Bulk Phase Separation Research Articles

Observation of Bulk Phase Separation and Coexistence in a Sheared Micellar Solution

We observe a shear-flow-induced phase transition in wormlike micelle solutions which depends on whether the applied stress or the applied shear rate is held constant. When the shear stress is held constant, the system macroscopically phase separates into two coexisting phases with a single stable interface between them. By contrast, when the shear rate is held constant, the system still undergoes a phase transition, but the steady states are homogeneous and no coexistence is observed. This leads to dramatically different rheological behavior and unusual long-lived metastable states.

Effect of EO Chain Length of Dodecanol Ethoxylates (C12En) on the Complexation of C12En/SDS Mixed Micelles with an Oppositely Charged Polyelectrolyte

Turbidimetry, dynamic light scattering, and capillary electrophoresis were used to study the complexation of polydiallyldimethylammonium chloride (PDADMAC) with mixed micelles of sodium dodecyl sulfate (SDS) and dodecanol ethoxylates (C12En). The effect of EO chain length and its distribution was examined using various combinations of C12En(n= 4, 6, 8, 12). The results show that the onset of the complexation of PDADMAC with the mixed micelles is affected by the EO chain length of C12En: the mole fraction (Y) of SDS in the mixed micelle required to form the complex (Yc) increases withn. The effect of EO chain length on the onset of bulk phase separation shows the same trend. AlthoughYcvaries withn, the electrophoretic mobility of mixed micelles with composition corresponding toYcis independent ofn. We propose that the effect of EO chain length has two aspects: (1) an increase in the average distance between bound polycation segments and the SDS sulfate groups, and (2) an increase in the distance between SDS head groups, which causes a decrease in the surface charge density (σ) of the micelle. Therefore, the electrical potential at the mean locus of bound polymer segments, ψ0, decreases with increasingn; in order for complexation to occur, this effect must be compensated for by a larger value ofY. Broader distributions of EO chain length lead to an increase in the range ofYover which the soluble complex is stable. We suggest that polycations initially bind to micelles which are rich in shorter EO chains and thus have higher “surface” potential, ψ0. However, additional SDS may go preferentially into micelles rich in longer chains with lower ψ0. This delays the formation of micelles which have sufficiently large ψ0to cause phase separation.

Partial Compatibility and the Formation of Thickened, Shrink-Resistant Thermoset Moulding Compounds

This paper describes a new class of thermosetting polymer blends. The blends are now used commercially in the manufacture of sheet moulding compounds (SMC) where they exhibit important advantages compared with conventional systems. The new compositions comprise a cross-linkable base resin and unsaturated monomer in which the base resin is dissolved and a saturated additive resin which has a crystalline melting temperature T m below the temperature T r at which cross-linking occurs. Between T r and T m the additive and base resin form a single liquid phase, which on cooling thickens reversibly to a leathery sheet with no bulk phase separation. The paper describes the mechanisms underlying this viscosity switch phenomenon and the partial compatibility criterion used to select the resin-additive combinations which exhibit this behaviour.

A new CSLM-based method for determination of the phase behaviour of aqueous mixtures of biopolymers

On mixing different types of high molecular weight (bio)polymers in an aqueous solution, phase separation often occurs. In some cases, the occurrence of phase separation may be readily observed, because due to density differences the heavier of the two phases is accumulated at the bottom of the vessel in which the mixture is contained. By using classical techniques, the composition of the two phases may then be determined. In the case where the density differences are not so large, and the viscosity of the system is high, the two phases remain intimately mixed. An alternative route to determine the phase behaviour of these systems might be a microscopic technique (Confocal Scanning Laser Microscopy, CSLM), using the fluorescence intensity of labelled biopolymers to quantify their concentration and phase volume in the system. Experiments were performed with several mixtures of sodium alginate, labelled with fluorescein, and sodium caseinate, fluorescently labelled with Texas Red. The viscosity of the mixtures studied was low enough to allow bulk phase separation of the phases by using an ultracentrifuge. Results of the phase volumes, and the composition of the phases, obtained independently by applying the two different methods (CSLM, or analysis of the separate phases after centrifugation) were compared and found to be in reasonable agreement.

