Coexistence Temperature Research Articles

Bubble and droplet motion in binary mixtures: Evaporation-condensation mechanism and Marangoni effect

Bubble and droplet motion in binary mixtures is studied in weak heat and diffusion fluxes and in gravity by solving the linearized hydrodynamic equations supplemented with appropriate surface boundary conditions. Without gravity, the velocity field is induced by evaporation and condensation at the interface and by the Marangoni effect due to a surface-tension gradient. In pure fluids, the latter nearly vanishes since the interface temperature tends to the coexistence temperature T_{cx}(p) even in heat flow. In binary mixtures, the velocity field can be much enhanced by the Marangoni effect above a crossover concentration c;{ *} inversely proportional to the radius R of the bubble or droplet. Here c;{ *} is usually very small for large R for non-azeotropic mixtures. The temperature and concentration deviations are also calculated.

Temperature-induced0−πcoexistence in clean superconductor-ferromagnet-superconductor Josephson junctions

A clean ballistic superconductor-ferromagnet metal-superconductor (SFS) junction is studied within the quasiclassical theory of superconductivity. We show that the temperature-induced coexistence of the 0 and $\ensuremath{\pi}$ states and the transition $0\text{\ensuremath{-}}\ensuremath{\pi}$ results from the temperature-induced transition between stable and metastable states of the junction. The $\ensuremath{\pi}$ shift occurs in small but finite critical intervals ${{Z}_{c}}$ of the magnetic barrier influence parameter $Z=2dh/\ensuremath{\hbar}{v}_{0}$, where $h$ is the exchange energy and $d$ the barrier thickness. The width of ${{Z}_{c}}$ and the coexistence temperature ${T}_{0\ensuremath{\pi}}$ depend also on the reduced barrier thickness $\overline{d}=d/{\ensuremath{\xi}}_{0}$, where ${\ensuremath{\xi}}_{0}$ is the zero-temperature superconducting coherence length. The interval ${{Z}_{c}}$ increases with $\overline{d}$. For $T\ensuremath{\ne}{T}_{0\ensuremath{\pi}}$ the junction is in the 0 or $\ensuremath{\pi}$ state. In contrast to the dirty case, the critical current ${I}_{c}$ depends monotonously on temperature, changing the sign but not the magnitude at the transition point. The nonvanishing critical current ${I}_{c}({T}_{0\ensuremath{\pi}})$ is related to the existence of the second harmonic in the current-phase relation, which could be observed experimentally.

Influence of Oxides on Microstructure and Notch Toughness of Weld Metal with Flux-cored Wires for High Strength Steel

In recent years, various steel structures tend to be larger and the recent trend of applying high strength steels makes it possible to reduce the structure weight. Notch toughness as well as strength is essential for materials used in the offshore structure and bridge construction industries and achieving both of them is a big challenge. While covered electrodes have been widely used in high strength steel structures in out-of-position welding, the usage of flux-cored wire (FCW) is desired because of its advantages over covered electrode in terms of operational efficiency and less skill requirement. TiO2 type FCW is most popular for all positional FCWs for mild steel and HT590MPa class steel and its excellent welding usability can be obtained with FCW for high strength steels up to HT780MPa. However, the weld metal made by TiO2 type FCW contains higher oxygen, making it more difficult to achieve superior notch toughness. By contrast, basic type FCW capable of achieving higher notch toughness is not good in usability in out-of-position welding. In this report, the effects of various oxides on microstructure, notch toughness, welding usability and oxygen content in the weld metal of high strength steel FCW are discussed.It has been clarified that notch toughness can be improved by decreasing the TiO2 activity and the density of dispersive oxide inclusions, leading to less oxygen content in the weld metal of TiO2 type FCW. In contrast to this, the oxygen content of the weld metal made by Al2O3 type FCW is high, and thus notch toughness cannot be improved even if its microstructure is refined. Furthermore, thermo-mechanical calculation has proved that welding usability in the vertical position becomes better when the solid-liquid coexistence temperature range of slag is wider.

