Phase Separation Of Binary Fluids Research Articles

Stability of Lamellar Domain in Two-Dimensional Phase-Separating Binary Fluid under Poiseuille Flow

We investigate effect of external flow on domains in two-dimensional phase-separating binary fluid. By use of the coupled Cahn–Hillard and Navier–Stokes equations, we study stability of a lamellar ...

The rheology and morphology of phase-separating fluids with viscosity contrast

In this paper, the effects of viscosity contrast between the components on the rheology and morphology of phase-separating binary fluids have been studied by numerically solving time-dependent Ginzburg–Landau equation and Navier–Stokes equation. It is found that, the viscosity contrast between the components strongly affects the bulk contribution to the overall rheological behavior. When the viscosity of the minor phase is higher, the trend of bulk contribution is contrary with that of interfacial contribution. Therefore, the non-Newtonian behavior is weakened. When the viscosity of the minor phases is lower, the evolution of bulk contribution is the same as interfacial contribution, and thus the non-Newtonian behavior is strengthened. However, the interfacial contribution still plays the crucial role in the overall rheological behavior. When the system contains two droplets or many domains, due to the merging of domains induced by shear flow, there is a decrease of interfacial volume fraction. Therefore, the interfacial contribution of rheology decreases and forms a peak at small shear strain. Correspondingly, the bulk contribution has also been altered. However, whether the bulk contribution increases or decreases will depend on the viscosity contrast.

Interplay between wetting and phase separation inbinary fluid mixtures: roles of hydrodynamics

Here we consider how phase-separation kinetics and morphology areaffected by the preferential wetting of a solid surface by onecomponent of a binary fluid mixture. The behaviour is cruciallydependent upon whether the spinodal decomposition is bicontinuous-type or droplet-type, i.e. the compositionsymmetry. Near a symmetric composition, wetting effects arestrongly delocalized by hydrodynamic effects unique to bicontinuousphase separation. We discuss the physical mechanism of theunusually fast lateral growth of wetting domains found by Wiltziusand Cumming, the thickening dynamics of wetting layers, and patternevolution under the influence of surface fields, focusing on theroles of hydrodynamics. We point out a novel possibility of doublephase separation: that the quick hydrodynamic reduction of theinterface area may spontaneously destabilize the phase-separatedmacroscopic domains and induce secondary phase separation. We alsoconsider effects of the preferential wetting of immobile and mobileparticles by one component of a fluid mixture on phase separationand the resulting complex pattern evolution. It is demonstratedthat hydrodynamics always plays crucial roles in the pattern evolutionof a phase-separating fluid mixture interacting with solid surfaces.

Surface-directed spinodal decomposition in binary fluid mixtures.

We consider the phase separation of binary fluids in contact with a surface, which is preferentially wetted by one of the components of the mixture. We review the results available for this problem and present numerical results obtained using a mesoscopic level simulation technique for the three-dimensional problem.

Effect of Stationary Particles on the Phase Separation of Binary Fluids

ABSTRACTPhase separating binary fluids with the addition of immobile particles, which act as osmotic force centres, were simulated using a Lattice Boltzmann model in two dimensions. In the hydrodynamic over-damped limit, where the flow is entirely driven by capillary effects, the presence of particles that are preferentially wetted by one of the fluid components significantly affects the kinetics of the growth of the fluid domains. The late time dynamics is governed by the wetting interactions and the final size of the domains can be tailored by varying the strength of the particles-fluid interaction as well as the particles concentration. These features are predicted within a simple theoretical model and are amenable of experimental checks.

The effect of shear flow on morphology and rheology of phase separating binary mixtures

The morphology and the corresponding rheological properties of phase separating binary mixtures under shear flow are studied by computer simulation based on the modified time-dependent Ginzburg–Landau (TDGL) model. In order to investigate the hydrodynamic effect, model H in three dimensions has been used to simulate the phase separation of binary fluids under shear flow. For the sake of comparison, the simulation has also been performed based on simple binary solid model (model B). It is found that, for deep and critical quench, the domain grows faster and the domain anisotropy is lower in binary fluids due to the internal flow field induced by hydrodynamic interaction. For deep and off-critical quench, the internal flow field makes the elongated domain quickly relax to their original spherical shape before they are mutually contacted each other. Thus, it reduces the domain merging probability. It is also found that, for deep and critical quench, there are two peaks appeared in the shear viscosity as a function of shear strain at low shear rate, which agrees with the experimentally observations quite well. For shallow quenching, the broader interfaces suppress the internal flow caused by hydrodynamic interaction and thus the difference between binary solids and binary fluids is small. All these observed unique characters have been explained according to the hydrodynamic interaction and the relaxation rate of the deformed interface.

