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PubMed2006

Dynamic phase transition in the kinetic spin- Blume-Capel model under a time-dependent oscillating external field.

We present a study, within a mean-field approach, of the stationary states of the kinetic spin-3/2 Blume-Capel model in the presence of a time-dependent oscillating external magnetic field. We use the Glauber-type stochastic dynamics to describe the time evolution of the system. We have found that the behavior of the system strongly depends on the crystal-field interaction. We can identify two types of solutions: a symmetric one where the magnetization (m) of the system oscillates in time around zero, which corresponds to a paramagnetic phase (P), and an antisymmetric one where m oscillates in time around a finite value different from zero, namely +/-3/2 and +/-1/2 that corresponds to the ferromagnetic-3/2 (F3/2) and the ferromagnetic-1/2 (F1/2) phases, respectively. There are coexistence regions of the phase space where the F3/2, F1/2, (F3/2 + F1/2), F3/2, P(F3/2 + P), F1/2, and P(F1/2 + P), F3/2, F1/2, P(F3/2 + F1/2 + P) phases coexist, hence the system exhibits seven different phases. We obtain the dynamic phase transition points and find six fundamental phase diagrams which exhibit one or three dynamic tricritical points. We have also calculated the Liapunov exponent to verify the stability of the solutions and the dynamic phase transition points.

Journal Article↗
PubMed2006

Thermodynamics of the liquid expanded to condensed phase transition of poly(L-lactic acid) in Langmuir monolayers.

Surface pressure-area per monomer (pi-A) isotherms show that poly(L-lactic acid) (PLLA) Langmuir monolayers exhibit a liquid expanded-to-condensed (LE/LC) phase transition at low surface pressure. Brewster angle microscopy images show circular domains where the LC phase is surrounded by the LE phase during phase coexistence. Morphology studies via atomic force microscopy show that well-ordered patterns are only observed for Langmuir-Blodgett films prepared in the LC phase, while no ordered features are observed in the LE phase. The morphological differences confirm that during the LE/LC phase transition PLLA molecules form well-ordered structures at the air/water interface. Analysis by the two-dimensional Clausius-Clapeyron equation is used to predict the critical parameters (X(c)). Both critical parameters, the critical temperature (T(c)) and the critical pressure (pi(c)), increase with increasing number average molar mass (M(n)) as X(c) = X(c,infinity) - KM(n)(-1), where X(c,infinity) is the value of the critical parameter at infinite molar mass and K is a constant. For PLLA T(c,infinity) = 36.2 +/- 0.3 degrees C and pi(c,infinity) = 4.53 +/- 0.06 mN x m(-1). This study provides a model polymer system for examining critical behavior in two dimensions.

Journal Article↗
PubMed2005

Hypopituitarism induced by traumatic brain injury in the transition phase.

Traumatic brain injury (TBI) has been associated with hypopituitarism in general and GH deficiency (GHD) in particular; the consequences of this on growth and development are likely to be critical in children and adolescents in the so-called "transition phase". In order to verify the consequences of TBI on pituitary function in the transition phase, we studied a population of adolescents and young adults 3 and 12 months after brain injury [no. = 23, 9 females, 14 males; age: 16-25 yr; body mass index (BMI): 21.9 +/- 0.6 kg/m2]. At 3 months, hypopituitarism was present in 34.6%. Total, multiple and isolated deficits were present in 8.6, 4.3 and 21.7%, respectively. Diabetes insipidus (DI) was present in 8.6% patients and mild hyperprolactinemia in 4.3%. At 12 months, hypopituitarism was present in 30.3%. Total, multiple and isolated deficits were present in 8.6, 4.3 and 17.4%, respectively. DI was present in 4.3% of patients and mild hyperprolactinemia in 4.3%. Total hypopituitarism was always confirmed at retesting. Multiple and isolated hypopituitarism were confirmed in 0/1 and 2/5, respectively. Two/23 patients showed isolated hypopituitarism at 12 months only; 1 patient with isolated at 3 months showed multiple hypopituitarism at retesting. GHD and secondary hypogonadism were the most common acquired pituitary deficits. These results show the high risk of TBI-induced hypopituitarism also in the transition age. Thus it is recommended that pediatric endocrinologists follow-up pituitary function of children and adolescents after brain injuries.

