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.

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.

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.

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.

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...

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 .

A Study of the Relation between Bicontinuous Microemulsions and Oil/Water Nano-emulsion Formation

Abstract: Oil/water (O/W) nano-emulsions have been formed in the system water/C16E6/mineral oil by the phase inversion temperature (PIT) method. The relation between the phase equilibria observed at the hydrophilic−lipophilic balance (HLB) temperature or the PIT (i.e., the nature, number, and relative volume fractions of the involved phases), the droplet sizes, and polydispersities of the resulting emulsions have been determined. Milky white emulsions were obtained when, at the HLB temperature, a three-phase equilibrium formed by water (W), shear-birefringent microemulsion (D), and oil (O) was observed. However, bluish transparent O/W nano-emulsions with droplet sizes as low as 40 nm were formed in a narrow range of oil-to-surfactant ratios in which a D or W + D phases were the initial equilibrium phases. In the W + D equilibria, droplet sizes were independent from the water content, indicating that nanodroplet formation is mainly controlled by the structure of the D phase. These results suggest that the main requi...

The Binary System Isotactic Polypropylene/Bis(3,4-dimethylbenzylidene)sorbitol: Phase Behavior, Nucleation, and Optical Properties

Abstract: The phase behavior of the binary system consisting of the commercial nucleating and clarifying agent 1,3:2,4-bis(3,4-dimethyldibenzylidene)sorbitol (DMDBS, Millad 3988) and isotactic polypropylene (i-PP) was investigated over the entire concentration range by means of differential scanning calorimetry (DSC), rheology, and optical microscopy. Experimental phase diagrams were constructed from data obtained in melting and crystallization studies, and a simple binary monotectic is advanced. Distinct regimes in the phase diagram, which apparently dictate nucleation and clarification of i-PP by DMDBS, are discussed. A maximum increase in the crystallization temperature of i-PP due to the nucleating action of DMDBS was observed in compositions containing between 0.2 and 1 wt % of the latter. Liquid−liquid phase separation was observed at elevated temperatures for i-PP/DMDBS mixtures comprising more than 2 wt % of DMDBS. A study of the optical properties of the i-PP/DMDBS system revealed that values for haze and ...

Liquid-Liquid Immiscibility in Membranes

Abstract: The observation of liquid-liquid immiscibility in cholesterol-phospholipid mixtures in monolayers and bilayers has opened a broad field of research into their physical chemistry. Some mixtures exhibit multiple immiscibilities. This unusual property has led to a thermodynamic model of "condensed complexes." These complexes are the consequence of an exothermic, reversible reaction between cholesterol and phospholipids. In this quantitative model the complexes are sometimes concentrated in a separate liquid phase. The phase separation into a complex-rich phase depends on membrane composition and intensive variables such as temperature. The properties of defined cholesterol-phospholipid mixtures provide a conceptual foundation for the exploration of a number of aspects of the biophysics and biochemistry of animal cell membranes.

Local structure of condensed zinc oxide

Abstract: The high-pressure local structure of zinc oxide has been studied at room temperature using combined energy-dispersive x-ray-diffraction and x-ray-absorption spectroscopy experiments. The structural parameter u and the lattice-parameter ratio $c/a$ of the wurtzite phase is given as a function of pressure and compared with results from ab initio calculations based on a plane-wave pseudopotential method within the density-functional theory. It is shown that an accurate study of ZnO requires the explicit treatment of the d electrons of Zn as valence electrons. In good agreement with present calculations, our experimental data do not show any variation of $u(P)$ in the low-pressure wurtzite phase between 0 and 9 GPa, pressure at which the phase transition to the rocksalt phase occurs. Moreover, no dramatic modification of the r-phase K-edge position up to $\ensuremath{\sim}20\mathrm{GPa}$ is observed, indicating the absence of metallization. In view of all these results, theoretical models identifying the wurtzite-to-rocksalt transition as an homogeneous path are discussed.

Nematic Elastomer Fiber Actuator

Abstract: We report the synthesis and physical studies of a liquid crystalline elastomer fiber consisting of two side-chain liquid crystalline acrylates and a nonmesogennic comonomer side group that acts as a reactive site for cross-linking. The terpolymer was synthesized by radical polymerization, and the cross-linking of the network was achieved by using a diisocyanate unit. The fiber formed shows good liquid crystal alignment texture under a cross-polarizer microscope. Thermoelastic response shows strain changes through the nematic−isotropic phase transition of about 30−35%. A retractive force of nearly 300 kPa was measured in the isotropic phase. Static work loop studies show the viscoelastic losses in these materials to be very small. We also present preliminary studies on the effect of doping carbon nanotubes on the induced strain at the nematic−isotropic transition.

Phase transitions in ternary caesium lead bromide

Abstract: Phase transitions in ternary caesium lead bromide (CsPbBr3) were studied by means of DSC, TMA and high temperature X-ray diffraction. The samples were prepared from the solution by water evaporation and from the melt. on the DSC curves as well as on the temperature dependence of the lattice constants of CsPbBr3 only two effects were found belonging to the earlier published phase transitions at 88 and 130°C and no further effects. Linear thermal expansion coefficient α of individual CsPbBr3 modifications were calculated from both TMA and high temperature X-ray diffraction. The structural parameters of the room temperature orthorhombic phase were refined and the results are presented. CsPbBr3 prepared from the solution contained about 10% of CsPb2Br5 and so the DSC curve of pure CsPb2Br5 was also measured and an effect at a temperature of 68.5°C was found.