Crossover from the hydrodynamic regime to the thermal fluctuation regime in a two-dimensional phase-separating binary fluid containing surfactants

Extensive simulations were carried out to investigate the crossover between the hydrodynamic regime at intermediate stage and the thermal fluctuation regime at late stage in a phase-separating binary fluid/surfactant system in two dimensions. The existence of the crossover and its dependence on the surfactant concentration were analyzed using Kawasaki and Ohta's interface kinetic equation [Physica A 118, 175 (1983)]. The analysis showed that there should exist a critical surfactant concentration, above which thermal fluctuations dominate phase separation at all times. Simulations suggested that the crossover exists and the hydrodynamic regime shrinks when surfactant concentration increases. Simulations also demonstrated that the trapped surfactants seen in a previous study [Phys. Rev. E 59, 2109 (1999)] can remain trapped for a time much longer than the time needed to form well segregated domains, in spite of the presence of significant thermal fluctuations.

MATCHING MACROSCOPIC PROPERTIES OF BINARY FLUIDS TO THE INTERACTIONS OF DISSIPATIVE PARTICLE DYNAMICS

We investigate the role played by conservative forces in dissipative particle dynamics (DPD) simulation of single-component and binary fluids. We employ equations from kinetic theory for matching the coefficients of DPD interparticle force to the macroscopic properties of fluid such as: density, temperature, diffusion coefficient, kinematic viscosity and sound velocity. The sound velocity c is coupled with scaling factor π1 of conservative component of the DPD collision operator. Its value sets up an upper limit on the mass S of a single particle in DPD fluid. The Kirkwood–Alder fluid–solid transition is observed for a sufficiently large S. We emphasize the role of the scaling factor π12 for particles of different types in simulating phase separation in binary fluids. The temporal growth of average domain size R(t) in the phase separation process depends on the value of immiscibility coefficient Δ = π12 - π1. For small immiscibility, R (t) ∝ tβ, where β ≈ 1/2 for R (t) < R H and β ≈ 2/3 for R (t) > R H , R H is the hydrodynamic length. Finally, both phases separate out completely. For larger immiscibility, R(t) increases exponentially at the beginning of simulation, while finally the domain growth process becomes marginal. We also observe the creation of emulsion-like structures. This effect results from an increase of the surface tension on the two-phase interface along with increasing immiscibility.

Molecular dynamics study of a phase-separating fluid mixture under shear flow

Molecular dynamics simulation is carried out to study domain structures and rheological properties of a two-dimensional phase-separating binary fluid mixture under shear flow. In the early stage of the phase separation, anisotropic composition fluctuations appear immediately after the quench. As the domain grows, the anisotropy in the composition fluctuations increases. The quenched system eventually reaches a dynamical steady state, in which anisotropic domain structures are preserved. In the steady state, the shortest characteristic length scale ${R}_{\ensuremath{\perp}}$ of domains decreases with increasing shear rate $\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}$ as ${R}_{\ensuremath{\perp}}\ensuremath{\sim}{\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}}^{\ensuremath{-}1/3}.$ Stringlike domain structures are observed in the strong shear regime, whereas randomly fluctuating patterns are observed in the weak shear regime. Moreover, the excess viscosity $\ensuremath{\Delta}\ensuremath{\eta}$ is found to decrease with increasing shear rate as $\ensuremath{\Delta}\ensuremath{\eta}\ensuremath{\sim}{\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}}^{\ensuremath{-}1/2},$ indicating that the phase-separating fluid mixtures are highly non-Newtonian because of domain deformations.

Interfacial curvature in phase-separating binary fluids: comparison between computer simulation and experiments

We present comparisons of results on the late stage dynamics of spinodal decomposition (SD) obtained by the computer simulation based on the time-dependent Ginzburg-Landau equation with hydrodynamic interaction and experiments of a polymer mixture of deuterated polybutadiene and polybutadiene. We especially focus on the curvatures of the interface of phase-separated structures formed by SD and show that the scaling functions of the curvature distributions for the two systems are in good agreement.