Quantum path-integral study of the phase diagram and isotope effects of neon

The phase diagram of natural neon has been calculated for temperatures in the range of 17-50 K and pressures between 10(-2) and 2 x 10(3) bar. The phase coexistence between solid, liquid, and gas phases has been determined by the calculation of the separate free energy of each phase as a function of temperature. Thus, for a given pressure, the coexistence temperature was obtained by the condition of equal free energy of coexisting phases. The free energy was calculated by using nonequilibrium techniques such as adiabatic switching and reversible scaling. The phase diagram obtained by classical Monte Carlo simulations has been compared to that obtained by quantum path-integral simulations. Quantum effects related to the finite mass of neon cause that coexistence lines are shifted toward lower temperatures when compared to the classical limit. The shift found in the triple point amounts to 1.5 K, i.e., about 6% of the triple-point temperature. The triple-point isotope effect has been determined for (20)Ne, (21)Ne, (22)Ne, and natural neon. The simulation data show satisfactory agreement to previous experimental results, which report a shift of about 0.15 K between triple-point temperatures of (20)Ne and (22)Ne. The vapor pressure isotope effect has been calculated for both solid and liquid phases at triple-point conditions. The quantum simulations predict that this isotope effect is larger in the solid than in the liquid phase, and the calculated values show nearly quantitative agreement to available experimental data.

Effects of sonoprocessing on microstructure and mechanical properties of A390 aluminium alloy

Ultrasound is defined in terms of human hearing and it is a sound having a frequency higher than that to which the human ear can respond. The attempts to use the ultrasound at casting process have been carried out and there are a lot of study results on grain refinement on microstructure of casting alloys. However, these studies almost have been focused on mould vibration at solidifying melts of solid and liquid coexistence temperature. In this study, high intensity ultrasound was injected in aluminium full melts, especially A390 alloy, to evaluate the effect of ultrasound on microstructure and mechanical properties of castings. The effect of ultrasonic injection on melts could be summarised as follows: reduction of mean size of primary silicon, variation of phase distribution, improvement of solute homogeneity and mechanical properties.

Reversible Phase Transition of Colloids in a Binary Liquid Solvent

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.

Composition Profile of a Wetting Film in a Binary Mixture

The composition profile at a solid surface in the one-phase region of a binary liquid mixture in the complete wetting regime has been studied with neutron reflectometry. A hexane-rich film adsorbs to an octadecyl-coated silicon surface from a n-hexane-d 14 /perfluoro-n-hexane mixture. Above the critical temperature, T c , of 14.4 °C, there is a comparatively thin (5-20 nm) film with an excess n-hexane content that decays exponentially toward the bulk, with the n-hexane content and decay length increasing as the temperature approaches T c . Below T c , as the coexistence temperature To of 8.9 °C is approached, a very long-range effect on the density profile becomes apparent and the n-hexane content of the film remains greater than that of bulk out to distances z > 100 nm. Although the data are consistent with a z -3 decay of the composition profile, as expected for van der Waals forces, we cannot unambiguously assign a decay to the long-range 80-120 nm region of the film. However, as for van der Waals forces, the n-hexane surface excess, r, increases as (T - T 0 ) -1/3 near coexistence, but more steeply closer to T c .

Solubility of selected dibasic carboxylic acids in water, in ionic liquid of [Bmim][BF 4], and in aqueous [Bmim][BF 4] solutions

The solubilities of three dibasic carboxylic acids (adipic acid, glutaric acid, and succinic acid) in water, in the ionic liquid of 1-butyl-3-methyl-imidazolim tetrafluoroborate ([Bmim][BF 4]), and in the aqueous [Bmim][BF 4] solutions have been measured by a solid-disapperance method. The binodal curve of water + [Bmim][BF 4] was also determined experimentally from solid–liquid–liquid coexistence temperature up to near the upper critical solution temperature. Experimental results showed that each acid-containing binary behaved as a simple eutectic system. The solid–liquid equilibrium (SLE) data were correlated with the NRTL model for each binary system. The NRTL model with these determined binary parameters predicted the solid-disappearance temperatures of the aqueous ternary mixtures containing [Bmim][BF 4] and the dibasic acids to within an average absolute deviation of 2.0%.