A Recent Advance In The Application of Blending Laws in Mixed Biopolymer Systems

Abstract A survey of the literature clearly indicates the potential of steric exclusion phenomena between proteins and polysaccharides in enhancing the functional properties of food/non-food products. However, in spite of the range and depth of research over the last fifty years, even the most recent reviewers accept that there are still very significant areas of obscurity in our understanding of how phase-separated proteins and polysaccharides contribute to the texture of industrial products. Recently, some progress has been made in the understanding of how biphasic gels behave in terms of phase continuity, phase inversion and, above all, solvent distribution between the two phases. It is based on the assumption that either bulk phase separation to equilibrium takes place first with gelation then occurring subsequently and independently in each phase or the fastest gelling component does so prior to the establishment of a true thermodynamic equilibrium with subsequent gelation of the second, slower gelli...

Comparison of the Effect of a Hydrocarbon Oil and a Terpene Tackifier on the Autohesion of Styrene-Butadiene Rubber

Abstract Blends of uncrosslinked styrene-butadiene rubber (SBR) with a terpene tackifier resin or a naphthenic oil have been characterized, and their autohesion and cohesion determined using a T-peel geometry. SBR/oil blends are homogeneous at all proportions, while SBR/resin blends, based on DSC and DMA analysis, undergo bulk phase separation at about 50% resin. However, migration of tackifier to the surface region is proposed at much lower resin contents. Compositions diluted with oil have autohesion similar to the neat SBR. This is attributed to compensating effects; although oil hastens self-bond formation by increasing chain mobility, this is nearly equally balanced by more facile chain separation during bond rupture. In short, oil-diluted compositions are soft and weak. On the other hand, SBR compositions containing small amounts of resin have high autohesion. Resin-diluted specimens deform easily at low strain, just as those containing oil, but intertwined chains of the former have greater resistan...

Double phase separation in a confined, symmetric binary mixture: Interface quench effect unique to bicontinuous phase separation.

It is found that for a geometrically confined, symmetric binary mixture the secondary phase separation is spontaneously induced by the extremely quick reduction of the interface area caused by the hydrodynamics unique to bicontinuous phase separation. This hydrodynamic coarsening is likely too quick for the concentration diffusion to establish the local equilibrium. This phenomenon is generally observed for nearly symmetric binary mixtures confined both in one-dimensional and in two-dimensional capillaries, under deep quench conditions. This interface quench effect and the resulting double phase separation could exist even for bicontinuous phase separation in bulk.

Entropically driven surface phase separation in binary colloidal mixtures.

We investigate the phase diagram of mixtures of charge stabilized polystyrene spheres of two different sizes in the hard-sphere limit. We observe bulk phase separation into two disordered phases and find a new ordered phase located on the cell walls. The ordered phase occurs at volume fractions as low as 0.2, much less than the value of 0.49 for hard-sphere freezing in monodisperse suspensions, and is qualitatively explained with simple geometric arguments.

Phase separation in the presence of a surface

We have studied the phase separation of a system with a conserved order parameter in the presence of a flat surface via Monte Carlo simulations. For various quench parameters, we observed the domain growth in a surface layer. The kinetics of this ordering was studied, both in the presence and absence of bulk phase separation. In the absence of bulk phase separation, the surface domains were found to follow model A growth kinetics. In the presence of bulk phase separation, a crossover in the surface kinetics between model A and model B dynamics was observed.

Alloy surfaces and surface alloys: from equilibrium to kinetics

We study the relation between the surface segregation at thermodynamical equilibrium in an alloy A c B 1− c and the kinetics of dissolution of A B or B A . This is possible within a new kinetic model based on the electronic structure: the kinetic tight-binding Ising model. In particular, the existence of a local equilibrium between the surface and the first underlayer is stressed. Some illustrations are given in the particular case of two systems, one presenting a tendency to bulk phase separation and surface layering transitions (CuAg) and the other a tendency to bulk ordering and surface concentration profile transitions (PtNi). The notion of “ surface alloy” is defined in the latter case.

Molecular dynamics of phase separation in narrow channels.

The results of molecular-dynamics simulations of the phase separation of binary fluids in narrow channels are presented. Two channel widths allowing for one or two fluids layers are considered. The one-layer case does not exhibit good scaling. The two-layer case does and the growth exponents are the same as that found earlier for bulk phase separation. Our results point to the importance of conservation laws and hydrodynamic modes in determining the growth dynamics.