Adolescent↗
PubMed2006

A reexamination of the ice III/IX hydrogen bond ordering phase transition.

Ice III is a hydrogen bond disordered crystal which when cooled 1 K / min or faster transforms to an antiferroelectric hydrogen bond ordered structure, ice IX. Throughout its region of stability, experiments indicate that the H bonds in ice III are, in fact, partially ordered, i.e., some proton arrangements are preferred. In addition, there has been evidence that the structure of ice IX retains some residual disorder after the transition. Diffraction experiments and calorimetry apparently conflict with regard to the degree of ordering at the ice III/IX transition. Mean field statistical mechanical theories have been used to link partial occupations from diffraction data with thermodynamics. In this work, we investigate the ice III/IX proton ordering phase transition using electronic density functional theory calculations for small unit cells, extended to simulate the phase transition in a large unit cell using graph invariants. In agreement with experiment, we observe partial ordering over a wide range of temperatures as ice III transforms to partially disordered ice IX, near 126 K, which becomes fully ordered at lower temperatures. We compare our results from full statistical mechanical simulations with mean field models, finding small errors for the low-temperature ice IX phase and much larger errors for the high-temperature ice III phase. The failure of mean field theories may explain the apparent conflict between diffraction experiments and calorimetry.

Journal Article↗
PubMed2000

Phase Transition Processes in Surfactant Adsorption Layers.

Adsorption data in the form of surface tension vs bulk concentration isotherms having strong changes in the slope have often been discussed in the literature in terms of phase transitions in the adsorption layer. It will be shown thermodynamically that these apparent kink points in the isotherms are unreal. A phase transition would cause a completely opposite change in the isotherm. Experimental surface tension data for sodium octanoate and dodecyl ammonium chloride obtained from the literature are reinterpreted on the basis of the Frumkin and reorientation isotherms. Copyright 2000 Academic Press.

Journal Article↗
PubMed2004

Strong-weak coupling self-duality in the two-dimensional quantum phase transition of p + ip superconducting arrays.

The 2D quantum phase transition that occurs in a square lattice of Josephson-coupled p +/- ip superconductors is an example of how four-body interactions in d = 2 reproduce nonperturbative effects caused by two-body interactions in d = 1. The ordered phase has an unconventional "bond order" of the local T-breaking variable. This problem can be analyzed using an exact self-duality; this duality in classical notation is the 3D generalization of the Kramers-Wannier duality of the 2D Ising model, and there are similar exact dualities in dimensions d > or = 3. We discuss the excitation spectrum and experimental signatures of the ordered and disordered phases, and the relationship between our model and previously studied behavior of 2D boson models with four-boson interactions.

Journal Article↗
PubMed1979

Approximate steady-state properties of lattices of interacting three-state enzyme molecules: a novel phase transition.

Previous work on the cooperative behavior of lattices of interacting two-state enzyme molecules at steady state is extended here to interacting three-state enzyme molecules with a one-way cycle. The Bragg-Williams (mean field) approximation is used. A phase-transition example with a bifurcation point is discussed. Compared to conventional phase transitions (with a van der Waals loop), several new and complicated features appear. A second paper on this subject will contain a number of other examples of three-state systems.

Enzymes↗
PubMed2002

Quantum phase transition from a superfluid to a Mott insulator in a gas of ultracold atoms.

