Showing papers on "Phase (matter) published in 2003"

Computational fluid dynamics of dispersed two-phase flows at high phase fractions

Abstract: This study describes the development and validation of Computational Fluid Dynamics (CFD) methodology for the simulation of dispersed two-phase flows. A two-fluid (Euler-Euler) methodology previously developed at Imperial College is adapted to high phase fractions. It employs averaged mass and momentum conservation equations to describe the time-dependent motion of both phases and, due to the averaging process, requires additional models for the inter-phase momentum transfer and turbulence for closure. The continuous phase turbulence is represented using a two-equation k − ε−turbulence model which contains additional terms to account for the effects of the dispersed on the continuous phase turbulence. The Reynolds stresses of the dispersed phase are calculated by relating them to those of the continuous phase through a turbulence response function. The inter-phase momentum transfer is determined from the instantaneous forces acting on the dispersed phase, comprising drag, lift and virtual mass. These forces are phase fraction dependent and in this work revised modelling is put forward in order to capture the phase fraction dependency of drag and lift. Furthermore, a correlation for the effect of the phase fraction on the turbulence response function is proposed. The revised modelling is based on an extensive survey of the existing literature. The conservation equations are discretised using the finite-volume method and solved in a solution procedure, which is loosely based on the PISO algorithm, adapted to the solution of the two-fluid model. Special techniques are employed to ensure the stability of the procedure when the phase fraction is high or changing rapidely. Finally, assessment of the methodology is made with reference to experimental data for gas-liquid bubbly flow in a sudden enlargement of a circular pipe and in a plane mixing layer. Additionally, Direct Numerical Simulations (DNS) are performed using an interface-capturing methodology in order to gain insight into the dynamics of free rising bubbles, with a view towards use in the longer term as an aid in the development of inter-phase momentum transfer models for the two-fluid methodology. The direct numerical simulation employs the mass and momentum conservation equations in their unaveraged form and the topology of the interface between the two phases is determined as part of the solution. A novel solution procedure, similar to that used for the two-fluid model, is used for the interface-capturing methodology, which allows calculation of air bubbles in water. Two situations are investigated: bubbles rising in a stagnant liquid and in a shear flow. Again, experimental data are used to verify the computational results.

Nanoparticle Assembly and Transport at Liquid-Liquid Interfaces

Abstract: The self-assembly of particles at fluid interfaces, driven by the reduction in interfacial energy, is well established. However, for nanoscopic particles, thermal fluctuations compete with interfacial energy and give rise to a particle-size-dependent self-assembly. Ligand-stabilized nanoparticles assembled into three-dimensional constructs at fluid-fluid interfaces, where the properties unique to the nanoparticles were preserved. The small size of the nanoparticles led to a weak confinement of the nanoparticles at the fluid interface that opens avenues to size-selective particle assembly, two-dimensional phase behavior, and functionalization. Fluid interfaces afford a rapid approach to equilibrium and easy access to nanoparticles for subsequent modification. A photoinduced transformation is described in which nanoparticles, initially soluble only in toluene, were transported across an interface into water and were dispersed in the water phase. The characteristic fluorescence emission of the nanoparticles provided a direct probe of their spatial distribution.

An Atomic-Level View of Melting Using Femtosecond Electron Diffraction

Abstract: We used 600-femtosecond electron pulses to study the structural evolution of aluminum as it underwent an ultrafast laser–induced solid-liquid phase transition. Real-time observations showed the loss of long-range order that was present in the crystalline phase and the emergence of the liquid structure where only short-range atomic correlations were present; this transition occurred in 3.5picoseconds for thin-film aluminum with an excitation fluence of 70 millijoules per square centimeter. The sensitivity and time resolution were sufficient to capture the time-dependent pair correlation function as the system evolved from the solid to the liquid state. These observations provide an atomic-level description of the melting process, in which the dynamics are best understood as a thermal phase transition under strongly driven conditions.

Pentacene thin film transistors on inorganic dielectrics: Morphology, structural properties, and electronic transport

Abstract: The structural and transport properties of evaporated pentacene organic thin film transistors (TFTs) are reported, and they show the influence of the deposition conditions with different inorganic dielectrics. Dielectrics compatible with large area fabrication were explored to facilitate low cost electronics on glass or flexible plastic substrates. X-ray diffraction and atomic force microscopy show a clear correlation between the morphology and the structure of the highly polycrystalline films for all dielectrics investigated. The roughness of the dielectric has a distinct influence on the morphology and the structural properties, whereas the films on smooth thermal oxide are in general highly ordered and independent of the deposition conditions. The ordered films exhibit a “thin film” and a bulk phase, and the bulk phase volume fraction increases with the deposition temperature and the film thickness. Careful control of the deposition conditions gives virtually identical films on thermal oxide and silico...