Steady State Thermodynamics

The present paper reports our attempt to search for a new universal framework in nonequilibrium physics. We propose a thermodynamic formalism that is expected to apply to a large class of nonequilibrium steady states including a heat conducting fluid, a sheared fluid, and an electrically conducting fluid. We call our theory steady state thermodynamics (SST) after Oono and Paniconi's original proposal. The construction of SST is based on a careful examination of how the basic notions in thermodynamics should be modified in nonequilibrium steady states. We define all thermodynamic quantities through operational procedures which can be (in principle) realized experimentally. Based on SST thus constructed, we make some nontrivial predictions, including an extension of Einstein's formula on density fluctuation, an extension of the minimum work principle, the existence of a new osmotic pressure of a purely nonequilibrium origin, and a shift of coexistence temperature. All these predictions may be checked experimentally to test SST for its quantitative validity.

Absence of Fluid-Ordered/Fluid-Disordered Phase Coexistence in Ceramide/POPC Mixtures Containing Cholesterol

The effect of temperature on the lateral structure of lipid bilayers composed of porcine brain ceramide and 1-palmitoyl 2-oleoyl-phosphatidylcholine (POPC), with and without addition of cholesterol, were studied using differential scanning calorimetry, Fourier transformed infrared spectroscopy, atomic force microscopy, and confocal/two-photon excitation fluorescence microscopy (which included LAURDAN generalized polarization function images). A broad gel/fluid phase coexistence temperature regime, characterized by the presence of micrometer-sized gel-phase domains with stripe and flowerlike shapes, was observed for different POPC/ceramide mixtures (up to ∼25 mol % ceramide). This observed phase coexistence scenario is in contrast to that reported previously for this mixture, where absence of gel/fluid phase coexistence was claimed using bulk LAURDAN generalized polarization (GP) measurements. We demonstrate that this apparent discrepancy (based on the direct comparison between the LAURDAN GP data obtained in the microscope and the fluorometer) disappears when the additive property of the LAURDAN GP function is taken into account to examine the data obtained using bulk fluorescence measurements. Addition of cholesterol to the POPC/ceramide mixtures shows a gradual transition from a gel/fluid to gel/liquid-ordered phase coexistence scenario as indicated by the different experimental techniques used in our experiments. This last result suggests the absence of fluid-ordered/fluid-disordered phase coexistence in the ternary mixtures studied in contrast to that observed at similar molar concentrations with other ceramide-base-containing lipid mixtures (such as POPC/sphingomyelin/cholesterol, which is used as a canonical raft model membrane). Additionally, we observe a critical cholesterol concentration in the ternary mixtures that generates a peculiar lateral pattern characterized by the observation of three distinct regions in the membrane.

Direct calculation of solid-vapor coexistence points by thermodynamic integration: Application to single component and binary systems

We present a new thermodynamic integration method that directly connects the vapor and solid phases by a reversible path. The thermodynamic integration in the isothermal-isobaric ensemble yields the Gibbs free energy difference between the two phases, from which the sublimation temperature can be easily calculated. The method extends to the binary mixture without any modification to the integration path simply by employing the isothermal-isobaric semigrand ensemble. The thermodynamic integration, in this case, yields the chemical potential difference between the solid and vapor phases for one of the components, from which the binary sublimation temperature can be calculated. The coexistence temperatures predicted by our method agree well with those in the literature for single component and binary Lennard-Jones systems.

