic incoherent scattering relaxation time, apparent localization length, and length scale of maximum nongaussian behavior. Multiple connections are predicted between thermodynamics, short-time dynamics in the nearly localized state, and long-time relaxation by entropic barrier crossing. A critical comparison of the fluid volume fraction dependence of the hopping time with fit formulas which contain ideal divergences has been performed. Application of the derivative Stickel analysis suggests that the fit functions do not provide an accurate description over a wide range of volume fractions. Generalization to treat the kinetic vitrification of more complex colloidal and nanoparticle suspensions, and thermal glass-forming liquids, is possible. ©2003 American Institute of Physics.

Activated hopping and dynamical fluctuation effects in hard sphere suspensions and fluids

Erica Saltzman — 2007-10-29T19:49:58-00:00

The Journal of Chemical Physics, Vol. 125, No. 4. (2006), pp. 044509-044509.

Single particle Brownian dynamics simulation methods are employed to establish the full trajectory level predictions of our nonlinear stochastic Langevin equation theory of activated hopping dynamics in glassy hard sphere suspensions and fluids. The consequences of thermal noise driven mobility fluctuations associated with the barrier hopping process are determined for various ensemble-averaged properties and their distributions. The predicted mean square displacements show classic signatures of transient trapping and anomalous diffusion on intermediate time and length scales. A crossover to a stronger volume fraction dependence of the apparent nondiffusive exponent occurs when the entropic barrier is of order the thermal energy. The volume fraction dependences of various mean relaxation times and rates can be fitted by empirical critical power laws with parameters consistent with ideal mode-coupling theory. However, the results of our divergence-free theory are largely a consequence of activated dynamics. The experimentally measurable alpha relaxation time is found to be very similar to the theoretically defined mean reaction time for escape from the barrier-dominated regime. Various measures of decoupling have been studied. For fluid states with small or nonexistent barriers, relaxation times obey a simple log-normal distribution, while for high volume fractions the relaxation time distributions become Poissonian. The product of the self-diffusion constant and mean alpha relaxation time increases roughly as a logarithmic function of the alpha relaxation time. The cage scale incoherent dynamic structure factor exhibits nonexponential decay with a modest degree of stretching. A nearly universal collapse of the different volume fraction results occurs if time is scaled by the mean alpha relaxation time. Hence, time-volume fraction superposition holds quite well, despite the presence of stretching and volume fraction dependent decoupling associated with the stochastic barrier hopping process. The relevance of other origins of dynamic heterogeneity (e.g., mesoscopic domains), and comparison of our results with experiments, simulations, and alternative theories, is discussed. ©2006 American Institute of Physics

Non-Gaussian effects, space-time decoupling, and mobility bifurcation in glassy hard-sphere fluids and suspensions

Erica Saltzman — 2007-10-29T19:47:05-00:00

Physical Review E, Vol. 74, No. 6. (2006)

Brownian trajectory simulation methods are employed to fully establish the non-Gaussian fluctuation effects predicted by our nonlinear Langevin equation theory of single particle activated dynamics in glassy hard-sphere fluids. The consequences of stochastic mobility fluctuations associated with the space-time complexities of the transient localization and barrier hopping processes have been determined. The incoherent dynamic structure factor was computed for a range of wave vectors and becomes of an increasingly non-Gaussian form for volume fractions beyond the (naive) ideal mode coupling theory (MCT) transition. The non-Gaussian parameter (NGP) amplitude increases markedly with volume fraction and is well described by a power law in the maximum restoring force of the nonequilibrium free energy profile. The time scale associated with the NGP peak becomes much smaller than the relaxation time for systems characterized by significant entropic barriers. An alternate non-Gaussian parameter that probes the long time relaxation process displays a different shape, peak intensity, and time scale of its maximum. However, a strong correspondence between the classic and alternate NGP amplitudes is predicted which suggests a deep connection between the early and final stages of cage escape. Strong space-time decoupling emerges at high volume fractions as indicated by a nondiffusive wave vector dependence of the relaxation time and growth of the translation-relaxation decoupling parameter. Displacement distributions exhibit non-Gaussian behavior at intermediate times, evolving into a strongly bimodal form with slow and fast subpopulations at high volume fractions. Qualitative and semiquantitative comparisons of the theoretical results with colloid experiments, ideal MCT, and multiple simulation studies are presented.

Structural Relaxation of Polymer Glasses at Surfaces, Interfaces, and In Between

Rodney Priestley — 2007-10-29T17:51:36-00:00

Science, Vol. 309, No. 5733. (15 July 2005), pp. 456-459.

