CiteULike: weeks's structure

Following microscopic motion in a two-dimensional glass-forming binary fluid

Matthew Downton — 2008-06-25T18:22:01-00:00

Journal of Physics: Condensed Matter, Vol. 20, No. 24. (2008), 244127.

The dynamics of a binary mixture of large and small discs is studied at temperatures approaching the glass transition using an analysis based on the topology of the Voronoi polygon surrounding each atom. At higher temperatures we find that the dynamics is dominated by fluid-like motion that involves particles entering and exiting the nearest neighbour shells of nearby particles. As the temperature is lowered, the rate of topological moves decreases and motion becomes localized to regions of mixed pentagons and heptagons. In addition we find that in the low temperature state particles may translate significant distances without undergoing changes in their nearest neighbour shell. These results have implications for dynamical heterogeneities in glass-forming liquids.

Direct observation of a local structural mechanism for dynamic arrest

Patrick — 2008-06-27T14:26:14-00:00

Nat Mater, Vol. 7, No. 7. (July 2008), pp. 556-561.

The mechanism by which a liquid may become arrested, forming a glass or gel, is a long-standing problem of materials science. In particular, long-lived (energetically) locally favoured structures (LFSs), the geometry of which may prevent the system relaxing to its equilibrium state, have long been thought to play a key role in dynamical arrest. Here, we propose a definition of LFSs which we identify with a novel topological method and directly measure with experiments on a colloidal liquid–gel transition. The population of LFSs is a strong function of (effective) temperature in the ergodic liquid phase, rising sharply approaching dynamical arrest, and indeed forms a percolating network that becomes the 'arms' of the gel. Owing to the LFSs, the gel is unable to reach equilibrium, crystal–gas coexistence. Our results provide direct experimental observation of a link between local structure and dynamical arrest, and open a new perspective on a wide range of metastable materials.

On the relationship between structure and dynamics in a supercooled liquid

Asaph Widmer-Cooper — 2008-02-15T14:48:10-00:00

Journal of Physics: Condensed Matter, Vol. 17, No. 49. (2005), pp. S4025-S4034.

We present the dynamic propensity distribution as an explicit measure of the degree to which the dynamics in a liquid over the time scale of structural relaxation is determined by the initial configuration. We then examine, for a binary mixture of soft discs in two dimensions, the correlation between the spatial distribution of propensity and that of two local measures of configuration structure: the local composition and local free volume. While the small particles dominate the high propensity population, we find no strong correlation between either the local composition or the local free volume and the propensity. It is argued that this is a generic failure of purely local structural measures to capture the inherently non-local character of collective behaviour.

Dense Packing and Symmetry in Small Clusters of Microspheres

Vinothan Manoharan — 2007-04-04T07:42:56-00:00

Science, Vol. 301, No. 5632. (25 July 2003), pp. 483-487.

When small numbers of colloidal microspheres are attached to the surfaces of liquid emulsion droplets, removing fluid from the droplets leads to packings of spheres that minimize the second moment of the mass distribution. The structures of the packings range from sphere doublets, triangles, and tetrahedra to exotic polyhedra not found in infinite lattice packings, molecules, or minimum-potential energy clusters. The emulsion system presents a route to produce newcolloidal structures and a means to study howdifferent physical constraints affect symmetry in small parcels of matter. 10.1126/science.1086189

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.

Frustration-limited clusters in liquids

Steven Kivelson — 2007-05-17T16:43:03-00:00

The Journal of Chemical Physics, Vol. 101, No. 3. (1994), pp. 2391-2397.

We present a continuum theory of frustration-limited clusters in one-component glass-forming liquids that accounts, in part, for the recently reported [Fischer et al., J. Non-Cryst. Solids, 131–133, 134 (1991)], and quite unexpected, presence in simple glass-forming liquids of stable clusters at low temperatures (T) and the even less expected persistence for very long times of these clusters at higher T's. The model is based on the idea that there is a local structure that is energetically preferred over simple crystalline packing, which is strained (frustrated) over large distances; although in a curved space the preferred packing could lead to “ideal” crystallization at temperatures that are usually above the actual freezing temperature, in “flat” space this transition is narrowly avoided. We are led to a new ansatz for the T dependence of the viscosity, which permits us to collapse data for many liquids onto a universal curve. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.

