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PubMed2002

Novel dynamical effects and persistent memory in phase separated manganites.

The time dependent response of the magnetic and transport properties of Fe-doped phase separated (PS) manganite La(0.5)Ca(0.5)MnO3 is reported. The nontrivial coexistence of ferromagnetic (FM) and non-FM regions induces a slow dynamics which leads to time relaxation and cooling rate dependence within the PS regime. This dynamics influences physical properties drastically. On one hand, metalliclike behavior, assumed to be a fingerprint of percolation, can be also observed before the FM phase percolates as a result of dynamical contributions. On the other hand, two novel effects for the manganites are reported, namely, the rejuvenation of the resistivity after aging and a persistent memory of low magnetic fields (<1 T), imprinted in the amount of the FM phase.

Journal Article↗

PubMed2002

Time-modulated oscillatory structures in phase-separating reactive mixtures.

We investigate the effect of temporal modulation on the spatiotemporal patterns produced by phase separation in chemically reactive ternary mixtures. The temporal modulation is introduced by making one of the reaction rates periodic in time. Our main concern is the stability of traveling waves, which appear above a Hopf-type bifurcation at a finite wave number and are stable in the absence of external modulation. It is shown by computer simulations in two dimensions that when the external modulation is present, various types of coherent standing waves emerge near the special point in the parameter space, where the Turing-type bifurcation line and the Hopf-type bifurcation line meet each other. We carry out a theoretical analysis to understand the phase diagram of the synchronized oscillatory structures obtained numerically.

Journal Article↗

PubMed1989

Theory of protein-induced lateral phase separation in lipid membranes.

An account is given of the current status of theoretical modeling of the phase equilibria in lipid membranes with intrinsic proteins. Special attention is paid to the description of lateral phase separation, which is important for membrane function since it may lead to biologically differentiated regions. We discuss in particular the mattress-model approach by Mouritsen and Bloom, who take matching between protein and lipid hydrophobic thicknesses as a determining factor for the phase behavior. The model has been developed in the framework of phenomenological thermodynamic solution theory. The predictions of the theory are compared to a variety of experimental measurements, including those of membrane recombinants of the protein content of the reaction center and antenna protein of the bacterial photosynthetic apparatus as well as the erythrocyte band 3 protein. The physical effects of lipid-protein interactions are contrasted to those of lipid-cholesterol interactions. The concept of hydrophobic matching is then used as a basis for discussing a possible relationship between membrane thickness and physiological function.

Bacteria, Aerobic↗

PubMed2001

Kinetics of phase separation in ternary mixtures.

We present detailed results from Monte Carlo simulations of the kinetics of phase separation in ternary mixtures. We focus on the case of ABV mixtures (where V denotes a vacancy) and investigate segregation kinetics resulting from V-mediated dynamics. We provide heuristic arguments for the existence of different morphologies in various parameter regimes. Furthermore, we present comprehensive numerical results for various characteristic features of the domain growth process, e.g., real-space correlation functions, domain-size distribution functions, and growth laws.

Journal Article↗

PubMed2005

Universal aspects of coulomb-frustrated phase separation.

We study the consequences of Coulomb interactions on a system undergoing a putative first order phase transition. In two dimensions (2D), near the critical density, the system is universally unstable to the formation of new intermediate phases, which we call "electronic microemulsion phases," which consist of an intermediate scale mixture of regions of the two competing phases. A corollary is that there can be no direct transition as a function of density from a 2D Wigner crystal to a uniform electron liquid. In 3D, if the strength of the Coulomb interactions exceeds a critical value, no phase separation occurs, while for the weaker Coulomb strength electronic microemulsions are inevitable. This tendency is considerably more pronounced in anisotropic (quasi-2D or quasi-1D) systems, where a devil's staircase of transitions is possible.

Journal Article↗

PubMed2003

Nifedipine loaded chitosan microspheres prepared by emulsification phase-separation.

A high yield of nifedipine-chitosan microspheres could be obtained using an emulsification phase-separation method. A high level of entrapment of nifedipine in the microspheres was achieved. The microspheres exhibited excellent swelling properties. Differential scanning calorimetry, X-ray diffractometry, and scanning electron microscopy confirmed that at 1.84% loading, nifedipine was dispersed molecularly. The microspheres exhibited faster release at low loadings compared to high loadings. Fitting the data to the coupled Fickian/case II equation, showed that at low loadings polymer relaxation coefficients (k2) were high. As the polymer content increased in the microspheres, the value of n (diffusional exponent characteristic of the release mechanism) approached one, which is indicative of zero order.

Chitin↗

PubMed2002

Surface-directed phase separation with off-critical composition: analytical and numerical results.

