ATOMIC FORCE MICROSCOPY STUDIES AND MODELING OF SURFACE STRUCTURES PATTERNED DURING OSTWALD RIPENING AT Fe/Mo MULTILAYER SYSTEMS

Fe/Mo multilayers which were grown by sputtering, annealed in high vacuum and analyzed by means of Atomic Force Microscopy and Scanning Electron Microscopy. Concentric circle-1ike patterns were observed after annealing. Two-dimensional Ostwald ripening mechanisms in immiscible systems of Fe/Mo may explain the formation of these structures. We simulated pattern formation in a late stage of the phase separation by applying the Thomas-Freundlich thermodynamic relation. Based on a two-dimensional mode1 in the framework of the Lifshitz-Slyozov theory, our modeling has been extended to include the diffusion limitation in a multi-cluster system.     

多组分磷脂巨囊泡的相结构和胆固醇对微畴成长的影响

摘要：巨囊泡作为细胞的简化模型,其分相与出芽机理及动力学规律已引起许多领域科学家的关注。在富含胆固醇的典型生物膜体系如二棕榈酰磷脂酰胆碱DPPC(2-dihexadecanoyl-rac-glycero-3phosphocholine)/二油酰磷脂酰胆碱DOPC(dioleoyl-phosphatidylcholine)/胆固醇(Chol)的三组分形成的巨囊泡作为模型,从高温退火至低温会发生相分离,形成微畴。实验中借助荧光显微镜观察生物膜体系侧向分离的相结构图。实验发现,体系各组分的不同会影响磷脂膜的相结构和膜内微畴的成长,固定 DOPC/DPPC为1:1的前提下,微畴尺寸随着胆固醇参入量的增加而变大。最后运用理论进一步分析了微畴的成长机理。     

多组分磷脂巨囊泡的相结构和胆固醇对微畴成长的影响

巨囊泡作为细胞的简化模型,其相分离与出芽动力学规律已引起许多领域科学家的关注.本实验采用DPPC/DOPC/Chol的三组分形成的巨囊泡作为模型,借助荧光显微镜观察该三组分体系侧向分离的相结构图,并对微畴的成长过程作了系统的观察研究和理论分析.实验发现:从高温的均相区域淬灭到低温的分相区域,膜表面发生侧向分离形成微畴.体系内胆固醇的掺入量的多少会影响磷脂膜的相结构和膜内微畴的成长,固定DOPC/DPPC为1:1的前提下,微畴尺寸随着胆固醇掺入量的增加而变大.     

Modeling phase decomposition and patterning in binary alloy systems subjected to neutron irradiation

We study processes of phase decomposition and patterning in a model of a binary alloy system subjected to sustained irradiation. We exploit the reaction rate theory and generalize the Darken approach of vacancy diffusion to describe generation, recombination, annihilation and spatial interaction of point defects. It is shown that an increase in the defect production rate phase, decomposition processes are replaced by disordering and patterning with vacancy clusters' formation. At elevated damage rates, both phase separation and patterning are accompanied by pattern selection processes. In the framework of numerical simulations, dynamics of phase decomposition and vacancy clusters formation is studied in detail. A change in the morphology of vacancy clusters during irradiation and their statistical properties are discussed.     

Viscoelastic phase separation in polymer blends

In this paper, the dynamics and morphology of viscoelastic phase separation in polymer blends is investigated based on the two-fluid model in two dimensions. At critical composition, we have carefully checked the role of shear modulus, without taking account of bulk modulus. The results show that the higher shear modulus component tends to form a dispersed phase in the intermediate stage of phase separation, if the difference between the shear moduli of the components is large enough. This is opposite to the role of bulk modulus, that the higher bulk modulus component forms a networklike pattern without taking account of the shear modulus even if it is the minority phase. The morphological formation is determined by the competition of opposite effects of shear modulus and bulk modulus. For polymer blends at critical composition, the bulk modulus difference leads to a networklike pattern formed by the higher modulus component in the intermediate stage of phase separation. But if the difference between the shear moduli of the components is large enough, a co-continuous structure is observed, resulting from the competition between shear and bulk moduli. For off-critical composition, difference in bulk modulus also leads to a networklike pattern of the component with higher bulk modulus in the intermediate stage of phase separation, but phase inversion is observed rapidly. A small difference between the shear moduli of the components can support the networklike pattern to continue for longer time. But the networklike pattern does not occur for large difference between shear moduli.Received: 9 September 2004, Published online: 10 November 2004PACS: 64.75. + g Solubility, segregation, and mixing; phase separation - 83.80.Tc Polymer blends     

