CiteULike: dchen's phase

Novel Phase Transformation in ZnO Nanowires under Tensile Loading

Ambarish Kulkarni — 2008-07-24T18:34:11-00:00

Physical Review Letters, Vol. 97, No. 10. (2006)

We predict a previously unknown phase transformation from wurtzite to a graphitelike (P63/mmc) hexagonal structure in [010]-oriented ZnO nanowires under uniaxial tensile loading. Molecular dynamics simulations and first principles calculations show that this structure corresponds to a distinct minimum on the enthalpy surfaces of ZnO for such loading conditions. This transformation is reversible with a low level of hysteretic dissipation of 0.16 J/m3 and, along with elastic stretching, endows the nanowires with the ability to recover pseudoelastic strains up to 15%.

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.

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

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.

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.

Phase behavior of wormlike rods

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

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

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

Reversible Phase Transition of Colloids in a Binary Liquid Solvent

Hua Guo — 2008-05-18T19:08:24-00:00

Physical Review Letters, Vol. 100, No. 18. (2008)

We report fluid-fluid and fluid-solid phase transitions of charge-stabilized polystyrene particles suspended in a binary liquid mixture of 3-methylpyridine and water. These thermally reversible phase transitions occur in the homogeneous phase of the binary liquid mixture below the coexistence temperature of the two liquids. Close density matching of the particles and the solvent allows us to follow the phase behavior until complete coexistence of macroscopic phases with temperature as the control parameter. We use small angle x-ray scattering to characterize these phases as colloidal gas, liquid, fcc crystal, and glass.

Phase Transition to Bundles of Flexible Supramolecular Polymers

BAH Huisman — 2008-05-05T21:58:29-00:00

Physical Review Letters, Vol. 100, No. 18. (2008)

We report Monte Carlo simulations of the self-assembly of supramolecular polymers based on a model of patchy particles. We find a first-order phase transition, characterized by hysteresis and nucleation, toward a solid bundle of polymers, of length much greater than the average gas phase length. We argue that the bundling transition is the supramolecular equivalent of the sublimation transition, which results from a weak chain-chain interaction. We provide a qualitative equation of state that gives physical insight beyond the specific values of the parameters used in our simulations.

Emulsification of Partially Miscible Liquids Using Colloidal Particles: Nonspherical and Extended Domain Structures

PS Clegg — 2008-05-15T00:03:54-00:00

Langmuir, Vol. 23, No. 11. (22 May 2007), pp. 5984-5994.

Abstract: We present microscopy studies of particle-stabilized emulsions with unconventional morphologies. The emulsions comprise pairs of partially miscible fluids and are stabilized by colloids. Alcohol-oil mixtures are employed; silica colloids are chemically modified so that they have partial wettability. We create our morphologies by two distinct routes: starting with a conventional colloid-stabilized emulsion or starting in the single-fluid phase with the colloids dispersed. In the first case temperature cycling leads to the creation of extended fluid domains built around some of the initial fluid droplets. In the second case quenching into the demixed region leads to the formation of domains which reflect the demixing kinetics. The structures are stable due to a jammed, semisolid, multilayer of colloids on the liquid-liquid interface. The differing morphologies reflect the roles in formation of the arrested state of heterogeneous and homogeneous nucleation and spinodal decomposition. The latter results in metastable, bicontinuous emulsions with frozen interfaces, at least for the thin-slab samples, investigated here.

Origin of the Complex Molecular Dynamics in Functionalized Discotic Liquid Crystals

MM Elmahdy — 2008-05-07T22:55:49-00:00

Physical Review Letters, Vol. 100, No. 10. (2008)

The molecular dynamics of three dipole functionalized hexa-peri-hexabenzocoronenes have been studied using site-specific NMR techniques and dielectric spectroscopy as a function of temperature and pressure. These probes (i) suggest that the thermodynamic state completely controls the dynamic response, (ii) clarify the origin of two dynamic processes associated with the presence of two glass temperatures, and (iii) provide the first phase diagram for substances of this kind.

In Situ Observation of Fringing-Field-Induced Phase Separation in a Liquid-Crystal--Monomer Mixture

Hongwen Ren — 2008-03-24T17:17:09-00:00

Physical Review Letters, Vol. 100, No. 11. (2008)

Fringing-field-induced phase separation dynamics in liquid-crystal–(LC-)monomer mixtures is investigated via a microscope. At a low LC concentration, the fringing field converts the randomly dispersed LC droplets to an ordered droplet array, while at a high LC concentration the fringing field converts the amorphous LC-monomer system to a composite film. Because the LC and monomer are immiscible, the converted morphologies are stable even after the voltage is removed. Using the fringing field-induced phase separation, it is possible to prepare different polymer-dispersed LC morphologies.