We study the interplay of wetting and phase separation in an unstable binary mixture (AB) with off-critical composition, placed in contact with a surface which prefers the component A. We consider surface potentials V(z) approximately z(-n), where z is the distance from the surface, and present analytical arguments and detailed numerical results to elucidate wetting-layer kinetics for arbitrary mixture compositions. If the preferred component is the minority phase, the wetting-layer thickness exhibits a potential-specific behavior at early times tau, R1 approximately tau(1/(n+2)), before crossing over to the universal growth law, R1 approximately tau(1/3). On the other hand, if the preferred component is the majority phase, there is a crossover from potential-specific growth (as before) to a slower growth regime.

Journal Article↗

PubMed1989

Phase separation in bimolecular mixed lipid membranes induced by polylysine.

We demonstrate, for the first time, polylysine-induced phase separation in a bimolecular lipid membrane of a lecithin/phosphatidylglycerol-mixture by analysing the single channel current fluctuations of gramicidin. The bimodal conductance histograms are direct evidence for the incorporation of the transport system into the two coexisting phases of different composition.

Dimyristoylphosphatidylcholine↗

PubMed1991

Phase separation of Triton X-100 micelle solution induced by osmotic stress.

We have found out that the phase separation of Triton X-100 micelle solution was caused by the addition of poly(ethylene glycol) (PEG) above a critical concentration. The critical concentration of PEG depended on its molecular weight and temperature, larger molecular weight or higher temperature giving lower critical concentration. These results were analyzed on the basis of the osmoelastic coupling theory recently proposed by us (Biochemistry (1989) 28, 3710-3715; Biochemistry (1989) 28, 5626-5630).

Chemical Phenomena↗

PubMed2005

Phase separation driven by surface diffusion: a Monte Carlo study.

We propose a kinetic Ising model to study phase separation driven by surface diffusion. This model is referred to as Model S, and consists of the usual Kawasaki spin-exchange kinetics (Model B) in conjunction with a kinetic constraint. We use multispin coding techniques to develop fast algorithms for Monte Carlo simulations of Models B and S. We use these algorithms to study the late stages of pattern dynamics in these systems, and compare properties of the evolution morphologies, e.g., growth laws, domain distribution functions, and spatial and temporal correlation functions.

Journal Article↗

PubMed2005

Current oscillation and low-field colossal magnetoresistance effect in phase-separated manganites.

Current-induced switching from a metallic to an insulating state is observed in phase-separated states of (La(1-y)Pr(y))0.7Ca0.3MnO3 (y=0.7) and Nd(0.5)Ca(0.5)Mn(1-z)Cr(z)O3 (z=0.03) crystals. The application of magnetic fields to this current-induced insulating state causes a pronounced low-field negative magnetoresistance effect [rho(H)/rho(0)=10(-3) at H=1 kOe]. The application of a constant voltage also causes the breakdown of the Ohmic relation above a threshold voltage. At voltages higher than this threshold value, oscillations in currents are observed. This oscillation is well reproduced by a simple model of local switching of a percolative conduction path.

Journal Article↗

PubMed2005

Kinetics of phase separation in ramified polymer blends of arbitrary topology.

We present here a theoretical study of kinetics of phase separation within a mixture made of two chemically incompatible ramified polymers. For simplicity, we assume that they have the same topology. We are interested in the variation of the relaxation rate, tau(q), versus the wave number q, in the vicinity of the spinodal temperature. The kinetics is governed by local (Rouse) and reptation motions (faster and slower modes). For qRG >> 1 (RG being the gyration radius), kinetics is entirely controlled by local motions where each chain moves inside its own tube, and we show that the corresponding characteristic frequency, tau(q)(-1), scales as tau(q)(-1) approximately kappaGq6, where kappaG is a known topological factor. For qR(G) << 1, however, kinetics is rather dominated by long-wavelength (reptation) motions where unlike ramified polymers creep inside a long tube. For this case, we find that tau(q)(-1) approximately delta (0) q2 (chi(c) - chi), where delta (0) is another known topological factor that represents the total mobility of free monomers belonging to connected chains and reticulation points, and chi(c) accounts for the critical value of the segregation parameter. Finally, the derived relaxation rate must be compared to that relative to a linear polymer mixture.

Journal Article↗

PubMed1993

Monte Carlo simulation of lipid mixtures: finding phase separation.

The nonideal mixing of phosphatidylserine (PS) and phosphatidylcholine (PC) binary lipid mixtures was studied by computer simulation based on a model wherein the excess energy of mixing is divided between an electrostatic term and one adjustable term delta Em that includes all other nonideal interactions. The lateral distribution of the lipids and the energy of the mixtures were obtained by using Kawasaki relaxation in a canonical ensemble. The Gibbs free energies were calculated by Kirkwood's coupling parameter method. The simulation results are strongly dependent on simulation size for sizes smaller than about 1000 lipids. Nonideal interaction between lipids can result in large scale separation of lipid phases of different composition at reasonable delta Em values as well as clustering of like lipids. In plots of total Gibbs free energy of mixing versus PS mole fraction in PS/PC, the boundaries of the two phase region could be accurately determined. The electrostatic interaction influences cluster size and shape, and also the composition of phases in the two-phase region.