Liquid-liquid phase transition in supercooled silicon.

Abstract: Silicon in its liquid and amorphous forms occupies a unique position among amorphous materials. Obviously important in its own right, the amorphous form is structurally close to the group of 4–4, 3–5 and 2–6 amorphous semiconductors that have been found to have interesting pressure-induced semiconductor-to-metal phase transitions1,2. On the other hand, its liquid form has much in common, thermodynamically, with water and other ‘tetrahedral network’ liquids that show density maxima3,4,5,6,7. Proper study of the ‘liquid–amorphous transition’, documented for non-crystalline silicon by both experimental and computer simulation studies8,9,10,11,12,13,14,15,16,17, may therefore also shed light on phase behaviour in these related materials. Here, we provide detailed and unambiguous simulation evidence that the transition in supercooled liquid silicon, in the Stillinger–Weber potential18, is thermodynamically of first order and indeed occurs between two liquid states, as originally predicted by Aptekar10. In addition we present evidence to support the relevance of spinodal divergences near such a transition, and the prediction3 that the transition marks a change in the liquid dynamic character from that of a fragile liquid to that of a strong liquid.

Surface-stress-induced phase transformation in metal nanowires.

Abstract: Several researchers have demonstrated, through experiments and analysis, that the structure and properties of nanometre-scale materials can be quite different to those of bulk materials due to the effect of surfaces. Here we use atomistic simulations to study a surface-stress-induced phase transformation in gold nanowires. The emergence of the transformation is controlled by wire size, initial orientation, boundary conditions, temperature and initial cross-sectional shape. For a initial crystal orientation and wire cross-sectional area below 4 nm(2), surface stresses alone cause gold nanowires to transform from a face-centred-cubic structure to a body-centred-tetragonal structure. The transformation occurs roughly when the compressive stress caused by tensile surface-stress components in the length direction exceeds the compressive stress required to transform bulk gold to its higher energy metastable crystal structure.

Composition and structure of the RuO2(110) surface in an O2 and CO environment: Implications for the catalytic formation of CO2

Abstract: The phase diagram of surface structures for the model catalyst ${\mathrm{RuO}}_{2}(110)$ in contact with a gas environment of ${\mathrm{O}}_{2}$ and CO is calculated by density-functional theory and atomistic thermodynamics. Adsorption of the reactants is found to depend crucially on temperature and partial pressures in the gas phase. Assuming that a catalyst surface under steady-state operation conditions is close to a constrained thermodynamic equilibrium, we are able to rationalize a number of experimental findings on the CO oxidation over ${\mathrm{RuO}}_{2}(110).$ We also calculated reaction pathways and energy barriers. Based on the various results the importance of phase coexistence conditions is emphasized as these will lead to an enhanced dynamics at the catalyst surface. Such conditions may actuate an additional, kinetically controlled reaction mechanism on ${\mathrm{RuO}}_{2}(110).$

Ordered and disordered patterns in two-phase flows in microchannels.

Abstract: We show that wetting properties crucially control the patterns in two-phase flows of immiscible fluids in microchannels. Ordered patterns, continuously entrained by the flow, are obtained when one phase completely wets the walls, while disordered patterns, intermittently adhering to the channel walls, are unavoidably produced when wetting is partial. A lower limit for the channel sizes capable of generating well structured objects (drops, pears, pearl necklaces, ...) is presented.

Anomalously Immobilized Water: A New Water Phase Induced by Confinement in Nanotubes

Abstract: Confinement can induce unusual behavior in the properties of matter. Using molecular dynamics simulations, we show here that water confined to carbon nanotubes of a critical size under ambient conditions (1 bar, 300 K) can undergo a transition into a state having icelike mobility with an amount of hydrogen bonding similar to that in liquid water. The onset of this behavior occurs rapidly, raising the possibility that confinement inside nanotubes, and perhaps even buckyballs, can provide an environment in which the dynamics of phase changes may be studied directly by simulation. Moreover, because of a variety of evidence suggesting that water ordering may modulate proton conductance via a “proton wire” hydrogen bonding network, the ability to modulate water ordering with geometry suggests a possible mechanism for a switchable nanoscale semiconductor.