Atomistic simulation of solid-liquid coexistence for molecular systems: Application to triazole and benzene

A method recently developed to rigorously determine solid-liquid equilibrium using a free-energy-based analysis has been extended to analyze multiatom molecular systems. This method is based on using a pseudosupercritical transformation path to reversibly transform between solid and liquid phases. Integration along this path yields the free energy difference at a single state point, which can then be used to determine the free energy difference as a function of temperature and therefore locate the coexistence temperature at a fixed pressure. The primary extension reported here is the introduction of an external potential field capable of inducing center of mass order along with secondary orientational order for molecules. The method is used to calculate the melting point of 1-H-1,2,4-triazole and benzene. Despite the fact that the triazole model gives accurate bulk densities for the liquid and crystal phases, it is found to do a poor job of reproducing the experimental crystal structure and heat of fusion. Consequently, it yields a melting point that is 100 K lower than the experimental value. On the other hand, the benzene model has been parametrized extensively to match a wide range of properties and yields a melting point that is only 20 K lower than the experimental value. Previous work in which a simple "direct heating" method was used actually found that the melting point of the benzene model was 50 K higher than the experimental value. This demonstrates the importance of using proper free energy methods to compute phase behavior. It also shows that the melting point is a very sensitive measure of force field quality that should be considered in parametrization efforts. The method described here provides a relatively simple approach for computing melting points of molecular systems.

The range of meta stability of ice-water melting for two simple models of water

A number of crystal structures of water have been ‘superheated’ in Monte Carlo simulations. Two well-known models for water were considered; namely the TIP4P model and the SPC/E model. By comparing the fluid–solid coexistence temperature to the temperature at which the solid becomes mechanically unstable and melts it is possible to determine the typical range of temperatures over which it is possible to superheat the ice phases in conventional simulation studies. It is found that the ice phases can be superheated to approximately 90 K beyond the fluid–solid coexistence temperature. Beyond this limit they spontaneously melt. This limit appears to depend weakly both on the type of ice phase considered and on the chosen model. Obviously only rigorous free energy calculations can determine the equilibrium fluid–solid coexistence of a model. However, a ‘rule of thumb’ is that, by subtracting 90 K from the mechanical stability limit of the ice phase one is provided with a first guess as to the equilibrium fluid–solid coexistence temperature.

Gibbs Ensemble Simulations of Vapor/Liquid Equilibrium Using the Flexible RWK2 Water Potential

Gibbs ensemble Monte Carlo simulations of water vapor/liquid equilibrium (VLE) using the flexible fixed charge RWK2 water-water potential are reported. The equilibrium densities, saturation pressures, and critical parameters calculated with this potential are in better agreement with experimental data than are values obtained from the SPC, MSPC/E, TIP4P, and TIP5P rigid fixed charge potentials as well as from polarizable interactions, such as those of the PPC, KJ, SPC/P, TIP4P/P, and SPCDP potentials. The agreement between predicted and experimental phase coexistence phase density versus temperature variation in the critical region is similar to that obtained from the SPC/E and the polarizable SPC-pol-1 potentials. However, the variation of the saturation pressure and heat of vaporization with temperature and the critical pressure predicted by the RWK2 potential are closer to experiment than predictions from SPC/E interaction and similar (for lower temperatures) to those of the MSPC/E potential. The water structure predictions of the RWK2 potential are substantially better than those from the SPC-pol-1 interaction. VLE data are not used to parametrize the RWK2 potential. Nevertheless, predictions for coexistence temperature versus phase density and saturation pressure versus temperature are similar to those of the recent potential of Errington and Panagiotopoulos (EP potential), which is adjusted to reproduce the VLE behavior of water. MD simulations using the RWK2 potential show much better agreement with the room-temperature liquid structure and second virial coefficient data than does the EP potential. Fixing the bond lengths and HOH angle in the RWK2 interaction at their isolated molecule minimum energy values leads to significant deterioration of the VLE predictions obtained.

Adsorption from alkane+perfluoroalkane mixtures at fluorophobic and fluorophilic surfaces. II. Crossover from critical adsorption to complete wetting

