Hydrodynamic effects on phase separation of binary mixtures with reversible chemical reaction

Lattice Boltzmann simulation for phase separation with chemical reaction controlled by thermal diffusion

This study investigates phase separation behavior and pattern formation in a binary fluid with chemical reaction controlled by thermal diffusion. By incorporating the Arrhenius equation into the lattice Boltzmann method (LBM), the coupling effects of the pre-exponential factor K, viscosity [Formula: see text], and thermal diffusion D on phase separation were successfully evaluated. The effect of the competition between thermal diffusion and concentration on the phase separation morphology and dynamics of binary mixtures under a chemically reacting controlled by slow cooling is assessed based on the extended LBM. The calculations indicated that increases in viscosity and thermal diffusion can obtain interconnected structures (ISs) and lamellar structures (LSs) for cases with small K. However, concentric phase-separated structures (CSs) were observed in cases with large K. The increase in the degree and efficiency of phase separation were significantly greater in cases with decreased viscosity and increased thermal diffusion.

Dissipative particle dynamics study of phase separation in binary fluid mixtures in periodic and confined domains

We investigate the phase separation behavior of binary mixtures in two-dimensional periodic and confined domains using dissipative particle dynamics. Two canonical problems of fluid mechanics are considered for the confined domains: square cavity with no-slip walls and lid-driven cavity with one driven wall. The dynamics is studied for both weakly and strongly separating mixtures and different area fractions. The phase separation process is analyzed using the structure factor and the total interface length. The dynamics of phase separation in the square cavity and lid-driven cavity are observed to be significantly slower when compared to the dynamics in the periodic domain. The presence of the no-slip walls and the inertial effects significantly influences the separation dynamics. Finally, we show that the growth exponent for the strongly separating case is invariant to changes in the inter-species repulsion parameter.

Molecular dynamics study of phase separation in fluids with chemical reactions

We present results from the first d=3 molecular dynamics (MD) study of phase-separating fluid mixtures (AB) with simple chemical reactions (A⇌B). We focus on the case where the rates of forward and backward reactions are equal. The chemical reactions compete with segregation, and the coarsening system settles into a steady-state mesoscale morphology. However, hydrodynamic effects destroy the lamellar morphology which characterizes the diffusive case. This has important consequences for the phase-separating structure, which we study in detail. In particular, the equilibrium length scale (ℓ(eq)) in the steady state suggests a power-law dependence on the reaction rate ε:ℓ(eq)∼ε(-θ) with θ≃1.0.

Suppression of Ostwald ripening in active emulsions

Emulsions consisting of droplets immersed in a fluid are typically unstable since they coarsen over time. One important coarsening process is Ostwald ripening, which is driven by the surface tension of the droplets. Stability of emulsions is relevant not only in complex fluids but also in biological cells, which contain liquidlike compartments, e.g., germ granules, Cajal bodies, and centrosomes. Such cellular systems are driven away from equilibrium, e.g., by chemical reactions, and thus can be called active emulsions. In this paper, we study such active emulsions by developing a coarse-grained description of the droplet dynamics, which we analyze for two different chemical reaction schemes. We first consider the simple case of first-order reactions, which leads to stable, monodisperse emulsions in which Ostwald ripening is suppressed within a range of chemical reaction rates. We then consider autocatalytic droplets, which catalyze the production of their own droplet material. Spontaneous nucleation of autocatalytic droplets is strongly suppressed and their emulsions are typically unstable. We show that autocatalytic droplets can be nucleated reliably and their emulsions stabilized by the help of chemically active cores, which catalyze the production of droplet material. In summary, different reaction schemes and catalytic cores can be used to stabilize emulsions and to control their properties.

