Lower Critical Point Research Articles

Molecular dynamics simulation of a binary mixture near the lower critical point.

2,6-lutidine molecules mix with water at high and low temperatures but in a wide intermediate temperature range a 2,6-lutidine/water mixture exhibits a miscibility gap. We constructed and validated an atomistic model for 2,6-lutidine and performed molecular dynamics simulations of 2,6-lutidine/water mixture at different temperatures. We determined the part of demixing curve with the lower critical point. The lower critical point extracted from our data is located close to the experimental one. The estimates for critical exponents obtained from our simulations are in a good agreement with the values corresponding to the 3D Ising universality class.

Effect of rolling temperature on the microstructure and mechanical properties of interstitial free steel

In the present research, to improve the mechanical of interstitial free steel, the hot rolling process has performed for 50% reduction in area at three different phase regions (below the lower critical point, above the upper critical point and intermediate region) of interstitial free steel. Samples were cooled down to room temperature by normal air cooling in the room temperature. The microstructure and mechanical properties of control rolled steel have been investigated by an optical microscope, Vickers hardness tester and tensile test. It has found that yield strength and ultimate tensile strength of sample rolled at a lower critical temperature (600°C) improved significantly without much significant change in ductility, due to ferrite grain refinement and an increase of dislocation density. Scanning electron microscope fractography analysis shows cup and cone fracture. Finally, it is suggested that a controlled rolling process of interstitial free steel in pure ferritic region conferred optimum properties.

The Analysis of Direct Numerical Simulation and Experiment in Critical Point of Transitional Flow

In order to study the lower critical point in transitional area of pipe, we used the method of direct numerical simulation to simulate fluid flow and contrasted it with experiment. The result showed that the flow state is close to laminar. Along the pipe axis, the change of pressure is not obviously. The changing rate of axial velocity U near wall region was significantly greater than in the mainstream area, it proved the important role of viscous force.

Probing layer localization in twisted graphene bilayers via cyclotron resonance

Electron wavefunctions in twisted bilayer graphene may have a strong single layer character or be intrinsically delocalized between layers, with their nature often determined by how energetically close they are to the Dirac point. In this paper, we demonstrate that in magnetic fields, optical absorption (cyclotron resonance) spectra contain signatures which may be used to distinguish the nature of these wavefunctions at low energies, as well as to locate low energy critical points in the zero-field energy spectrum. Optical absorption for two different configurations -- electric field parallel and perpendicular to the bilayer -- are calculated, which are shown to have different selection rules with respect to which states are connected by the perturbation. Interlayer bias further distinguishes transitions involving states of a single layer nature from those with support in both layers. For doped systems, a sharp increase in intra-Landau level absorption occurs with increasing field as the level passes through the zero-field saddle point energy, where the states change character from single layer to bilayer.

Two stage ecological hybrid sorption–compression refrigeration cycle

Low critical point of CO2 results in transcritical refrigeration cycles with low efficiency and high discharge pressure. H2O has a high triple point temperature that limits its application to temperatures above 0 °C. In this paper the two stage hybrid cycle is presented, where the Low Temperature (LT) stage is the compression cycle with CO2 and the High Temperature (HT) stage is the H2O sorption cycle. The choice between absorption and adsorption technology depends on the application and most of all is related to the temperature available for the desorber. This paper presents the adsorption technology on the HT stage powered by solar thermal energy. The combined hybrid cycle has been tested for two years at the Politechnika Krakowska, and a summary of the results is shown in this paper. The experimental results have been compared with two-refrigerants, two stage cycles and CO2 - only cycles.

Simple Model of Splitting Instability in Swollen Membranes

A simple one-dimensional mechanical model is proposed for splitting instability in swollen membranes. The splitting instability occurs by ring constriction. The bifurcation can be both subcritical and supercritical, depending on the slenderness of spheroidal membranes. The lower critical point is estimated theoretically for the simplest circular system. Necking instability occurs in a spheroidal membrane when the twist angle in the boundary conditions is increased.

The isotropic-nematic and nematic-nematic phase transition of binary mixtures of tangent hard-sphere chain fluids: an analytical equation of state.