For a system at a temperature of absolute zero, all thermal fluctuations are frozen out, while quantum fluctuations prevail. These microscopic quantum fluctuations can induce a macroscopic phase transition in the ground state of a many-body system when the relative strength of two competing energy terms is varied across a critical value. Here we observe such a quantum phase transition in a Bose-Einstein condensate with repulsive interactions, held in a three-dimensional optical lattice potential. As the potential depth of the lattice is increased, a transition is observed from a superfluid to a Mott insulator phase. In the superfluid phase, each atom is spread out over the entire lattice, with long-range phase coherence. But in the insulating phase, exact numbers of atoms are localized at individual lattice sites, with no phase coherence across the lattice; this phase is characterized by a gap in the excitation spectrum. We can induce reversible changes between the two ground states of the system.

Journal Article↗
PubMed1988

Fluorometric detection of the bilayer-to-hexagonal phase transition in liposomes.

We have used the fluorescent probe N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (NBD-PE) to detect the bilayer-to-hexagonal phase transition. The fluorescence intensity of the probe was found to increase during the bilayer-to-hexagonal transition. The bilayer-to-hexagonal transitions of various types of phosphatidylethanolamine or cardiolipin measured by this method are consistent with results obtained by differential scanning calorimetry. To establish this method for wider use, agents known to alter the bilayer-to-hexagonal transition were examined, and the results are comparable with the published data. The added advantage of this fluorometric method over other currently available techniques is that it is applicable not only for aggregated lipid samples but also for dilute liposome suspensions. This is especially important when one of the components of the system under study can partition between lipid and aqueous phase. Since NBD is located near the headgroup region of the bilayer, it most likely detects the change of the environment surrounding that region. On the basis of our present study, it appears that NBD-PE is sufficiently sensitive to detect bilayer-to-hexagonal phase transition.

Fluorescent Dyes↗
PubMed1994

The effect of N-acyl ethanolamines on phosphatidylethanolamine phase transitions studied by laurdan generalised polarisation.

The effect of N-lauroylethanolamine (N-LEA) and N-oleoylethanolamine (N-OEA) on the thermal behaviour of fully hydrated egg phosphatidylethanolamine (TPE) was investigated by the steady-state fluorescence of 2-dimethylamino-(6-lauroyl)-naphtalene (laurdan) and 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). The parameter generalised polarisation (GP), calculated by exciting laurdan at 340 and 410 nm, revealed the gel to liquid crystalline lamellar (L alpha) as well as the L alpha to inverse hexagonal (HII) phase transitions of TPE. The L alpha to HII phase transition was not detected in TPE/N-OEA system, probably because of the formation of an intermediate Q224 cubic phase. The formation of Q224 phase in TPE/N-OEA and TPE/N-LEA systems was previously demonstrated by X-ray diffraction, but neither laurdan generalised polarisation nor TMA-DPH steady-state fluorescence anisotropy measurements revealed the presence of this phase. It is suggested that the lack of detection of the cubic phase is probably due to the similarity in dynamic characteristics and hydration levels of phospholipid headgroups in the bilayer and cubic phases.

2-Naphthylamine↗
PubMed2005

Thermotropic and barotropic phase transitions of N-methylated dipalmitoylphosphatidylethanolamine bilayers.

