arXiv subjects

Akira Onuki

Publications and source records attributed to Akira Onuki.

At least 55 records · Page 3Linked to original sources

arXiv2011

Phase Transitions in Soft Matter Induced by Selective Solvation

We review our recent studies on selective solvation effects in phase separation in polar binary mixtures with a small amount of solutes. Such hydrophilic or hydrophobic particles are preferentially attracted to one of the solvent components. We discuss the role of antagonistic salt composed of hydrophilic and hydrophobic ions, which undergo microphase separation at water-oil interfaces leading to mesophases. We then discuss phase separation induced by a strong selective solvent above a critical solute density np, which occurs far from the solvent coexistence curve. We also give theories of ionic surfactant systems and weakly ionized polyelectrolytes including solvation among charged particles and polar molecules. We point out that the Gibbs formula for the surface tension needs to include an electrostatic contribution in the presence of an electric double layer.

cond-mat.soft↗

arXiv2010

Solvation Effects in Phase Transitions in Soft Matter

Phase transitions in polar binary mixtures can be drastically altered even by a small amount of salt. This is because the preferential solvation strongly depends on the ambient composition. Together with a summary of our research in this problem, we present some detailed results on the role of antagonistic salt composed of hydrophilic and hydrophobic ions. These ions tend to segregate at liquid-liquid interfaces and selectively couple to water-rich and oil-rich composition fluctuations, leading to mesophase formation. In our two-dimensional simulation, the corasening of the domain structures can be stopped or slowed down, depending on the interaction parameter (or the temperature) and the salt density. We realize stripe patterns at the critical composition and droplet patterns at off-critical compositions. In the latter case, charged droplets emerge with considerable size dispersity in a percolated region. We also give the structure factors among the ions, accounting for the Coulomb interaction and the solvation interaction mediated by the composition fluctuations.

cond-mat.soft↗

arXiv2010

Precipitation in aqueous mixtures with addition of strongly hydrophilic or hydrophobic solute

We examine phase separation in aqueous mixtures due to preferential solvation with a low-density solute (hydrophilic ions or hydrophobic particles). For hydrophilic ions, preferential solvation can stabilize water domains enriched with ions. This precipitation occurs in wide ranges of the temperature and the average composition above a critical solute density $n_p$, where the mixture solvent would be in a one-phase state without solute. The volume fraction of precipitated domains tends to zero as the average solute density $\bar n$ is decreased to $n_p$ or as the interaction parameter $χ$ is decreased to a critical value $χ_p$. If we start with one-phase states with ${\bar n}>n_p$ or $χ>χ_p$, precipitation proceeds via homogeneous nucleation or via heterogeneous nucleation, for example, around suspended colloids. In the latter case, colliod particles are wrapped by thick wetting layers. We also predict a first-order prewetting transition for $\bar n$ or $χ$ slightly below $n_p$ or $χ_p$.

cond-mat.soft↗

arXiv2010

Self-organization in He4 near the superfluid transition in heat flow and gravity

We investigate the nonlinear dynamics of He4 slightly below the superffluid transition by integrating model F equations in three dimensions. When heated from above under gravity, a vortex tangle and a sheetlike phase slip are generated near the bottom plate. Then a self-organized superfluid containing high-density vortices and phase slips grows upward into an ordinary superfluid. The thermal resistance due to these defects produces a constant temperature gradient equal to the gradient of the pressure-dependent transition temperature $T_λ(p)$. In this self-organized region, the temperature deviation $T-T_λ(p)$ consists of a negative constant independent of the height and time-dependent fluctuations. Its time-average is calculated in good agreement with the experimental value (W.A. Moeur {\it et al.}, Phys. Rev. Lett. 78, 2421 (1997)).