Some Interface Properties of the Phase

Abstract: This squib has two goals: to identify evidence for (strong) phases (Chomsky 2000, 2001), and to use this evidence to investigate the extensional definition of a phase. Chomsky (2000) states that CP is a phase, whereas TP is not, and (transitive) vP is a phase, whereas passive and unaccusative verb phrases (VPs) are not. The A. argues here that unaccusative and passive VPs are phases as well

Formation of Co-continuous Structures in Melt-Mixed Immiscible Polymer Blends

Abstract: Co-continuous structures can be regarded as the coexistence of at least two continuous structures within the same volume. Blends with co-continuous structures may combine the properties of both components in a favorable way, for example, mechanical moduli. This review article deals with the identification, characterization, and properties of co-continuous structures as well as with the development of co-continuous structures during the melt blending process. Co-continuous structures usually can be formed within a composition region about the phase inversion composition, which mainly depends on the viscosity ratio. On the other hand, co-continuous structures can be found independent of composition as intermediate stages during the initial state of morphology development and during phase inversion process in blends in which the component finally forming the dispersed phase forms the matrix in early mixing states. In addition, even at low volume fractions of one component, stable co-continuous morph...

Phase Coexistence in Driven One-Dimensional Transport

Abstract: We study a one-dimensional totally asymmetric exclusion process with random particle attachments and detachments in the bulk. The resulting dynamics leads to unexpected stationary regimes for large but finite systems. Such regimes are characterized by a phase coexistence of low and high density regions separated by domain walls. We use a mean-field approach to interpret the numerical results obtained by Monte Carlo simulations, and we predict the phase diagram of this nonconserved dynamics in the thermodynamic limit.

Anomalously Immobilized Water: A New Water Phase Induced by Confinement in

Abstract: Confinement can induce unusual behavior in the properties of matter. Using molecular dynamics simulations, we show here that water confined to carbon nanotubes of a critical size under ambient conditions (1 bar, 300 K) can undergo a transition into a state having icelike mobility with an amount of hydrogen bonding similar to that in liquid water. The onset of this behavior occurs rapidly, raising the possibility that confinement inside nanotubes, and perhaps even buckyballs, can provide an environment in which the dynamics of phase changes may be studied directly by simulation. Moreover, because of a variety of evidence suggesting that water ordering may modulate proton conductance via a “proton wire” hydrogen bonding network, the ability to modulate water ordering with geometry suggests a possible mechanism for a switchable nanoscale semiconductor.

Gemini Surfactants : Synthesis, Interfacial and Solution-Phase Behavior, and Applications

Abstract: Synthesis of gemini (dimeric) and related surfactants, Isao Ikeda models of gemini surfactants, H. Diamant and D. Andelman adsorption and surface tension behaviour of gemini surfactants at air/water, oil/water and solid/water interfaces, E. Franses, M.R. Infante, L. Perez, A. Pinazo and A. Prosser state of gemini surfactants in solution at concentrations below the CMC, Raoul Zana gemini (dimeric) surfactants in water - solubility, CMC and thermodynamics of micellization and interaction with water soluble polymers, Raoul Zana properties of micelles and of micellar solutions of gemini (dimeric) surfactants, Raoul Zana rheology of solutions of gemini surfactants M. In phase behaviour of gemini surfactants, Raoul Zana and M. In mixed micellization between dimeric (gemini) surfactants and conventional surfactants, Raoul Zana and Jiding Xia special gemini surfactants - Nonionic, Zwitterionic, fluorinated and amino-acid based, T. Davey structure-performance relationships in gemini surfactants, Yun-Peng Zhu applications of gemini surfactants, Jiding Xia and Raoul Zana.

Metal-insulator transition in two-dimensional electron systems

Abstract: The interplay between strong Coulomb interactions and randomness has been a long-standing problem in condensed matter physics. According to the scaling theory of localization, in two-dimensional systems of noninteracting or weakly interacting electrons, the ever-present randomness causes the resistance to rise as the temperature is decreased, leading to an insulating ground state. However, new evidence has emerged within the past decade indicating a transition from insulating to metallic phase in two-dimensional systems of strongly interacting electrons. We review earlier experiments that demonstrate the unexpected presence of a metallic phase in two dimensions, and present an overview of recent experiments with emphasis on the anomalous magnetic properties that have been observed in the vicinity of the transition.

Dynamics of the plume formation and parameters of the ejected clusters in short-pulse laser ablation

Abstract: The dynamics of the early stages of the ablation plume formation and the mechanisms of cluster ejection are investigated in large-scale molecular dynamics simulations The cluster composition of the ablation plume has a strong dependence on the irradiation conditions and is defined by the interplay of a number of processes during the ablation plume evolution At sufficiently high laser fluences, the phase explosion of the overheated material leads to the formation of a foamy transient structure of interconnected liquid regions that subsequently decomposes into a mixture of liquid droplets, gas-phase molecules, and small clusters The ejection of the largest droplets is attributed to the hydrodynamic motion in the vicinity of the melted surface, especially active in the regime of stress confinement Spatially resolved analysis of the dynamics of the plume formation reveals the effect of segregation of the clusters of different sizes in the expanding plume A relatively low density of small/medium clusters is observed in the region adjacent to the surface, where large clusters are being formed Medium-size clusters dominate in the middle of the plume and only small clusters and monomers are observed near the front of the expanding plume Despite being ejected from deeper under the surface, the larger clusters in the plume have substantially higher internal temperatures as compared to the smaller clusters The cluster-size distributions can be relatively well described by a power law Y(N)∼N-τ with exponents different for small, up to ∼15 molecules, and large clusters The decay is much slower in the high-mass region of the distribution