In order to understand the effect of polar head group modification on the thermotropic and barotropic phase behavior of phospholipid bilayer membranes, the phase transitions of dipalmitoylphosphatidylethanolamine (DPPE), dipalmitoylphosphatidyl-N-methylethanolamine (DPMePE), dipalmitoylphosphatidyl-N,N-dimethylethanolamine (DPMe2PE) and dipalmitoylphosphatidylcholine (DPPC) bilayer membranes were observed by differential scanning calorimetry and high-pressure optical methods. The temperatures of the so-called main transition from the gel (L(beta)) or ripple gel (P(beta)') phase to the liquid crystalline (L(alpha)) phase were almost linearly elevated by applying pressure. The slope of the temperature-pressure boundary, dT/dp, was in the range of 0.220-0.264 K MPa(-1) depending on the number of methyl groups in the head group of lipids. The main-transition temperatures of N-methylated DPPEs decreased with increasing size of head group by stepwise N-methylation. On the other hand, there was no significant difference in thermodynamic quantities of the main transition between the phospholipids. With respect to the transition from the subgel (L(c)) phase to the lamellar gel (L(beta) or L(beta)') phase, the transition temperatures were also elevated by applying pressure. In the case of DPPE bilayer the L(c)/L(beta) transition appeared at a pressure higher than 21.8 MPa. At a pressure below 21.8 MPa the L(c)/L(alpha) transition was observed at a temperature higher than the main-transition temperature. The main (L(beta)/L(alpha)) transition can be recognized as the transformation between metastable phases in the range from ambient pressure to 21.8 MPa. Polymorphism in the gel phase is characteristic of DPPC bilayer membrane unlike other lipid bilayers used in this study: the L(beta)', P(beta)' and pressure-induced interdigitated gel (L(beta)I) phases were observed only in the DPPC bilayer. Regarding the bilayers of DPPE, DPMePE and DPMe2PE, the interdigitation of acyl chain did not appear even at pressures as high as 200 MPa.

Calorimetry, Differential Scanning↗
PubMed1996

Cell adhesion to substratum and activation of tyrosine kinases are essentially required for G1/S phase transition in BALB/c 3T3 fibroblasts.

Cell adhesion to substratum and activation of tyrosine kinases are essential for the progression of cell cycle through G1 phase in mammalian cells. The kinetic studies of mouse BALB/c 3T3 fibroblasts showed that serum was no longer required for the progression of G1/S phase transition. In contrast, cell adhesion was essentially required in late G1 phase, especially at the period of G1/S transition. Among the kinase inhibitors used to elucidate the signal transduction caused by cell adhesion, tyrosine kinase inhibitors, genistein and herbimycin A, blocked the G1/S transition most effectively when cells were exposed to the inhibitors at the period of G1/S transition. Cell adhesion was not critically required for cells to undergo DNA synthesis once they had passed the G1/S boundary, and the effects of tyrosine kinase inhibitors on the progression of S phase were also not critical. The expressions of histone H2B and dihydrofolate reductase (DHFR) genes (S phase specific genes) and also the transcription factor E2F-1 gene (an activator of DHFR gene) were suppressed when cells were cultured without adhesion or exposed to the tyrosine kinase inhibitors. These results suggest that cell adhesion to substratum plays an important role in the G1/S phase transition of mouse BALB/c 3T3 fibroblasts through the activation of tyrosine kinases other than growth factor receptor-tyrosine kinases.

3T3 Cells↗
PubMed2004

Nucleation and growth in pressure-induced phase transitions from molecular dynamics simulations: mechanism of the reconstructive transformation of NaCl to the CsCl-type structure.

We perform path sampling molecular dynamics on the pressure-induced reconstructive phase transition from NaCl to CsCl type structure. Unlike the molecular dynamics simulations prior to this work our approach does not drive the process by applying elevated pressure. As a consequence, we are able to observe nucleation events that initiate the successive transformation of the crystal. The competing phases are separated by an interface exhibiting a well-defined structure that propagates through the crystal during phase transition.

Journal Article↗
PubMed1990

Infrared spectra of phospholipid membranes: interfacial dehydration by volatile anesthetics and phase transition.