An analytical equation of state (EoS) is derived to describe the isotropic (I) and nematic (N) phase of linear- and partially flexible tangent hard-sphere chain fluids and their mixtures. The EoS is based on an extension of Onsager's second virial theory that was developed in our previous work [T. van Westen, B. Oyarzún, T. J. H. Vlugt, and J. Gross, J. Chem. Phys. 139, 034505 (2013)]. Higher virial coefficients are calculated using a Vega-Lago rescaling procedure, which is hereby generalized to mixtures. The EoS is used to study (1) the effect of length bidispersity on the I-N and N-N phase behavior of binary linear tangent hard-sphere chain fluid mixtures, (2) the effect of partial molecular flexibility on the binary phase diagram, and (3) the solubility of hard-sphere solutes in I- and N tangent hard-sphere chain fluids. By changing the length bidispersity, two types of phase diagrams were found. The first type is characterized by an I-N region at low pressure and a N-N demixed region at higher pressure that starts from an I-N-N triphase equilibrium. The second type does not show the I-N-N equilibrium. Instead, the N-N region starts from a lower critical point at a pressure above the I-N region. The results for the I-N region are in excellent agreement with the results from molecular simulations. It is shown that the N-N demixing is driven both by orientational and configurational/excluded volume entropy. By making the chains partially flexible, it is shown that the driving force resulting from the configurational entropy is reduced (due to a less anisotropic pair-excluded volume), resulting in a shift of the N-N demixed region to higher pressure. Compared to linear chains, no topological differences in the phase diagram were found. We show that the solubility of hard-sphere solutes decreases across the I-N phase transition. Furthermore, it is shown that by using a liquid crystal mixture as the solvent, the solubility difference can by maximized by tuning the composition. Theoretical results for the Henry's law constant of the hard-sphere solute are in good agreement with the results from molecular simulation.

Modeling and simulation of an activated carbon–CO2 four bed based adsorption cooling system

In this study, a transient mathematical model of a 4-bed adsorption chiller using Maxsorb III as the adsorbent and CO2 as the refrigerant has been analyzed. The performances of the cyclic-steady-state system are presented for different heating and cooling water inlet temperatures. It is found that the desorption pressure has a big influence in the performances due to the low critical point of CO2 (Tc=31°C). With 80kg of Maxsorb III, the CO2 based adsorption chiller produces 2kW of cooling power and presents a COP of 0.1, at driving heat source temperature of 95°C along with a cooling temperature of 27°C and at optimum desorption pressure of 79bar. The present thermal compression air-conditioning system could be driven with solar energy or waste heat from internal combustion engines and therefore is suitable for both residential and mobile air-conditioning applications.

Communication: Protein dynamical transition vs. liquid-liquid phase transition in protein hydration water

In this work, we compare experimental data on myoglobin hydrated powders from elastic neutron scattering, broadband dielectric spectroscopy, and differential scanning calorimetry. Our aim is to obtain new insights on the connection between the protein dynamical transition, a fundamental phenomenon observed in proteins whose physical origin is highly debated, and the liquid-liquid phase transition (LLPT) possibly occurring in protein hydration water and related to the existence of a low temperature critical point in supercooled water. Our results provide a consistent thermodynamic/dynamic description which gives experimental support to the LLPT hypothesis and further reveals how fundamental properties of water and proteins are tightly related.

Two-patch colloidal model with re-entrant phase behaviour

We propose a second-order thermodynamic perturbation theory for a hard-sphere patchy colloidal model with two doubly bondable patches of type A and B. AB bonding results in the formation of a three-dimensional network of the particles and AA and BB bonding promotes chain formation. The theory is applied to study the phase behaviour of the model at different values of the potential model parameters. Competition between network and chain formation gives rise to a re-entrant phase behaviour with upper and lower critical points. The model with an additional van der Waals type of interaction may have a re-entrant phase diagram with three critical points and two separate regions of the liquid-gas phase coexistence. We analyze our results in terms of the fractions of the particles in different bonding states and conclude that re-entrant phase coexistence can be seen as a coexistence between a gas phase rich in chain ends and a liquid phase rich in branch points.

Dicke–Hepp–Lieb superradiant phase transition and independent modes model in quantum optomechanics

We show that in the strong coupling linear regime, the dispersive opto-mechanical Hamiltonian system can undergo a Dicke–Hepp–Lieb superradiant phase transition with two critical points. The higher critical point is the usual second order Dicke superradiant phase transition while the lower critical point is a new effect showing similarity to the usual Dicke superradiance. We also show that the mechanical mode near its quantum ground state completely decouples from the optical mode and the optical mode shows similarity to the Bogoluibov like excitations.