Variable reactivity and phase separation in patchy particle systems

ABSTRACT

Using statistical model, we study mechanisms of phase separation in a solution consisting of patchy particles, which are capable to form directed and saturated thermoreversible bonds. We focus on the impact of variable reactivity of patchy particles on the form of miscibility gap. We show that the variation of model parameters determining features of interparticle interaction makes it possible to obtain miscibility gaps of different types within the unified formalism. In particular, we uncover two different mechanisms of the formation of phase separation curves with lower critical solution temperature. The first mechanism is realised in the case of positive bonding energy; the second one can takes place when the energy of formation of two-bonded particles is lower than that for all other m-bonded ones. We conclude that the most interesting and non-trivial phase behavior is observed in the case of patchy particles with variable reactivity. Using rigorous statistical model, we uncover new mechanisms of phase separation in a solution consisting of patchy particles, which are capable to form directed and saturated thermoreversible bonds. This topic corresponds to state of the art in modern chemical physics. The results obtained shed light on interplay between features of non-isotropic interactions and phase behavior in both molecular and nanoparticle systems. We conclude that the most interesting and non-trivial phase behavior is observed in the case of patchy particles with variable reactivity.     

Smoothing of an atomically rough vicinal surface – STM observation and MC simulation

Smoothing of an atomically rough vicinal surface of SrTiO₃ is studied by scanning tunneling microscope (STM) observation and by Monte Carlo (MC) simulation. A complex step pattern that resembles a two-dimensional phase separation pattern is observed on the surface. Analysis of the step pattern during annealing obtained by the STM in comparison with the MC simulation reveals an asymmetry of the relaxation pattern between islands and holes. The asymmetry is attributed to the difference of the mobility of an adatom and an atomic hole, and the asymmetry is enhanced by the step edge diffusion barrier. Values of an effective bond energy and an effective diffusion barrier as well as the surface diffusion coefficient are deduced from the relaxation pattern.     

Simulation of diffusion and phase formation during isothermal annealing of lamellar Fe-Zr systems

Results of the simulation of diffusion and phase formation in lamellar Fe-Zr systems subject to sequential isothermal annealing are presented. The simulation is performed in the framework of a physical model whose basis is Darken’s phenomenological theory and the mechanism of mutual diffusion of components along continuous phase channels in two-phase regions of the system. The results of simulation agree well with the data obtained using Mossbauer spectroscopy and X-ray diffractometry; this indicates that the character of phase transformations in the studied lamellar systems is determined by the change of local component concentration in the course of mutual diffusion and corresponds to the specific features of phase diagram of equilibrium states of the binary system.     

Switch and template pattern formation in a discrete reaction-diffusion system inspired by the Drosophila eye

We examine a spatially discrete reaction-diffusion model based on the interactions that create a periodic pattern in the Drosophila eye imaginal disc. This model is known to be capable of generating a regular hexagonal pattern of gene expression behind a moving front, as observed in the fly system. In order to better understand the novel “switch and template” mechanism behind this pattern formation, we present here a detailed study of the model's behavior in one dimension, using a combination of analytic methods and numerical searches of parameter space. We find that patterns are created robustly, provided that there is an appropriate separation of timescales and that self-activation is sufficiently strong, and we derive expressions in this limit for the front speed and the pattern wavelength. Moving fronts in pattern-forming systems near an initial linear instability generically select a unique pattern, but our model operates in a strongly nonlinear regime where the final pattern depends on the initial conditions as well as on parameter values. Our work highlights the important role that cellularization and cell-autonomous feedback can play in biological pattern formation.     

A comparative simulation study on three lattice systems for the phase separation of polymer-dispersed liquid crystals

This article reports a comparative study of the phase separation process in a polymer-dispersed liquid crystal, based on a Metropolis Monte Carlo simulation study of three lattice systems. We propose a model for the different processes occurring in the formation of polymer-dispersed liquid crystals (PDLCs). The mechanism of PDLC is studied as a function of quench temperature, concentration and degree of polymerization of liquid crystals and polymers. The obtained resultant phase diagrams of the three systems are approximated and compared with the Flory-Huggins theory, and show a good agreement. It has been observed in the simulation results that among all the three systems, the 40 × 40 × 40 lattice showed the most accurate, reliable and stable results.      