Biophysical Phenomena↗

PubMed2000

Chemical speciation of nickel in fly ash by phase separation and carbon paste electrode voltammetry

In a risk-based approach to cost-effective management of power plant emissions, chemical speciation of fly ash is required. A quantitative but indirect separation of nickel phases by sequential extraction was undertaken in conjunction with direct analysis of the sulfidic nickel phase by carbon paste electrode voltammetry (CPEV). Four ash samples produced in a laboratory combustion system from burning high- and low-sulfur residual oil at excess O(2) of 1 and 2-3 mol% were studied. The extractions yielded five phases of nickel. The CPEV analysis of sulfidic nickel in the ash and extraction residues was performed in pH 5 acetate. The anodic peak (-0.1 V) unique to Ni(3)S(2), distinguishing it from NiS and NiS(2), was found to be absent from these ash samples. The CPEV method was consistent with phase extraction, which showed NiSO(4) being predominant with very low proportions of sulfidic nickel.

Journal Article↗

PubMed2005

Pairing and phase separation in a polarized Fermi gas.

We report the observation of pairing in a gas of atomic fermions with unequal numbers of two components. Beyond a critical polarization, the gas separates into a phase that is consistent with a superfluid paired core surrounded by a shell of normal unpaired fermions. The critical polarization diminishes with decreasing attractive interaction. For near-zero polarization, we measured the parameter beta = -0.54 +/- 0.05, describing the universal energy of a strongly interacting paired Fermi gas, and found good agreement with recent theory. These results are relevant to predictions of exotic new phases of quark matter and of strongly magnetized superconductors.

Journal Article↗

PubMed2001

Performances of holographic gratings monitored by laser-induced phase separation in liquid mixtures.

We theoretically describe and experimentally explore the kinetics of holographic grating formation resulting from different laser-induced phase separation mechanisms. Our method makes use of two interfering c.w. laser waves to quench binary mixtures in composition, and to optically trap the nucleated domains on the fringes. Essentially, two different processes can lead to these variations in concentration: electrostriction and thermodiffusion. The former originates from induced dipolar couplings in a field gradient; as photopolymerization, this is a local process which is essentially sensitive to the q=q(0) Fourier mode forced by the fringe modulation. The latter corresponds to a variation in composition driven by a small thermal gradient; as solvent evaporation and thermal heating techniques, it is nonlocal and behaves as 1/q(2) because of its dissipative origin. By making experiments in both cases, we show that this q dependence on excitation has a strong influence on the performance of holographic gratings. While in the first case reflectivity saturates because the phase transition is confined by the fringes which behave as separated optical boxes with "soft walls" which calibrate the droplet size, blurring is expected for fringe-trapped domains induced by a nonlocal phase transition because the transition is governed by the Gaussian shape of the pump beams, and nucleated domains can reach a much larger size than the fringe spacing. The good agreement observed with our general model clearly illustrates how to make the difference between local and nonlocal excitations, and offers a first step towards a unified description of holographic grating formation monitored by phase transitions.

Biophysics↗

PubMed1999

Phase separation of rigid-rod suspensions in shear flow.

We analyze the behavior of a suspension of rigid-rod-like particles in shear flow using a modified version of the Doi model, and construct diagrams for phase coexistence under conditions of constant imposed stress and constant imposed strain rate, among paranematic, flow-aligning nematic, and log-rolling nematic states. We calculate the effective constitutive relations that would be measured through the regime of phase separation into shear bands. We calculate phase coexistence by examining the stability of interfacial steady states and find a wide range of possible "phase" behaviors.

Journal Article↗

PubMed1981

Geometry of phase-separated domains in phospholipid bilayers by diffraction-contrast electron microscopy.

The sizes and shapes of solidus (gel) phase domains in the hydrated molecular bilayers of dilauroylphosphatidylcholine/dipalmitoylphasphatidylcholine (DLPC/DPPC) (1:1) and phosphatidylserine (PS)/DPPC (1:2) are visualized directly by low dose diffraction-contrast electron microscopy. The temperature and humidity of the bilayers are controlled by an environmental chamber set in an electron microscope. The contrast between crystalline domains is enhanced by electron optical filtering of the diffraction patterns of the bilayers. The domains are seen as a patchwork in the plane of the bilayer, with an average width of 0.2-0.5 micrometer. The percentage of solidus area measured from diffraction-contrast micrographs at various temperatures agrees in general with those depicted by known phase diagrams. The shape and size of the domains resemble those seen by freeze-fracture in multilamellar vesicles. Temperature-related changes in domain size and in phase boundary per unit area are more pronounced in the less miscible DLPC/DPPC mixture. No significant change in these geometric parameters with temperature is found in the PS/DPPC mixture. Mapping domains by their molecular diffraction signals not only verifies the existance of areas of different molecular packing during phase separation but also provides a quantitative measurement of structural boundaries and defects in lipid bilayers.

Biophysical Phenomena↗