Polymerization-induced spinodal decomposition of ethylene glycol∕phenolic resin solutions under electric fields

Temporal evolution of polymerization-induced spinodal decomposition (PISD) under electric fields was investigated numerically in ethylene glycol∕phenolic resin solutions with different initial composition. A model composed of the nonlinear Cahn-Hilliard-Cook equation for spinodal decomposition and a rate equation for curing reaction was utilized to describe the PISD phenomenon. As initial composition varied, deformed droplet-like and aligned bi-continuous structures were observed in the presence of an electric field. Moreover, the anisotropic parameter (D), determined from the 2D-FFT power spectrum, was employed to quantitatively characterize the degree of morphology anisotropy. The value of D increased quickly in the early stage and then decreased in the intermediate stage of spinodal decomposition, which was attributed to the resistance of coarsening process to morphology deformation and the decline of electric stress caused by polymerization reaction. The results can also provide a guidance on how to control the morphology of monolithic porous polymer and carbon materials with anisotropic structures.

Traveling kinks in cubic nonlinear Ginzburg-Landau equations

Nonlinear cubic Euler-Lagrange equations of motion in the traveling variable are usually derived from Ginzburg-Landau free energy functionals frequently encountered in several fields of physics. Many authors considered in the past damped versions of such equations, with the damping term added by hand simulating the friction due to the environment. It is known that even in this damped case kink solutions can exist. By means of a factorization method, we provide analytic formulas for several possible kink solutions of such equations of motion in the undriven and constant field driven cases, including the recently introduced Riccati parameter kinks, which were not considered previously in such a context. The latter parameter controls the delay of the switching stage of the kinks. The delay is caused by antikink components that are introduced in the structure of the solution through this parameter.

Complex phase separation in poly(acrylonitrile-butadiene-styrene)-modified epoxy/4,4'-diaminodiphenyl sulfone blends: generation of new micro- and nanosubstructures

The epoxy system containing diglycidyl ether of bisphenol A and 4,4'-diaminodiphenyl sulfone is modified with poly(acrylonitrile-butadiene-styrene) (ABS) to explore the effects of the ABS content on the phase morphology, mechanism of phase separation, and viscoelastic properties. The amount of ABS in the blends was 5, 10, 15, and 20 parts per hundred of epoxy resin (phr). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were employed to investigate the final morphology of ABS-modified epoxy blends. Scanning electron microscopic studies of 15 phr ABS-modified epoxy blends reveal a bicontinuous structure in which both epoxy and ABS are continuous, with substructures of the ABS phase dispersed in the continuous epoxy phase and substructures of the epoxy phase dispersed in the continuous ABS phase. TEM micrographs of 15 phr ABS-modified epoxy blends confirm the results observed by SEM. TEM micrographs reveal the existence of nanosubstructures of ABS in 20 phr ABS-modified epoxy blends. To the best of our knowledge, to date, nanosubstructures have never been reported in any epoxy/thermoplastic blends. The influence of the concentration of the thermoplastic on the generated morphology as analyzed by SEM and TEM was explained in detail. The evolution and mechanism of phase separation was investigated in detail by optical microscopy (OM) and small-angle laser light scattering (SALLS). At concentrations lower than 10 phr the system phase separates through nucleation and growth (NG). However, at higher concentrations, 15 and 20 phr, the blends phase separate through both NG and spinodal decomposition mechanisms. On the basis of OM and SALLS, we conclude that the phenomenon of complex substructure formation in dynamic asymmetric blends is due to the combined effect of hydrodynamics and viscoelasticity. Additionally, dynamic mechanical analysis was carried out to evaluate the viscoelastic behavior of the cross-linked epoxy/ABS blends. Finally, apparent weight fractions of epoxy and ABS components in epoxy- and ABS-rich phases were evaluated from T(g) analysis.

Lattice Boltzmann simulations of phase separation in chemically reactive binary fluids

We use a lattice Boltzmann method to study pattern formation in chemically reactive binary fluids in the regime where hydrodynamic effects are important. The coupled equations solved by the method are a Cahn-Hilliard equation, modified by the inclusion of a reactive source term, and the Navier-Stokes equations for conservation of mass and momentum. The coupling is twofold, resulting from the advection of the order parameter by the velocity field and the effect of fluid composition on pressure. We study the evolution of the system following a critical quench for a linear and for a quadratic reaction source term. Comparison is made between the high and low viscosity regimes to identify the influence of hydrodynamic flows. In both cases hydrodynamics is found to influence the pathways available for domain growth and the eventual steady states.