Complexity in Strongly Correlated Electronic Systems

Elbio Dagotto — 2008-05-05T22:04:34-00:00

Science, Vol. 309, No. 5732. (8 July 2005), pp. 257-262.

A wide variety of experimental results and theoretical investigations in recent years have convincingly demonstrated that several transition metal oxides and other materials have dominant states that are not spatially homogeneous. This occurs in cases in which several physical interactions--spin, charge, lattice, and/or orbital--are simultaneously active. This phenomenon causes interesting effects, such as colossal magnetoresistance, and it also appears crucial to understand the high-temperature superconductors. The spontaneous emergence of electronic nanometer-scale structures in transition metal oxides, and the existence of many competing states, are properties often associated with complex matter where nonlinearities dominate, such as soft materials and biological systems. This electronic complexity could have potential consequences for applications of correlated electronic materials, because not only charge (semiconducting electronic), or charge and spin (spintronics) are of relevance, but in addition the lattice and orbital degrees of freedom are active, leading to giant responses to small perturbations. Moreover, several metallic and insulating phases compete, increasing the potential for novel behavior. 10.1126/science.1107559

Hexagonal Order in Crystalline and Columnar Phases of Hard Rods

Eric Grelet — 2008-05-05T21:53:20-00:00

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

We report a study of colloidal suspensions of highly monodisperse semiflexible chiral rodlike viruses, denoted fd, in the range of high concentrations. Small angle x-ray scattering experiments reveal the existence of two hexagonal phases: the first one is crystalline and the second one is hexatic columnar, as shown by its short-range positional order. The suspension of rodlike viruses is the first experimental system showing the whole phase sequence with increasing particle concentration theoretically predicted for systems of hard rods, ranging from the chiral nematic via the smectic to columnar and crystalline phases.

Microevaporators for Kinetic Exploration of Phase Diagrams

Jacques Leng — 2008-05-05T00:02:09-00:00

Physical Review Letters, Vol. 96, No. 8. (2006)

We use pervaporation-based microfluidic devices to concentrate species in aqueous solutions with spatial and temporal control of the process. Using experiments and modeling, we quantitatively describe the advection-diffusion behavior of the concentration field of various solutions (electrolytes, colloids, etc.) and demonstrate the potential of these devices as universal tools for the kinetic exploration of the phases and textures that form upon concentration.

Phase behaviour of concentrated suspensions of nearly hard colloidal spheres

PN Pusey — 2005-08-23T18:57:37-00:00

Nature, Vol. 320, No. 6060. (27 March 1986), pp. 340-342.

The relationship between liquid, supercooled and glassy water

Osamu Mishima — 2008-04-29T00:31:18-00:00

Nature, Vol. 396, No. 6709. (26 November 1998), pp. 329-335.

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), 6900.

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.

A comparative study on the phase behaviour of highly charged colloidal spheres in a confining wedge geometry

Ana Fontecha — 2007-09-27T16:53:09-00:00

Journal of Physics: Condensed Matter, Vol. 17, No. 31. (2005), pp. S2779-S2786.

We studied the structures formed in aqueous dispersions of charged colloidal spheres under a constant low salt concentration of c = 6 × 10[?]6 mol l[?]1. Particles of diameter 2a = 1000 nm were confined to a low angle wedge geometry with plate separation 0<S<50 µm and observed with video microscopy. Irrespective of the initial particle density n we reproducibly observe the particles to migrate to the narrow wedge side on the timescale of a few days. Thereby an interface between a crystalline structure and a near particle free region is formed, which propagates slowly until the dilute region is exhausted of particles. While the origin of this separation is still unclear, the final extension of the crystalline region is stable on the timescale of months. Within the crystal phase we observe a characteristic sequence of structures with increasing plate separation similar to that seen in previous experiments on hard sphere-like systems but here with non-touching particles. Moreover, we find that mechanical equilibrium is a necessary prerequisite for observing the full richness of different phases. A detailed comparison to recent theoretical calculations for bilayers was performed and semi-quantitative agreement with the predictions observed.

Formation of Glasses from Liquids and Biopolymers

CA Angell — 2007-05-20T17:03:13-00:00

Science, Vol. 267, No. 5206. (31 March 1995), pp. 1924-1935.