Synthesis and Characterization of Hydroxyapatite Nanopowders by Emulsion Technique

Abstract: Nanocrystalline HAp powder was synthesized using surfactant template systems. Composition of the microemulsion and synthesis parameters had significant effect on the formation of HAp nanopowder and their surface area and morphology. Powders were prepared with a surface area of 130 m2/g and particle size between 30 and 50 nm with needle shape and spherical morphology. Nanocrystalline hydroxyapatite (HAp) powder was synthesized using the reverse micelle-processing route. Cyclohexane was used as the oil phase, mixed poly(oxyethylene)5 nonylphenol ether (NP-5) and poly(oxyethylene)12 nonylphenol ether (NP-12) as the surfactant phase, and a solution of Ca(NO3)2 and H3PO4 was used as the aqueous phase. The powders were characterized by BET surface area analyzer, powder X-ray diffraction, and transmission electron microscopy. It was found that experimental conditions such as aqueous/organic phase volume ratio, pH, aging time, aging temperature, and metal ion concentration in the aqueous phase affected the crysta...

Phase behavior of the phytantriol/water system

Abstract: Phytantriol, 3,7,11,15-tetramethyl-1,2,3-hexadecanetriol, is frequiently used as a cosmetic ingredient; however, very little is known about its physical and chemical properties. Here, we present the phase behavior of phytantriol in water, as determined by X-ray diffraction. At room temperature, the phase sequence upon increasing the water concentration is reversed micellar, lamellar, cubic phase Q230, and cubic phase Q224. At 44 °C, the cubic liquid crystals are transformed into a reversed hexagonal phase. The temperature−composition phase diagram of phytantriol/water mixtures is, thus, qualitatively similar to that of aqueous glycerol monooleate. The chemical stability of phytantriol makes it an interesting alternative to glycerol monooleate in exploiting various scientific and technical applications of, in particular, the cubic liquid crystalline phases.

“Pulling” Nanoparticles into Water: Phase Transfer of Oleic Acid Stabilized Monodisperse Nanoparticles into Aqueous Solutions of α-Cyclodextrin

Abstract: This paper describes a general method to drastically improve the dispersity of oleic acid stabilized nanoparticles in aqueous solutions. We use oleic acid stabilized monodisperse nanoparticles of iron oxides and silver as model systems, and have modified the surface properties of these nanoparticles through the formation of an inclusion complex between surface-bound surfactant molecules and α-cyclodextrin (α-CD). After the modification, the nanoparticles of both iron oxide and Ag can transfer from hydrophobic solvents, such as hexane, to α-CD aqueous phase. The efficiency of the phase transfer to the aqueous solutions depends on the initial α-CD concentration. The α-CD/oleic acid complex stabilized nanoparticles can be stable for long periods of time in aqueous phase under ambient atmospheric conditions. Transmission electron microscopy (TEM), ultraviolet−visible (UV−vis) spectroscopy, Fourier transform-infrared (FT-IR) spectroscopy, and colorimetric methods have been used in the characterization of these...

Active nematics on a substrate: Giant number fluctuations and long-time tails

Abstract: We construct the equations of motion for the coupled dynamics of order parameter and concentration for the nematic phase of driven particles on a solid surface, and show that they imply i) giant number fluctuations, with a standard deviation proportional to the mean in dimension d = 2 of primary relevance to experiment, and ii) long-time tails $\sim t^{-d/2}$in the autocorrelation of the particle velocities despite the absence of a hydrodynamic velocity field. Our predictions can be tested in experiments on aggregates of amoeboid cells as well as on layers of agitated granular matter.

Is the fragility of a liquid embedded in the properties of its glass

Abstract: When a liquid is cooled below its melting temperature, it usually crystallizes. However, if the quenching rate is fast enough, the system may remain in a disordered state, progressively losing its fluidity upon further cooling. When the time needed for the rearrangement of the local atomic structure reaches approximately 100 seconds, the system becomes “solid” for any practical purpose, and this defines the glass transition temperature T g . Approaching this transition from the liquid side, different systems show qualitatively different temperature dependencies of the viscosity, and accordingly they have been classified by introducing the concept of “fragility.” We report experimental observations that relate the microscopic properties of the glassy phase to the fragility. We find that the vibrational properties of the glass well below T g are correlated with the fragility value. Consequently, we extend the fragility concept to the glassy state and indicate how to determine the fragility uniquely from glass properties well below T g .