Phase Separated Microstructure and its Stability in InGaAs Epitaxial Layers Grown by LPE

In 0.53 Ga 0.47 As layers were grown by LPE on (001) InP substrates in the temperature range of 480~780°C. The fine speckle contrast, which is attributed to two-dimensional phase separation, was observed in layers grown at temperatures as high as 780°C. Since the critical temperature for bulk miscibility gap is predicted to be around 450°C, this suggests that in the presence of the surface the critical temperature for the two-dimensional surface decomposition is higher than that for the bulk. The wavelength of the fine modulations does not change with the growth temperature. This could be due to the fast diffusion kinetics in the solid-liquid interface and the balance between the driving force and the gradient energy. To examine the stability of the above microstructures, zinc diffusion experiments were carried out in the temperature range of 390 to 540°C using the ampoule technique. The diffused layers exhibit homogenous microstructures. This demonstrates that critical temperature for phase separation in bulk is below 390°C and is comparable to that predicted by theory.

Mechanisms which can Accelerate the Growth of Phase Separating Domains Near a Wall

AbstractThough the dynamics of bulk phase separation during spinodal decomposition is fairly well understood nowadays, recent experiments have shown that the same process occuring near a wall introduces some surprisingly fast domain growth kinetics. Studies of both polymer mixtures and simple binary fluids at critical composition quenched into the unstable regime reveal the presence of a fast mode associated with domains coarsening at the wall. The scattering data indicates that while the bulk domains grow in time as t1/3, as dictated by usual diffusion driven dynamics, the surface domains coarsen with exponents ranging from about 1.1 to 1.5 depending on the quench depth. We construct a model in which the average size of domains in a binary mixture undergoing spinodal decomposition near a wall can achieve growth exponents much larger than 1/3. The accelerated growth is associated with the domains of the non-wetting phase coarsening anisotropically against a wall coated with the wetting phase.The faster growth evolves from coupling Lifshitz-Slyozov type coarsening for critical volume fractions, modified to include the geometric constraint of growth near a wall, the process of domain coalescence. Included are some predictions for experimental tests of these ideas.

Transmission electron microscopy studies of microstructural evolution, defect structure, and phase transitions in polycrystalline and epitaxial Ti 1− xAl xN and TiN films grown by reactive magnetron sputter deposition

The microstructural evolution, as revealed by plan-view and cross-sectional transmission electron microscopy of polycrystalline NaCl-structure Ti 1− x Al x N ( x≤0.5) and TiN films deposited by reactive magnetron sputter deposition is similar. At low temperatures ( T s ≤500° C) and in the absence of ion bombardment, the film are underdense and exhibit a pronounced columnar morphology. The addition of low energy ( E i ⪅200 eV)ion irradiation during deposition using incident ion-to-neutral ratios J i J n≤1 in mixed Ar N 2 plasmas results in film densification, defect incorporation and a more equiaxed grain structure. At higher incident flux ratios ( J i J n⩾4 ) , low-energy ion irradiation provides increased apparent adatom mobility and, hence, larger grain size. The epitaxial temperature for μm-thick films of both Ti 1− x Al x N( x≤0.5) and TiN, deposited in pure N 2 discharges, on Mg(100) substrates, is ≈ 500°C. The density of {111} dislocation loops n d, the primary defects, was found to depend both on T s and E i. For example n d was decreased by several orders of magnitude using low-energy ion irradiation. However, the use of higher-energy (≥ 300 eV) ion irradiation gave rise to the formation of Ar and/or N 2 gas bubbles due to precipitation of trapped ions. Plastic deformation in the films took place through glide of edge dislocations. Phase transition reaction paths were determined for Ti 1− x Al x N as a function of x and T s. In the case of epitaxial Ti 0.5Al 0.5N, increasing T s resulted in surface-initiated spinodal decomposition during growth in the range T s = 540−560 °C with the formation of compositionally modulated NaCl-structure plateles along [100] as a precursor to bulk phase separation of wurtzite-structure AlN and NaCl-structure TiN at higher T s. In the case of Ti 1− x Al x N grown on oxidized silicon at T s =500 ° C, increasing x resulted in single-phase NaCl-structure alloys for 0⩽ x⪅0.4, two-phase TiN and AlN-structure for 0.4 < x⪅0.9, and single-phase AlN for x>0.9. Pseudomorphic forces associated with growth on MgO(001) substrates extended the stability range for the NaCl-structure phase to x⩾0.5 at T s °C.