Using neutron reflectometry, adsorption from an equimolar mixture of hexane + perfluorohexane to a fluorophobic, octadecyl-coated, silicon substrate has been investigated as a function of temperature in the one-phase region upon approach to liquid-liquid coexistence. The composition of the investigated mixture, x(F) = 0.50, is well removed from the critical composition of x(F) = 0.36, where x(F) is the perfluorohexane mole fraction. To aid the modeling, mixtures with three different neutron refractive index contrasts have been used: namely, mixtures of C(6)H(14) + C(6)F(14) (H-F), C(6)D(14) + C(6)F(14) (D-F), and a mixture of C(6)H(14) + C(6)D(14) + C(6)F(14) which has been adjusted to have the same refractive index as silicon (CMSi). For all three contrasts, the principal features of the composition profile normal to the interface follow similar trends as the temperature T is reduced towards T(0), the coexistence temperature. These features consist of: (i) a hexane-rich primary adsorption layer appended to the octadecyl coupled layer. This primary layer is 22 +/- 5 A thick and becomes increasingly enriched in hexane as T(0) is approached. (ii) A tail that decays exponentially towards the bulk composition with a characteristic decay length zeta. As T(0) is approached, zeta increases. The scattering length density profiles have been converted to volume fraction profiles and the surface excess of hexane Gamma has been determined as a function of temperature for all three contrasts. As T(0) is approached Gamma increases, and its behavior can be represented using the scaling law Gamma approximately |T - T(0)|(-m). The resulting values of m are 0.71 +/- 0.09, 0.68 +/- 0.04, and 0.68 +/- 0.06 for the D-F, H-F, and CMSi contrasts, respectively. The behavior of Gamma with temperature does not adhere to the Gamma approximately |T - T(0)|(-1/3) law expected for complete wetting in systems with van der Waals interactions nor does it correspond to Gamma approximately |T - T(c)|(-0.305) expected for critical adsorption. The magnitude of the exponent m indicates that the adsorption resides in the crossover region between critical adsorption and complete wetting.

Pressure effects for crystal growth in a closed system

We analyze the growth of a crystal from its supercooled liquid in a closed domain (constrained growth), taking into account the effects due to the different densities rho(s) and rho(l) of the solid and liquid phases. We assume rho(l) > rho(s), i.e., the liquid expands upon solidification. Then, the growth is contrasted by an increasing pressure, which results in a continuous decrease of the coexistence temperature and the effective supercooling. These phenomena have been simulated in two dimensions through a modified version of the classic phase-field model. We observe that for spherical growth the interface temperature reflects almost instantaneously the change of the coexistence temperature. For dendritic growth, we observed a relaxation time for the dendrite tip velocity and the tip radius which is comparable to the characteristic time of the process; however, after the first fast transient, the growth dynamics seems to follow the changing pressure with no appreciable lag. The onset of the morphological instability is slightly anticipated in respect to free growth.

The evaporation/condensation transition of liquid droplets.

The condensation of a supersaturated vapor enclosed in a finite system is considered. A phenomenological analysis reveals that the vapor is found to be stable at densities well above coexistence. The system size at which the supersaturated vapor condenses into a droplet is found to be governed by a typical length scale which depends on the coexistence densities, temperature and surface tension. When fluctuations are neglected, the chemical potential is seen to show a discontinuity at an effective spinodal point, where the inhomogeneous state becomes more stable than the homogeneous state. If fluctuations are taken into account, the transition is rounded, but the slope of the chemical potential versus density isotherm develops a discontinuity in the thermodynamic limit. In order to test the theoretical predictions, we perform a simulation study of droplet condensation for a Lennard-Jones fluid and obtain loops in the chemical potential versus density and pressure. By computing probability distributions for the cluster size, chemical potential, and internal energy, we confirm that the effective spinodal point may be identified with the occurrence of a first order phase transition, resulting in the condensation of a droplet. An accurate equation of state is employed in order to estimate the droplet size and the coexisting vapor density and good quantitative agreement with the simulation data is obtained. The results highlight the need of an accurate equation of state data for the Laplace equation to have predictive power.