Glasses can be formed by many routes. In some cases, distinct polyamorphic forms are found. The normal mode of glass formation is cooling of a viscous liquid. Liquid behavior during cooling is classified between "strong" and "fragile," and the three canonical characteristics of relaxing liquids are correlated through the fragility. Strong liquids become fragile liquids on compression. In some cases, such conversions occur during cooling by a weak first-order transition. This behavior can be related to the polymorphism in a glass state through a recent simple modification of the van der Waals model for tetrahedrally bonded liquids. The sudden loss of some liquid degrees of freedom through such first-order transitions is suggestive of the polyamorphic transition between native and denatured hydrated proteins, which can be interpreted as single-chain glass-forming polymers plasticized by water and cross-linked by hydrogen bonds. The onset of a sharp change in d<r2>dT(<r2> is the Debye-Waller factor and T is temperature) in proteins, which is controversially indentified with the glass transition in liquids, is shown to be general for glass formers and observable in computer simulations of strong and fragile ionic liquids, where it proves to be close to the experimental glass transition temperature. The latter may originate in strong anharmonicity in modes ("bosons"), which permits the system to access multiple minima of its configuration space. These modes, the Kauzmann temperature TK, and the fragility of the liquid, may thus be connected. 10.1126/science.267.5206.1924

Physics of the Granular State

HM Jaeger — 2008-03-06T18:37:24-00:00

Science, Vol. 255, No. 5051. (20 March 1992), pp. 1523-1531.

Granular materials display a variety of behaviors that are in many ways different from those of other substances. They cannot be easily classified as either solids or liquids. This has prompted the generation of analogies between the physics found in a simple sandpile and that found in complicated microscopic systems, such as flux motion in superconductors or spin glasses. Recently, the unusual behavior of granular systems has led to a number of new theories and to a new era of experimentation on granular systems. 10.1126/science.255.5051.1523

Coexistence of Liquid and Solid Phases in Flowing Soft-Glassy Materials

P Coussot — 2008-03-31T13:15:36-00:00

Physical Review Letters, Vol. 88, No. 21. (May 2002), 218301.

Magnetic-resonance-imaging rheometrical experiments show that concentrated suspensions or emulsions cannot flow steadily at a uniform rate smaller than a critical value ( γ̇ c ). As a result; a “liquid” region (sheared rapidly; i.e.; at a rate larger than γ̇ c ) and a “solid” region (static) coexist. The behavior of the fluid in the liquid region follows a simple power-law model; while the extent of the solid region increases with the degree of jamming of the material.

Phase switching of ordered arrays of liquid crystal emulsions

D Rudhardt — 2008-04-24T19:51:28-00:00

Applied Physics Letters, Vol. 82, No. 16. (2003), pp. 2610-2612.

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Universal features of the ﬂuid to solid transition for attractive colloidal particles

Prasad — 2008-04-24T18:37:13-00:00

Faraday Discuss., 2003

Attractive colloidal particles can exhibit a fluid to solid phase transition if the magnitude of the attractive interaction is sufficiently large, if the volume fraction is sufficiently high, and if the applied stress is sufficiently small. The nature of this fluid to solid transition is similar for many different colloid systems, and for many different forms of interaction. The jamming phase transition captures the common features of these fluid to solid translations, by unifying the behavior as a function of the particle volume fraction, the energy of interparticle attractions, and the applied stress. This paper describes the applicability of the jamming state diagram, and highlights those regions where the fluid to solid transition is still poorly understood. It also presents new data for gelation of colloidal particles with an attractive depletion interaction, providing more insight into the origin of the fluid to solid transition.

Phase Behavior and Charge Regulation of Weak Polyelectrolyte Grafted Layers

Peng Gong — 2008-03-19T17:31:41-00:00

Physical Review Letters, Vol. 98, No. 1. (2007)

The stability of weak polyelectrolytes end grafted to a planar surface has been studied with a molecular theory. The effective quality of the solvent is found to depend on the interplay between polymer grafting density, acid-base equilibrium, and salt concentration. Our results reveal that increasing salt concentration results in a thermodynamically more stable layer. This reverse salt effect is due to the competition between the solvent quality and the dual role of the ionic strength in screening the electrostatic interactions (reducing stability with increasing salt concentration), and regulating the charge on the polymer (increasing charge with increasing salt concentration). Grafted weak polyelectrolyte layers are found to be thermodynamically unstable at intermediate surface coverages. Additionally, it is established that the increased solubility of the layer at low surface coverage is due to the relatively large charge of the grafted polymers. The range of stability of the film with regard to polymer surface coverage, temperature, bulk pH and salt concentration is demonstrated.