Search for a correlation length in a simulation of the glass transition

Richard Ernst — 2007-07-30T15:27:17-00:00

Physical Review B, Vol. 43, No. 10. (1 April 1991), pp. 8070-8080.

We have looked for evidence of a correlation length in a molecular-dynamics simulation of the glass transition. We have studied the correlation functions of both the translational order of particle positions and the orientational order of nearest-neighbor bond angles; and have seen no indication of a diverging length scale. We also present data from the simulation; which extend recent laboratory measurements of the frequency-dependent specific heat and thermal conductivity.

Void Structure in Colloidal Dispersions

Kensaku Ito — 2007-09-28T21:09:17-00:00

Science, Vol. 263, No. 5143. (7 January 1994), pp. 66-68.

The time evolution of void structures in highly purified polymer latex dispersions was studied with a confocal laser scanning microscope. In such dispersions, which were initially homogeneous, the voids grew with time when the dispersions were kept standing and formed more quickly in the internal material than in material close to the glass-dispersion interface. Void formation is thus not an artifact arising from the presence of the interface. A similar structural inhomogeneity, in apparently homogeneous systems, is discussed for simple ionic solutions, ionic polymer solutions, and Langmuir-Blodgett films. 10.1126/science.263.5143.66

Localized ordered structure in polymer latex suspensions as studied by a confocal laser scanning microscope

Hiroshi Yoshida — 2007-09-28T21:06:12-00:00

Physical Review B, Vol. 44, No. 1. (1 July 1991), pp. 435-438.

By using a confocal laser scanning microscope and a digital image analyzer; the influence of a glass-suspension interface on colloidal crystals was studied. Particular attention was paid to the interparticle distance (2 D expt ) in the ordered structure and the crystallization process. The 2 D expt was insensitive to the distance from the interface up to about 40 μm. The crystallization process of the ordered structure was characterized by a strong anisotropy originating at the glass-suspension interface.

Structure of colloidal glasses calculated by the molecular-dynamics method and measured by light scattering

I Snook — 2007-09-28T20:36:00-00:00

Physical Review A, Vol. 43, No. 12. (15 June 1991), pp. 6900-6907.

Static structure factors S ( q ) were measured by laser light scattering for very concentrated systems of spherical; near-monosized; sterically stabilized particles dispersed in nonpolar liquids. A range of systems with particle concentrations beyond that corresponding to the disorder to order transition configurations were found to have amorphous structures. As the particles were stabilized by means of very short chain polymers; these systems were thought to closely approximate the fundamentally important amorphous; hard-sphere system. Subsequent analysis of S ( q ) carried out by means of data generated by the molecular-dynamics method for very concentrated; amorphous states of the hard-sphere system confirmed this interpretation. Thus we were able; by a combination of experiment and simulation; to give an extensive analysis and description of the structure of the amorphous state of a system of hard spheres. This study complements past work on the thermodynamic and transport properties of metastable; amorphous states of a system of hard spheres.

The Bakerian Lecture, 1962. The Structure of Liquids

JD Bernal — 2007-09-28T17:40:11-00:00

Proceedings of the Royal Society of London. Series A, Vol. 280, No. 1382. (28 July 1964), pp. 299-322.

Weak Correlations between Local Density and Dynamics near the Glass Transition

JC Conrad — 2006-07-03T19:08:55-00:00

J. Phys. Chem. B, Vol. 109, No. 45. (17 November 2005), pp. 21235-21240.

We perform experiments on two different dense colloidal suspensions with confocal microscopy to probe the relationship between local structure and dynamics near the glass transition. We calculate the Voronoi volume for our particles and show that this quantity is not a universal probe of glassy structure for all colloidal suspensions. We correlate the Voronoi volume to displacement and find that these quantities are only weakly correlated. We observe qualitatively similar results in a simulation of a polymer melt. These results suggest that the Voronoi volume does not predict dynamical behavior in experimental colloidal suspensions; a purely structural approach based on local single particle volume likely cannot describe the colloidal glass transition.