We analyzed the glassy-state structural relaxation of polymers near surfaces and interfaces by monitoring fluorescence in multilayer films. Relative to that of bulk, the rate of structural relaxation of poly(methyl methacrylate) is reduced by a factor of 2 at a free surface and by a factor of 15 at a silica substrate interface; the latter exhibits a nearly complete arresting of relaxation. The distribution in relaxation rates extends more than 100 nanometers into the film interior, a distance greater than that over which surfaces and interfaces affect the glass transition temperature. 10.1126/science.1112217

A coupling model analysis of dynamics of concentrated colloidal suspensions

KL Ngai — 2007-10-29T14:52:12-00:00

Philosophical Magazine B, Vol. 77, No. 2. (February 1998), pp. 621-631.

Recent dynamic light scattering measurement on concentrated suspensions of 'hard-sphere' colloidal particles by Segre and Pusey (1996, Phys. Rev. Lett ., 77, 771) are compared with the theoretical results calculated by the coupling model. The calculated normalized intermediate scattering function decays exponentially at both short and long times, f (Q, short t) proportional exp[-DS(Q)Q2t] and f(Q, long t) proportional exp[-DL(Q)Q2t] respectively, with DL(Q) and DS(Q) having the same Q dependence. The ratio DL(Q)/DS(Q) decreases rapidly with volume fraction phi. At intermediate times, the decay of f(Q, t) is slower and nonexponential, becomes increasing slower and extends to longer times as phi increases. There is good agreement between theory and experiment. It is also pointed out that the diffusional dynamics found in concentrated colloidal suspensions are general and shared by other densely packed interacting systems.

Direct imaging of repulsive and attractive colloidal glasses

Laura Kaufman — 2007-06-25T01:35:08-00:00

The Journal of Chemical Physics, Vol. 125, No. 7. (2006), pp. 074716-074716.

Coherent anti-Stokes Raman scattering microscopy is performed on glassy systems of poly(methylmethacrylate) colloidal particles in density- and refractive-index-matched solvents. Samples are prepared with varying amounts of linear polystyrene, which induces a depletion driven attraction between the nearly hard-sphere particles. Images collected over several hours confirm the existence of a reentrant glass transition. The images also reveal that the dynamics of repulsive and attractive glasses are qualitatively different. Colloidal particles in repulsive glasses exhibit cage rattling and escape, while those in attractive glasses are nearly static while caged but exhibit large displacements upon (infrequent) cage escape. ©2006 American Institute of Physics

From Micro- to Nanofabrication with Soft Materials

Stephen Quake — 2007-09-27T08:45:33-00:00

Science, Vol. 290, No. 5496. (24 November 2000), pp. 1536-1540.

Soft materials are finding applications in areas ranging from microfluidic device technology to nanofabrication. We review recent work in these areas, discuss the motivation for device fabrication with soft materials, and describe applications of soft materials. In particular, we discuss active microfluidic devices for cell sorting and biochemical assays, replication-molded optics with subdiffraction limit features, and nanometer-scale resonators and wires formed from single-molecule DNA templates as examples of how the special properties of soft materials address outstanding problems in device fabrication.

Laser tweezer microrheology of a colloidal suspension.

Alexander Meyer — 2007-08-18T23:11:42-00:00

Journal of Rheology, Vol. 50, No. 1. (2006)

The microrheology of a colloidal suspension is measured using laser tweezers. Suspensions of refractive index-matched fluorinated ethylene propylene (FEP) particles are seeded with index-mismatched polystyrene or silica probe particles. Laser trapped probes are then subjected to steady uniform flows, enabling measurements of the suspension microviscosity as a function of FEP volume fraction and flow velocity. The microrheology results agree with bulk rheology, and both exhibit the same volume fraction dependence of the Krieger-Dougherty relationship for hard spheres. As volume fraction increases, the microrheology more closely agrees with the infinite shear bulk viscosity. In this regime, measurements using small probes exhibit additional shear thinning. Using confocal microscopy and fluorescent poly(methylmethacrylate) dispersions, we demonstrate that the nonlinear microrheology is consistent with the development of an anisotropic nonequilibrium pair distribution function between the

Particle Tracking Microrheology of Complex Fluids

TG Mason — 2006-08-30T16:25:45-00:00

Physical Review Letters, Vol. 79, No. 17. (27 October 1997), 3282.

We present a new method for measuring the linear viscoelastic shear moduli of complex fluids. Using photodiode detection of laser light scattered from a thermally excited colloidal probe sphere; we track its trajectory and extract the moduli using a frequency-dependent Stokes-Einstein equation. Spectra obtained for polyethylene oxide in water are in excellent agreement with those found mechanically and using diffusing wave spectroscopy. Since only minute sample volumes are required; this method is well suited for biomaterials of high purity; as we demonstrate with a concentrated DNA solution.