Surface segregation near the temperature of bulk phase separation: Incomplete wetting in Cu(Ag) alloys.

Surface segregation in alloys should present drastic modifications near a bulk phase transition. In particular, when the phase diagram exhibits a miscibility gap, one can wonder to what extent surface segregation could be viewed as the first step towards bulk phase separation. We show, in the particular case of very dilute Cu(Ag) alloys, that the actual situation is even more complex. More precisely, using simultaneously an energetic model based on the electronic structure (tight-binding Ising model) and a mean-field approximation formulated as an area-preserving map, we found evidence of incomplete wetting, i.e., a finite succession of layering transitions from almost pure Cu to almost pure Ag planes when the bulk Ag concentration approaches the solubility limit. This theoretical result compares satisfactorily with the experimental surface segregation isotherms derived from kinetics studies using Auger-electron spectroscopy, which indeed exhibit, at least, the first (surface) layering transition. Moreover, the experimentally observed hysteresis between the segregation and dissolution isotherms can be interpreted by taking into account the dependence of the size effect with respect to the surface concentration.

Surface segregation and bulk phase separation in segregating alloys

Surface segregation in segregating alloys with a surface has been investigated on the basis of the pairwise interaction model. The theoretical treatment has been carried out within the framework of the Bragg-Williams approximation. Each layer parallel to the surface has been found to undergo a transition between two phases with different layer concentrations. With decreasing temperature, the transitions occur successively above the bulk phase separation temperature. We discuss the mechanism of bulk phase separation with focus on the successive phase transitions of the layers.

Surface segregation and bulk phase separation in segregating alloys

Surface segregation and bulk phase separation in segregating alloys

Defect structure and phase transitions in epitaxial metastable cubic Ti0.5Al0.5N alloys grown on MgO(001) by ultra-high-vacuum magnetron sputter deposition

Ti0.5Al0.5N alloy films, typically 1.5 μm thick, were grown on MgO(001) at temperatures Ts between 400 and 850 °C by ultra-high-vacuum reactive magnetron sputtering in pure N2. Films grown at Ts between ≂480 and 560 °C were single crystals in which the lattice misfit strain was partially relieved by glide of 〈001〉 misfit dislocations, with Burgers vector =a0/2〈011〉, on {011̄} planes. Cross-sectional transmission electron microscopy investigation showed no evidence of residual extended defects in the films until thicknesses of ≂150 nm at which point threading dislocations, oriented along the [001] growth direction, were observed. Surface-initiated spinodal decomposition, resulting in the formation of compositionally modulated NaCl-structure platelets along [001] with width ≂1 nm, occurred over a narrow growth temperature range between 540 and 560 °C as a precursor to bulk phase separation of wurtzite-structure AlN at Ts≥560 °C. The alloy was continuously depleted of AlN at higher growth temperatures until the equilibrium two-phase structure, cubic TiN and wurtzite AlN, was obtained at Ts≥750 °C.

Dynamics of interfaces near a wall during phase separation

The connection between the growth of a wetting layer and bulk phase separation is investigated in a binary liquid system undergoing spinodal decomposition near a critical point. Gravity effects are suppressed by using a density-matched mixture of partially deuterated cyclohexane and methanol. The wetting phase which grows at the walls of the container is found to be rippled and connected to the bulk, and layering of spatially anisotropic domains of the nonwetting phase is observed near the wall. Growth laws are determined and compared with a dimensional analysis of the Navier-Stokes equations.

Dynamics of wetting and phase separation

The connection between the growth of a wetting layer and bulk phase separation is investigated in a liquid system undergoing spinodal decomposition. The wetting phase, which grows at the walls of the container, is found to be rippled and connected to the bulk, and layering of spatially anisotropic domains of the nonwetting phase is observed near the wall. Growth laws are determined and compared with a dimensional analysis of the Navier-Stokes equations.