Temperature-Controlled Structure and Kinetics of Ripple Phases in One- and Two-Component Supported Lipid Bilayers

Temperature-controlled atomic force microscopy (AFM) has been used to visualize and study the structure and kinetics of ripple phases in one-component dipalmitoylphosphatidylcholine (DPPC) and two-component dimyristoylphosphatidylcholine-distearoylphosphatidylcholine (DMPC-DSPC) lipid bilayers. The lipid bilayers are mica-supported double bilayers in which ripple-phase formation occurs in the top bilayer. In one-component DPPC lipid bilayers, the stable and metastable ripple phases were observed. In addition, a third ripple structure with approximately twice the wavelength of the metastable ripples was seen. From height profiles of the AFM images, estimates of the amplitudes of the different ripple phases are reported. To elucidate the processes of ripple formation and disappearance, a ripple-phase DPPC lipid bilayer was taken through the pretransition in the cooling and the heating direction and the disappearance and formation of ripples was visualized. It was found that both the disappearance and formation of ripples take place virtually one ripple at a time, thereby demonstrating the highly anisotropic nature of the ripple phase. Furthermore, when a two-component DMPC-DSPC mixture was heated from the ripple phase and into the ripple-phase/fluid-phase coexistence temperature region, the AFM images revealed that several dynamic properties of the ripple phase are important for the melting behavior of the lipid mixture. Onset of melting is observed at grain boundaries between different ripple types and different ripple orientations, and the longer-wavelength metastable ripple phase melts before the shorter-wavelength stable ripple phase. Moreover, it was observed that the ripple phase favors domain growth along the ripple direction and is responsible for creating straight-edged domains with 60° and 120° angles, as reported previously.

Ripples and the Formation of Anisotropic Lipid Domains: Imaging Two-Component Supported Double Bilayers by Atomic Force Microscopy

Direct visualization of the fluid-phase/ordered-phase domain structure in mica-supported bilayers composed of 1,2-dimyristoyl- sn-glycero-3-phosphocholine/1,2-distearoyl- sn-glycero-3-phosphocholine mixtures is performed with atomic force microscopy. The system studied is a double bilayer supported on a mica surface in which the top bilayer (which is not in direct contact with the mica) is visualized as a function of temperature. Because the top bilayer is not as restricted by the interactions with the surface as single supported bilayers, its behavior is more similar to a free-standing bilayer. Intriguing straight-edged anisotropic fluid-phase domains were observed in the fluid-phase/ordered-phase coexistence temperature range, which resemble the fluid-phase/ordered-phase domain patterns observed in giant unilamellar vesicles composed of such phospholipid mixtures. With the high resolution provided by atomic force microscopy, we investigated the origin of these anisotropic lipid domain patterns, and found that ripple phase formation is directly responsible for the anisotropic nature of these domains. The nucleation and growth of fluid-phase domains are found to be directed by the presence of ripples. In particular, the fluid-phase domains elongate parallel to the ripples. The results show that ripple phase formation may have implications for domain formation in biological systems.

Phase diagrams classification of thermoreversibly associating systems with due regard for mesoscopic cyclization effects

We consider systems of f-functional monomers Af, capable of thermoreversible associating with an equilibrium Arrhenius association constant k=g0 exp(−E/T). Effects of the parameters f, g0, and E on the global phase behavior of these systems are analyzed within two theoretical approaches differing in the way to allow for the presence of closed trails of labile bonds (cycles). Within the Flory approximation, which takes into account the trails closed at infinity only (i) sol–gel transition (emergence of the infinite cluster of labile bonds) is only a geometric transition; (ii) the phase diagrams with one, two, or three critical solution temperatures could exist; (iii) there are no diagrams with three phase coexistence temperatures (triple points). On the contrary, as consistent within our new mesoscopic cyclization (MsC) approach (i) the sol–gel transition is a genuine first order phase transition accompanied by a heat effect and phase separation; (ii) the phase diagrams are more varied and could possess triple points as well as some other peculiarities. An explicit topological classification of all types of phase diagrams is given for both approaches via building (i) the phase portraits, i.e., separation of the plane (ln g0,E) into the regions corresponding to topologically similar phase diagrams, and (ii) typical phase diagrams on the (volume fraction, T) and (pressure, T) planes for all regions of these phase portraits. For MsC approach, the latent phase transition heats are also presented. Possible changes of the presented phase diagram classification for more complicated models (in particular, for systems with concurrent association) are discussed.