Red Blood Cells and Other Nonspherical Capsules in Shear Flow: Oscillatory Dynamics and the Tank-Treading-to-Tumbling Transition

JM Skotheim — 2008-03-19T16:44:43-00:00

Physical Review Letters, Vol. 98, No. 7. (2007)

We consider the motion of red blood cells and other nonspherical microcapsules dilutely suspended in a simple shear flow. Our analysis indicates that depending on the viscosity, membrane elasticity, geometry, and shear rate, the particle exhibits either tumbling, tank-treading of the membrane about the viscous interior with periodic oscillations of the orientation angle, or intermittent behavior in which the two modes occur alternately. For red blood cells, we compute the complete phase diagram and identify a novel tank-treading-to-tumbling transition as the shear rate decreases. Observations of such motions coupled with our theoretical framework may provide a sensitive means of assessing capsule properties.

Kinetic Pathways for the Isotropic-Nematic Phase Transition in a System of Colloidal Hard Rods: A Simulation Study

Alejandro Cuetos — 2008-03-19T16:18:47-00:00

Physical Review Letters, Vol. 98, No. 9. (2007)

We study the kinetic pathways for the isotropic-to-nematic transition in a fluid of colloidal hard rods. In order to follow the formation of the nematic phase, we develop a new cluster criterion that distinguishes nematic clusters from the isotropic phase. Applying this criterion in Monte Carlo simulations, we find spinodal decomposition as well as nucleation and growth depending on the supersaturation. We determine the height of the nucleation barrier and we study the shape and structure of the cluster. More specifically, we find ellipsoidal nematic clusters with an aspect ratio of about 1.7 and a homogeneous nematic director field. Our results are consistent with theoretical predictions on the shape and director field of nematic tactoids. Classical nucleation theory gives reasonable predictions for the height of the nucleation barrier and the critical nucleus size.

Hydrodynamics and Rheology of Active Liquid Crystals: A Numerical Investigation

D Marenduzzo — 2008-03-19T01:42:05-00:00

Physical Review Letters, Vol. 98, No. 11. (2007)

We report numerical studies of the hydrodynamics and rheology of an active liquid crystal. We confirm the existence of a transition between a passive and an active phase, with spontaneous flow in steady state. We explore how the velocity profile changes with activity, and we point out the difference in behavior for flow-aligning and tumbling materials. We find that an active material can thicken or thin under a flow, or even exhibit both behaviors as the forcing changes.

Disappearance of the Gas-Liquid Phase Transition for Highly Charged Colloids

AP Hynninen — 2007-05-08T12:03:35-00:00

Physical Review Letters, Vol. 98, No. 19. (2007)

We calculate the full phase diagram of spherical charged colloidal particles using Monte Carlo free energy calculations. The system is described using the primitive model, consisting of explicit hard-sphere colloids and point counterions in a uniform dielectric continuum. We show that the gas-liquid critical point becomes metastable with respect to a gas-solid phase separation at colloid charges Q20 times the counterion charge. Approximate free energy calculations with only one and four particles in the fluid and solid phases, respectively, are used to determine the critical line for highly charged colloids up to Q=2000. We propose the scaling law Tc*" align="middle">~Q1/2 for this critical temperature.

“Sticky” Hard Spheres: Equation of State, Phase Diagram, and Metastable Gels

Stefano Buzzaccaro — 2007-09-13T03:11:31-00:00

Physical Review Letters, Vol. 99, No. 9. (2007)

A large variety of engaging phenomena, ranging from crystallization in protein solutions to the formation of colloidal gels and glasses via depletion forces, stems from the occurrence of very short-ranged attractive forces. From depolarized light scattering measurements of equilibrium sedimentation profiles, we obtain an accurate description of the equation of state and of the phase diagram of colloids where depletion forces are tuned by the addition of a surfactant. For weak depletion, a colloidal fluid fully described by Baxter's “sticky” hard sphere model coexists with ultradense colloidal crystals. For stronger attractive interactions, kinetically arrested looser gels form, showing an elastic modulus that scales as a power law of the local particle concentration.

Insights into phase transition kinetics from colloid science

Valerie Anderson — 2006-07-19T21:09:52-00:00

Nature, Vol. 416, No. 6883. (25 April 2002), pp. 811-815.

Colloids display intriguing transitions between gas, liquid, solid and liquid crystalline phases. Such phase transitions are ubiquitous in nature and have been studied for decades. However, the predictions of phase diagrams are not always realized; systems often become undercooled, supersaturated, or trapped in gel-like states. In many cases the end products strongly depend on the starting position in the phase diagram and discrepancies between predictions and actual observations are due to the intricacies of the dynamics of phase transitions. Colloid science aims to understand the underlying mechanisms of these transitions. Important advances have been made, for example, with new imaging techniques that allow direct observation of individual colloidal particles undergoing phase transitions, revealing some of the secrets of the complex pathways involved.