Asymmetric pairing of realistic mass quarks and color neutrality in the Polyakov–Nambu–Jona-Lasinio model of QCD

We investigate the effects of realistic quark masses and local color neutrality on quark pairing in the three-flavor Polyakov--Nambu--Jona-Lasinio model. While prior studies have indicated the presence of light flavor quark (2SC) or symmetric color-flavor-locked (CFL) pairing at low temperatures, we find that in the absence of a local color neutrality constraint the inclusion of the Polyakov loop gives rise to phases in which all quark colors and flavors pair, but with unequal magnitudes. We study this asymmetric color-flavor-locked (ACFL) phase, which can exist even for equal mass quarks, identifying its location in the phase diagram, the order of the associated phase transitions, and its symmetry breaking pattern, which proves to be the intersection of the symmetry groups of the 2SC and CFL phases. We also investigate the effects of the strange quark mass on this new phase and the QCD phase diagram generally. Finally, we analyze the effect of a local color neutrality constraint on these phases of asymmetric pairing. We observe that for massless quarks the neutrality constraint renders the 2SC phase energetically unfavorable, eliminating it at low temperatures, and giving rise to the previously proposed low temperature critical point, with associated continuity between the hadronic and ACFL phases. For realistic strange quark masses, however, the neutrality constraint shrinks the 2SC region of the phase diagram, but does not eliminate it, at T=0.

The definition of urban stormwater tolerance threshold and its conceptual estimation: an example from Taiwan

The combination of climate change and urbanization is worsening urban flooding problems. Estimating the amount of rainfall that a city can tolerate without flooding is a fundamental task that is difficult to perform, although large amounts of resources are invested in urban flood control. The purpose of this study is to determine the tolerance threshold for stormwater in a city. Based on hydrometeorological characteristics and existing flood control facilities, the urban adaptive water capacity is analyzed to determine the critical rainfall loading. Different critical levels are defined. The low critical point represents the beginning of the water accumulation, while the intermediate and high critical points are defined as flooding with heights of 300 and 600 cm, respectively, in low-lying areas. This study adopts a simple conceptual method to illustrate the critical levels instead of applying complex hydrologic and hydraulic modeling, which require high-resolution spatial data. Three cities and one township in Taiwan are used as urban case studies and to verify the conceptual method. As the capital, Taipei City utilizes the highest flood control engineering technology of our case studies; it is also the site in which the lowest rainfall thresholds cause the accumulation of water to reach the intermediate and high critical points because its small ‘internal water areas’ increase the height of floods rapidly. Conversely, Taichung City has a large internal water area that can disperse accumulating waters without increasing flood height. The estimations of urban storm tolerance thresholds increase the understanding of the limitations of water protection facilities. These estimations may be combined with rainfall forecasts to increase early warning functions and provide a reference point for subsequent planning related to urban flood adaptation strategies.

Three-dimensional patchy lattice model for empty fluids

The phase diagram of a simple model with two patches of type A and ten patches of type B (2A10B) on the face centred cubic lattice has been calculated by simulations and theory. Assuming that there is no interaction between the B patches the behavior of the system can be described in terms of the ratio of the AB and AA interactions, r. Our results show that, similarly to what happens for related off-lattice and two-dimensional lattice models, the liquid-vapor phase equilibria exhibit reentrant behavior for some values of the interaction parameters. However, for the model studied here the liquid-vapor phase equilibria occur for values of r lower than 1/3, a threshold value which was previously thought to be universal for 2AnB models. In addition, the theory predicts that below r=1/3 (and above a new condensation threshold which is <1/3) the reentrant liquid-vapor equilibria are so extreme that it exhibits a closed loop with a lower critical point, a very unusual behavior in single-component systems. An order-disorder transition is also observed at higher densities than the liquid-vapor equilibria, which shows that the liquid-vapor reentrancy occurs in an equilibrium region of the phase diagram. These findings may have implications in the understanding of the condensation of dipolar hard spheres given the analogy between that system and the 2AnB models considered here.

R-CRDSA: Reservation-Contention Resolution Diversity Slotted ALOHA for Satellite Networks

In this letter, a reservation scheme with contention resolution diversity slotted ALOHA (CRDSA), called R-CRDSA, is proposed. Although CRDSA promises excellent performance compared to slotted ALOHA (SA), there are two barriers to be overcome for its use in satellite networks. The first is a low critical point of an offered load; i.e., at this point, as the offered load increases, the throughput will decrease. CRDSA has a lower critical point than that of SA. The second barrier is that CRDSA has a lower capacity efficiency compared to SA; i.e., on average, CRDSA requires more power or bandwidth than SA for transmission. R-CRDSA can overcome these barriers at the same cost as CRDSA, especially for multi-packet messages. The analytic performances are derived by the observation of the transition between two successive frames, and the average number of packets in a message. The simulation results show that the analytic derivations are valid, and they confirm that the barriers are resolved by R-CRDSA.