Lateral diffusion in equimolar mixtures of natural sphingomyelins with dioleoylphosphatidylcholine

Cellular membranes of mammals are composed of a complex assembly of diverse phospholipids. Sphingomyelin (SM) and phosphatidylcholine (PC) are important lipids of eukaryotic cellular membranes and neuronal tissues, and presumably participate in the formation of membrane domains, known as "rafts," through intermolecular interaction and lateral microphase decomposition. In these two-dimensional membrane systems, lateral diffusion of lipids is an essential dynamic factor, which might even be indicative of lipid phase separation process. Here, we used pulsed field gradient nuclear magnetic resonance to study lateral diffusion of lipid components in macroscopically oriented bilayers composed of equimolar mixtures of natural SMs of egg yolk, bovine brain, bovine milk and dipalmitoylphosphatidylcholine (DPPC) with dioleoylphosphatidylcholine (DOPC). In addition, differential scanning calorimetry was used as a complementary technique to characterize the phase state of the lipid bilayers. In fully liquid bilayers, the lateral diffusion coefficients in both DOPC/DPPC and DOPC/SM systems exhibit mean values of the pure bilayers. For DOPC/SM bilayer system, this behavior can be explained by a model where most SM molecules form short-lived lateral domains with preferential SM-SM interactions occurring within them. However, for bilayers in the presence of their low-temperature gel phase, lateral diffusion becomes complicated and cannot simply be understood solely by a simple change in the liquid phase decomposition.     

Fermi surface and band mapping of the cerium/palladium surface alloy

X-ray and UV excitation angle-resolved photoemission spectroscopy of ultra-thin films of cerium deposited on Pd(1 1 1) single-crystal surface has been carried out. Photoelectron diffraction pattern showed that deposition of 1 ML of Ce led to a formation of Ce-Pd substitutional alloy. Valence band spectra measured with high angular resolution permitted to plot valence band maps and Fermi surface scans and showed formation of surface alloy exhibiting d- and f-electron orbital hybridization. A shift of Pd 4d-derived states to higher binding energy in the Ce-Pd systems was observed.     

Statistical description of the sol-gel transition in systems with thermoreversible chemical bonds

A general statistical model is proposed for describing network-forming systems. The model is based on the representation of the partition function for all possible configurations of a thermoreversible network in the form of a functional integral over a scalar field. According to this model, two types of first-order phase transitions can occur in the systems under consideration: macroscopic phase separation with the structural phase transition due to the change in the configuration of the spatial network and the sol-gel transition due to the formation of a thermoreversible percolation cluster consisting of bound structural units. A detailed analysis is performed of the thermodynamic and structural properties of a solution of monomers that have f functional groups and can form thermoreversible chemical bonds. The influence of specific features of the chemical and volume interactions on the phase diagram of the system is investigated. The mutual position of the sol-gel transition line and the phase diagram is determined for different model parameters. It is revealed that two substantially different regimes of the behavior of the sol-gel transition line in the “temperature-volume fraction of structural units” plane are observed with a change in the rigidity of chemical bonds.     

Forces that Drive Nanoscale Self-assembly on Solid Surfaces

Experimental evidence has accumulated in the recent decade that nanoscale patterns can self-assemble on solid surfaces. A two-component monolayer grown on a solid surface may separate into distinct phases. Sometimes the phases select sizes about 10 nm, and order into an array of stripes or disks. This paper reviews a model that accounts for these behaviors. Attention is focused on thermodynamic forces that drive the self-assembly. A double-welled, composition-dependent free energy drives phase separation. The phase boundary energy drives phase coarsening. The concentration-dependent surface stress drives phase refining. It is the competition between the coarsening and the refining that leads to size selection and spatial ordering. These thermodynamic forces are embodied in a nonlinear diffusion equation. Numerical simulations reveal rich dynamics of the pattern formation process. It is relatively fast for the phases to separate and select a uniform size, but exceedingly slow to order over a long distance, unless the symmetry is suitably broken.     

SIMS studies of polymeric tertiary structures in monolayers: Polysiloxane helical coil structures

A static SIMS study was performed using poly(dimethyl siloxane) (PDMS) as a model system to investigate the effects of tertiary structure on the ion formation mechanism and ion formation probability of fragments in the high mass region (>1000 Da). PDMS produced from anionic polymerization with a narrow polydispersity, can form well-ordered helically coiled monolayers using Langmuir-Blodgett methods at the air water interface, either in a hairpin configuration, if the endgroups are functional or as flat helically coiled chains. Reflection absorption Fourier transform infrared (RA-FTIR) spectroscopic analysis shows the changes in bonding between flat helically coiled PDMS and cast films from dilute solvents, which produces a random coil configuration. Differences between the model systems in infrared spectrum show evidence of changes in structure, producing variation in band formation. Also observed are changes in the band shape and relative peak area between the model systems. These polymeric changes translate into differences in the relative intensities of fragments formed in the repeating pattern of clusters in the high mass ToF-SIMS spectra of the ordered versus the random cast films. Both statistical chain break analysis and molecular mechanics simulations of structure are used to support the analysis.     