Stability of cylindrical domains in phase-separating binary fluids in shear flow

The stability of a long cylindrical domain in a phase-separating binary fluid in an external shear flow is investigated by linear stability analysis. Using the coupled Cahn-Hilliard and Stokes equations, the stability eigenvalues are derived analytically for long-wavelength perturbations, for arbitrary viscosity contrast between the two phases. The shear flow is found to suppress and sometimes completely stabilize both the hydrodynamic Rayleigh instability and the thermodynamic instability of the cylinder against varicose perturbations, by mixing with nonaxisymmetric perturbations. The results are consistent with recent observations of a ``string phase'' in phase-separating fluids in shear.

Breakdown of Scale Invariance in the Coarsening of Phase-Separating Binary Fluids

We present evidence, based on lattice Boltzmann simulations, to show that the coarsening of the domains in phase separating binary fluids is not a scale-invariant process. Moreover we emphasise that the pathway by which phase separation occurs depends strongly on the relation between diffusive and hydrodynamic time scales.

Effect of shear flow on the stability of domains in two-dimensional phase-separating binary fluids

We perform a linear stability analysis of extended domains in phase-separating fluids of equal viscosity in two dimensions. Using the coupled Cahn-Hilliard and Stokes equations, we derive analytically the stability eigenvalues for long wavelength fluctuations. In the quiescent state we find an unstable varicose mode that corresponds to an instability towards coarsening. This mode is stabilized when an external shear flow is imposed on the fluid. The effect of the shear is seen to be qualitatively similar to that found in experiments.

Spinodal decomposition in binary fluids under shear flow

We discuss the effects of shear rates on the structure, rheology and kinetics of phase separation in binary fluids through numerical Langevin simulations in both two- and three-dimensions. Our major findings are as follows: (1) shear flow distorts the isotropy properties in normal spinodal decomposition. Under stronger shear, a string phase appears; (2) domains grow differently in directions parallel and perpendicular to the flow direction; (3) excess shear viscosities due to shear flow have a characteristic peak as a function of shear rates. We compare our findings with the experimental data and simulation results which do not incorporate hydrodynamics.

Some recent applications of cell dynamical modelling to phase ordering dynamics

We study experimentally relevant effects in phase ordering dynamics using Cell Dynamical System (CDS) models. In particular, we present representative numerical results for phase ordering in random magnets and phase separation in binary fluids.

Phase separation of binary fluids in porous media: Asymmetries in pore geometry and fluid composition.

Phase separation of a binary Lennard-Jones fluid confined in a variety of two-dimensional pores is studied using molecular dynamics simluations. We consider a simple strip pore, an uneven single pore, and a junction made out of two pores. We have studied dynamics of phase separation for both critical and off-critical composition of the binary fluid. We find the existence of long-lived metastable states in the presence of asymmetries both in the geometry and the composition of the fluid. The results are compared with theoretical predictions and with experimental observations.

Phase separation of binary fluids confined in a cylindrical pore: A molecular dynamics study.

Phase separation of a binary Lennard-Jones fluid confined in a cylindrical pore is studied by molecular dynamics simulations. The wetting interaction of the fluid molecules with the walls of the single pore is derived from the Lennard-Jones potential. Existence of long-lived metastable states is found even in the presence of hydrodynamic modes. Other effects of the hydrodynamic interactions on the formation of wetting layers are discussed. Our results are consistent with recent experimental observations.

Dynamics of phase-separated fluids under external fields

Phase separation in external fields has attracted much attention recently. The reason is twofold. Since kinetics of phase separation and morphology of growing domains can be controlled by external fields, it is of technological importance. The other is that existence of mesoscale domains causes curious dynamical properties in fields, which provides us with a fundamental statistical dynamic problem. One example is a phase separation of binary fluids under shear flow. Phase-separated domains are deformed under the field, which causes burst, fusion, and reconnection of domains so that extra energy dissipation occurs in these processes. Because of this large deformation of domains, the system exhibits quite unusual rheological behavior. The kinetics of phase separation of binary fluids is also influenced by an external electric field when the new phases have different dielectric constants. Deformation and interaction of domains in an electric field are investigated by means of an interfacial approach.

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.

Dynamics of phase separation of binary fluids.

The results of molecular-dynamics studies of surface-tension-dominated spinodal decomposition of initially well-mixed binary fluids in the absence and presence of gravity are presented. The growth exponent for the domain size and the decay exponent of the potential energy of interaction between the two species with time are found to be 0.6\ifmmode\pm\else\textpm\fi{}0.1, inconsistent with scaling arguments based on dimensional analysis.