Fourier-transform infrared attenuated total reflection (ATR) spectroscopy was used to study the effect of volatile anesthetics on fully hydrated dipalmitoylphosphatidylcholine (DPPC) vesicle membranes. The main phase transition was monitored by the change in the C-H2 asymmetric stretching frequencies of the lipid tails. The surface property was analyzed by the changes in the P = O stretching, (CH3)3-N+ stretching of the hydrophilic head, and C = O stretching of the glycerol skeleton. The partial pressures of those agents that decreased the transition temperature 1.0 C degree were halothane 0.75, enflurane 1.90 and CCl4 0.85 kPa. At a 2:1 lipid/anesthetic mole ratio, the polar anesthetics, halothane and enflurane, increased the ratio of (P = O stretching band area)/((CH3)3-N+ stretching band area) by 26.3% and 21.1%, respectively, whereas apolar CCl4 increased it 10.5%. The water molecules bound to the P = O moiety are apparently replaced by the anesthetic molecules. The deconvoluted C = O spectra showed two peaks: free sn-1 that is closer to the lipid core and hydrogen-bonded sn-2 that is closer to the polar head. Addition of halothane and enflurane, but not CCl4, increased the number of peaks to three. The third peak is free sn-2, formed by disrupting hydrogen-bonding to water. Because the temperature-induced spectral change was limited to C-H2 stretching at the main phase transition, the effects of anesthetics on the lipid membrane structure are not identical to temperature elevation. Among anesthetics, the effects of apolar and polar molecules on the interfacial properties are different.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2002

Pion propagation near the QCD chiral phase transition.

We point out that, in analogy with spin waves in antiferromagnets, all parameters describing the real-time propagation of soft pions at temperatures below the QCD chiral phase transition can be expressed in terms of static correlators. This allows, in principle, the determination of the soft pion dispersion relation on the lattice. Using scaling and universality arguments, we determine the critical behavior of the parameters of pion propagation. We predict that, when the critical temperature is approached from below, the pole mass of the pion drops despite the growth of the pion screening mass. This fact is attributed to the decrease of the pion velocity near the phase transition.

Journal Article↗
PubMed2006

Defect evolution in block copolymer thin films via temporal phase transitions.

We study the details of the defect dynamics in thin films of a cylinder-forming polystyrene-block-polybutadiene (SB) diblock copolymer melt. The high temporal resolution of in-situ scanning force microscopy (SFM) uncovers elementary dynamic processes of structural rearrangements on time scales not accessible so far. Short-term interfacial undulations and the formation of transient phases (spheres, perforated lamellae, and lamellae) are observed. We demonstrate that the well-known structural defects are annihilated by short-term phase transitions into what may be considered excited states. These temporary phase transitions are reproduced in simulations based on dynamic self-consistent field theory. We discuss the role of the observed structural evolution in the context of the equilibrium phase behavior in SB thin films.

Journal Article↗
PubMed2005

Phase transition in random catalytic networks.

The notion of (auto)catalytic networks has become a cornerstone in understanding the possibility of a sudden dramatic increase of diversity in biological evolution as well as in the evolution of social and economical systems. Here we study catalytic random networks with respect to the final outcome diversity of products. We show that an analytical treatment of this long-standing problem is possible by mapping the problem onto a set of nonlinear recurrence equations. The solution of these equations shows a crucial dependence of the final number of products on the initial number of products and the density of catalytic production rules. For a fixed density of rules we can demonstrate the existence of a phase transition from a practically unpopulated regime to a fully populated and diverse one. The order parameter is the number of final products. We are able to fully understand the origin of this phase transition as a crossover from one set of solutions from a quadratic equation to the other. We observe a remarkable similarity of the solution of the system and the PVT diagrams in standard thermodynamics.

Algorithms↗
PubMed2005

Phase transition with the Berezinskii-Kosterlitz-Thouless singularity in the Ising model on a growing network.

We consider the ferromagnetic Ising model on a highly inhomogeneous network created by a growth process. We find that the phase transition in this system is characterized by the Berezinskii-Kosterlitz-Thouless singularity, although critical fluctuations are absent and the mean-field description is exact. Below this infinite order transition, the magnetization behaves as exp((-const/square root of(Tc-T)). We show that the critical point separates the phase with the power-law distribution of the linear response to a local field and the phase where this distribution rapidly decreases. We suggest that this phase transition occurs in a wide range of cooperative models with a strong infinite-range inhomogeneity.

Journal Article↗