Real-Space Imaging of Nucleation and Growth in Colloidal Crystallization

U Gasser — 2007-09-20T04:28:54-00:00

Science, Vol. 292, No. 5515. (13 April 2001), pp. 258-262.

Crystallization of concentrated colloidal suspensions was studied in real space using laser scanning confocal microscopy. Direct imaging in three dimensions allowed identification and observation of nucleation and growth of crystalline regions, providing the first experimental measure of properties of the nucleating crystallites. By following their evolution, critical nuclei were identified, nucleation rates were determined, and the average surface tension of the crystal-liquid interface was measured. The structure of the nuclei was the same as the bulk solid phase, random-hexagonal-close-packed (rhcp), and their average shape was rather aspherical, with rough rather than faceted surfaces.

Phase Behavior and 3D Structure of Strongly Attractive Microsphere-Nanoparticle Mixtures

JF Gilchrist — 2007-09-20T04:13:37-00:00

Langmuir, Vol. 21, No. 24. (22 November 2005), pp. 11040-11047.

Abstract: We investigate the phase behavior and 3D structure of strongly attractive mixtures of silica microspheres and polystyrene nanoparticles. These binary mixtures are electrostatically tuned to promote a repulsion between like-charged (microsphere-microsphere and nanoparticle-nanoparticle) species and a strong attraction between oppositely charged (microsphere-nanoparticle) species. Using confocal fluorescence scanning microscopy, we directly observe the 3D structure of colloidal phases assembled from these mixtures as a function of varying composition. In the absence of nanoparticle additions, the charged-stabilized microspheres assemble into a polycrystalline array upon sedimentation. With increasing nanoparticle volume fraction, nanoparticle bridges form between microspheres, inducing their flocculation. At even higher nanoparticle volume fractions, the microspheres become well coated with nanoparticles, leading to their charge reversal and subsequent restabilization. We demonstrate how this fluid-gel-fluid transition can be utilized to control the morphology of the colloidal phases formed under gravity-driven sedimentation.

Interparticle Interactions and Direct Imaging of Colloidal Phases Assembled from Microsphere-Nanoparticle Mixtures

CJ Martinez — 2007-09-20T04:12:36-00:00

Langmuir, Vol. 21, No. 22. (25 October 2005), pp. 9978-9989.

Abstract: We investigate the interparticle interactions, phase behavior, and structure of microsphere-nanoparticle mixtures that possess high size and charge asymmetry.1 We employ a novel Monte Carlo simulation scheme2 to calculate the effective microsphere interactions in suspension, yielding new insight into the origin of the experimentally observed behavior.3 The initial settling velocity, final sediment density, and three-dimensional structure of colloidal phases assembled from these binary mixtures via gravitational settling of silica microspheres in water and index-matched solutions exhibit a strong compositional dependence. Confocal laser scanning microscopy is used to directly image and quantify their structural evolution during assembly. Below a lower critical nanoparticle volume fraction (nano < L,C), the intrinsic van der Waals attraction between microspheres leads to the formation of colloidal gels. These gels exhibit enhanced consolidation as nano approaches L,C. When nano exceeds L,C, an effective repulsion arises between microspheres due to the formation of a dynamic nanoparticle halo around the colloids. From this stable fluid phase, the microspheres settle into a crystalline array. Finally, above an upper critical nanoparticle volume fraction (nano > U,C), colloidal gels form whose structure becomes more open with increasing nanoparticle concentration due to the emergence of an effective microsphere attraction,3 whose magnitude exhibits a superlinear dependence on nano.

Invariance of Structure in an Aging Colloidal Glass

Gianguido Cianci — 2007-09-20T04:07:03-00:00

AIP Conference Proceedings, Vol. 832, No. 1. (2006), pp. 21-25.

We study concentrated colloidal suspensions, a model system which has a glass transition. The non-equilibrium nature of the glassy state is most clearly highlighted by aging — the dependence of the system's properties on the time elapsed since vitrification. Fast laser scanning confocal microscopy allows us to image a colloidal glass and track the particles in three dimensions. We analyze the static structure in terms of tetrahedral packing. We find that while the aging of the suspension clearly affects its dynamics, none of the geometrical quantities associated with tetrahedra change with age.