Analysis of miscibility gaps based on the Redlich–Kister polynomial for binary solutions

The conditions for miscibility gaps are examined for the binary solutions of which the excess molar Gibbs energy is described by the Redlich–Kister polynomials of the terms, LA,B(n)xAxB(xA−xB)n (n=0, 1, 2, …, v), and LA,B(n) is given by AA,B(n)+BA,B(n)T. For the binary solutions specified by the first three R–K terms, the domains of miscibility gaps and of no miscibility gap at 0K have been successfully defined on the coordinate plane of LA,B(2)/LA,B(0) vs. LA,B(1)/LA,B(0). For the binary regular solution at high temperatures (|AA,B(0)|≪|BA,B(0)T|), the domain of miscibility gap with lower critical points and that with upper critical points have been successfully defined on the coordinate plane of AA,B(0) vs. BA,B(0) for the first time. For the case of single R–K terms at high temperatures, a rough guiding range of BA,B(n) for no miscibility gap is found to be given by −2R≤BA,B(n)≤2R. Both analytical approaches and numerical calculations were exploited in the present study.

Depolarized light scattering in a 2.6-lutidine-water solution near the critical point

The spectra of depolarized Rayleigh light scattering, i.e., the Rayleigh line wing (RLW), and Raman scattering in a solution with the lower critical point in the phase separation diagram is experimentally studied. The Rayleigh line wing is studied to 30 cm−1 from the exciting light frequency; the Raman scattering line (RSL) is studied in the near profile region. In the vicinity of the lower critical point, as well as in solutions with the upper critical point, strong narrowing of the RLW and RSL is observed. In this case, the conditions arise for the manifestation of the profile in the Rayleigh line spectrum, which is caused by the interaction of concentration and anisotropy fluctuations. Previously, this spectral line was theoretically predicted by I. A. Chaban in [1].

Axial anomaly and the three-flavor Nambu–Jona-Lasinio model with confinement: Constructing the QCD phase diagram

We investigate the phase structure of massless three-flavor QCD by extending the Nambu--Jona-Lasinio model to include the effects of confinement and the axial anomaly. We study the interplay between the chiral and diquark condensates induced by the axial anomaly, as well as their relationship with the Polyakov loop, which parameterizes confinement. By minimizing the thermodynamic potential we construct the QCD phase diagram and investigate the possibility of realizing a recently discovered low temperature critical point and an associated BEC-BCS crossover. We also perform a Ginzburg-Landau expansion of the thermodynamic potential, comparing our results to a prior analysis based purely on symmetry considerations, in order to assess the lowest-order effects of the condensate-confinement couplings.

Phase Behaviour of Palm Oil Fatty Acid Components in Supercritical Carbon Dioxide

In the supercritical CO2 method of extraction of palm oil, many processes in conventional method, such as degumming, deodorization, refining and bleaching processes, are eliminated. The supercritical method allows palm oil to be extracted and fractionated simultaneously, which not only reduces the cost of processing, but also provides a more environmental-friendly processing alternative. In this research, the high-pressure phase behaviour of the binary system between supercritical carbon dioxide (SC CO2) and palm oil fatty acid components were investigated. The phase transition is observed from a camera which is connected to a high-pressure variable-volume view cell. Carbon dioxide has high solvating power, nontoxic, inflammable and low critical points. The determination of phase behaviour could offer an insight to the right operating condition of palm oil supercritical fluid carbon dioxide extraction process in order to acquire the desired extraction selectivity and an optimum yield. The phase boundaries of some fatty acids components, lauric acid (C12), stearic acid (C18), and oleic acid (C18) in compressed supercritical carbon dioxide were determined at temperatures of 313.15 K, 323.15 K, 333.15 K, 343.15 K and 353.15 K under pressures between 10 MPa and 60 MPa.

An apparent halt to the decline ofSimulium woodiin the Usambara foci of onchocerciasis in Tanzania

An apparent halt to the decline of<i>Simulium woodi</i>in the Usambara foci of onchocerciasis in Tanzania