X-ray photoelectron studies of changes in the electronic structure upon phase transitions in Co–Ni and Co–Fe alloys

The changes in the electronic structure of Co–Ni and Co–Fe systems upon phase transitions are studied. X-ray photoelectron study of the valence-band spectra and the parameters of the multiplet splitting of Co, Ni and Fe 3s spectra is carried out at different temperatures. It is established that the ordering–separation phase transition in Co–Ni alloys takes place in the temperature range of 600–700°C. As opposed to Co–Ni alloys, in the Fe–Co alloy, ordering–separation–ordering phase transitions are observed. High-temperature ordering of the Fe₅₀Co₅₀ alloy is observed above 1200°C. The transition from ordering to separation is shown to lead to changes in the d electron spectra of the valence band and in the parameters of the multiplet splitting of the 3s spectra.     

Simulation of thermally-induced processes of diffusion and phase formation in layered metal-metalloid systems

A physical model of thermally induced processes of diffusion and phase formation in layered metal-metalloid systems is offered. The software for the model is created, which allows one to quantitatively describe the kinetics of these processes under arbitrary modes of annealing. The results of theoretical computations are compared with the data of experimental studies of a ⁵⁷Fe:O⁺ layered implantation system using the methods of M?ssbauer spectroscopy. The agreement between the numerical and experimental results indicates that the nature of phase transformations in the investigated system is determined by a change in the local concentration of the metalloid during its interstitial diffusion and corresponds to peculiarities of the phase equilibrium diagram of the Fe-O binary system.     

Phase separation in binary systems with competing internal and external multiplicative noises

We have considered phase separation processes in binary stochastic systems with thermal diffusion and ballistic mixing representing irradiation influence. Introducing fluctuations of thermal flux and an external source of atom relocations due to ballistic diffusion into dynamics of a globally conserved field, we have shown that there are two competing mechanisms of phase transitions type of “order-disorder”: thermally assisted diffusion and irradiation induced atomic exchange. We have studied dynamics of the structure function at early stages of decomposition. In the framework of the mean field theory we have derived the effective Fokker-Planck equation to describe phase separation processes. It was shown that the ordering processes can be controlled by both regular and stochastic parts of external source influence. A reentrant behavior of a mean field order parameter versus the external noise intensity and fluctuations correlation radius is found.     

Unusual Phase Separation Kinetics in Poly(Methyl Methacrylate)/Poly(Styrene-co-Acrylonitrile) (PMMA/SAN) Blends with PMMA of Different Molecular Weights

The influence of molecular weight of poly (methyl methacrylate) (PMMA) on the thermodynamics and dynamics of phase separation in PMMA/poly (styrene-co-acrylonitrile) (SAN) blends was investigated via optical microscopy, time-resolved small-angle light scattering (SALS), and dynamic rheological measurements. It was found that the cloud point temperature of the blends decreased with an increase in the molecular weight of the PMMA. The phase separation rates of PMMA 48K/SAN and PMMA 85K/SAN blends with the near-critical composition were almost the same at small quench depths due to the limited mobility of molecular chains at low temperatures. However, an unexpected phase separation dynamics was observed at larger quench depths. Not only the morphology evolution but also the apparent diffusion coefficient D_app calculated from SALS revealed that the phase separation rate was faster in the PMMA 85K/SAN blend than in the PMMA 48K/SAN blend. The possible reasons for this unusual rapid kinetics of phase separation observed in the higher molecular weight blend were discussed in terms of molecular mobility and viscoelasticity.     

Reaction-Induced Phase Separation and Structure Formation in Polymer Blends

The peculiarities of reaction-induced phase separation and the structure formation in semi- and full interpenetrating polymer networks and in the blends of linear polymers formed in situ are analyzed. It is shown that for most of these systems phase separation proceeds viathe spinodal decomposition mechanism resulting in the formation of interconnected spatially periodic structures. The possible ways for the structure regulation of the composites produced are considered.