Correlations of structure and dynamics in an aging colloidal glass

Gianguido Cianci — 2007-09-20T04:05:51-00:00

Solid State Communications, Vol. 139, No. 11-12. (September 2006), pp. 599-604.

We study concentrated colloidal suspensions, a model system which has a glass transition. Samples in the glassy state show aging, in that the motion of the colloidal particles slows as the sample ages from an initial state. We study the relationship between the static structure and the slowing dynamics, using confocal microscopy to follow the three-dimensional motion of the particles. The structure is quantified by considering tetrahedra formed by quadruplets of neighboring particles. We find that while the sample clearly slows down during aging, the static properties as measured by tetrahedral quantities do not vary. However, a weak correlation between tetrahedron shape and mobility is observed, suggesting that the structure facilitates the motion responsible for the sample aging.

Aging of tetrahedral structure in a Lennard-Jones glass

Gianguido Cianci — 2007-09-20T04:02:45-00:00

Vol. 19 (2007), pp. 51-56.

We study the aging of a glassy Lennard-Jones binary mixture with molecular dynamics simulations. We follow the evolution of the packing as a function of the system's age tw, the time passed since the system is quenched to below its glass transition temperature. We focus on simple properties of all tetrahedra formed by the majority of particles. We find that both the averages and the distributions of the edge length of tetrahedra and of their standard deviation monotonically evolve over time: they age. Specifically the aging process decreases the irregularity of tetrahedra while loosening them up. This is in stark contrast with previous experiments on slightly charged hard-sphere colloidal suspension where tetrahedral geometry was found to be a poor indicator of age. Furthermore, we confirm that tetrahedral packings sample microscopic structure in a non-trivial way.

Structure of dense colloidal liquids in tight spaces

Eric Weeks — 2007-09-20T04:00:14-00:00

Vol. 19 (2007), pp. 45-49.

We use three-dimensional confocal microscopy to study the structure of a dense colloidal liquid confined between two parallel glass plates. The colloidal sample is at a volume fraction of 50\% and is a binary mixture of 2~$μ$m and 3~$μ$m diameter particles to prevent crystallization. The plate separation ranges from 50 small particle diameters to 3 small particle diameters. While particles form layers immediately adjacent to the confining walls, we otherwise see little influence of the confinement on structure.

Confocal microscopy of colloids

V Prasad — 2007-09-19T15:10:32-00:00

Journal of Physics: Condensed Matter, Vol. 19 (2007), pp. 113102-113102.

Colloids have increasingly been used to characterize or mimic many aspects of atomic and molecular systems. With confocal microscopy these colloidal particles can be tracked spatially in three dimensions with great precision over large time scales. This review discusses equilibrium phases such as crystals and liquids, and non-equilibrium phases such as glasses and gels. The phases that form depend strongly on the type of particle interaction that dominates. Hard-sphere-like colloids are the simplest, and interactions such as the attractive depletion force and electrostatic repulsion result in more nontrivial phases which can better model molecular materials. Furthermore, shearing or otherwise externally forcing these colloids while under microscopic observation helps connect the microscopic particle dynamics to the macroscopic flow behavior. Finally, directions of future research in this field are discussed.

Real-Space Structure of Colloidal Hard-Sphere Glasses

A van Blaaderen — 2007-05-17T16:29:59-00:00

Science, Vol. 270 (November 1995), pp. 1177-1179.

The real-space structure of hard-sphere glasses quenched from colloidal liquids in thermodynamic equilibrium has been determined. Particle coordinates were obtained by combining the optical sectioning capability of confocal fluorescence microscopy with the structure of specially prepared fluorescent silica colloids. Both the average structure and the local structure of glasses, with volume fractions ranging from 0.60 to 0.64, were in good agreement with glasses and random close packings generated by computer simulations. No evidence of a divergent correlation length was found. The method used to obtain the three-dimensional particle coordinates is directly applicable to other colloidal structures, such as crystals, gels, and flocs.<p>