cond-mat.stat-mech↗

arXiv2010

Spreading with evaporation and condensation in one-component fluids

We investigate the dynamics of spreading of a small liquid droplet in gas in a one-component simple fluid, where the temperature is inhomogeneous around 0.9Tc and latent heat is released or generated at the interface upon evaporation or condensation (with Tc being the critical temperature). In the scheme of the dynamic van der Waals theory, the hydrodynamic equations containing the gradient stress are solved in the axisymmetric geometry. We assume that the substrate has a finite thickness and its temperature obeys the thermal diffusion equation. A precursor film then spreads ahead of the bulk droplet itself in the complete wetting condition. Cooling the substrate enhances condensation of gas onto the advancing film, which mostly takes place near the film edge and can be the dominant mechanism of the film growth in a late stage. The generated latent heat produces a temperature peak or a hot spot in the gas region near the film edge. On the other hand, heating the substrate induces evaporation all over the interface. For weak heating, a steady-state circular thin film can be formed on the substrate. For stronger heating, evaporation dominates over condensation, leading to eventual disappearance of the liquid region.

cond-mat.soft↗

arXiv2010

Plastic deformations in crystal, polycrystal, and glass in binary mixtures under shear: Collective yielding

Using molecular dynamics simulation, we examine the dynamics of crystal, polycrystal, and glass in a Lennard-Jones binary mixture composed of small and large particles in two dimensions. The crossovers occur among these states as the composition c is varied at fixed size ratio. Shear is applied to a system of 9000 particles in contact with moving boundary layers composed of 1800 particles. The particle configurations are visualized with a sixfold orientation angle alpha_j(t) and a disorder variable D_j(t) defined for particle j, where the latter represents the deviation from hexagonal order. Fundamental plastic elements are classified into dislocation gliding and grain boundary sliding. At any c, large-scale yielding events occur on the acoustic time scale. Moreover, they multiply occur in narrow fragile areas, forming shear bands. The dynamics of plastic flow is highly hierarchical with a wide range of time scales for slow shearing. We also clarify the relationship between the shear stress averaged in the bulk region and the wall stress applied at the boundaries.

cond-mat.soft↗

arXiv2010

Jammed Particle Configurations and Dynamics in High-Density Lennard-Jones Binary Mixtures in Two Dimensions

We examine the changeover in the particle configurations and the dynamics in dense Lennard-Jones binary mixtures composed of small and large particles. By varying the composition at a low temperature, we realize crystal with defects, polycrystal with small grains, and glass with various degrees of disorder. In particular, we show configurations where small crystalline regions composed of the majority species are enclosed by percolated amorphous layers composed of the two species. We visualize the dynamics of configuration changes using the method of bond breakage and following the particle displacements. In quiescent jammed states, the dynamics is severely slowed down and is highly heterogeneous at any compositions. In shear, plastic deformations multiply occur in relatively fragile regions, growing into large-scale shear bands where the strain is highly localized. Such bands appear on short time scales and change on long time scales with finite life times.

cond-mat.soft↗

arXiv2010

Phase field model of solid-liquid and liquid-liquid phase transitions in flow and elastic fields in one-component systems

We construct a phase field model including hydrodynamics and elasticity in one-component systems. It can be used to investigate solid-liquid and liquid-liquid phase transitions. Upon first-order phase transition, a velocity field is induced around interfaces in the presence of a density difference between the two phases even without applied shear flow. As applications, we present simulation results on two cases of melting, where a solid domain is placed on a heated wall in one case and is suspended in a warmer liquid under shear flow in the other. We find that the solid domain moves or rotates as a whole due to elasticity, releasing latent heat. We also examine the liquid-liquid phase transition of a highly viscous domain into a less viscous liquid on a heated wall, where an inhomogeneous velocity field is induced within a projected part of the domain. In these phase transitions, the interface temperature is nearly equal to the coexisting temperature $T_{\rm cx}(p)$ away from the heated wall in the presence of heat flow in the surrounding liquid.

cond-mat.soft↗

arXiv2010

Precipitation of water from aqueous mixtures with addition of hydrophilic ions

We examine phase separation in aqueous mixtures at fixed amounts of hydrophilic monovalent ions. When water is the minority component, preferential solvation can stabilize water domains enriched with ions. This ion-induced precipitation occurs in wide ranges of the temperature and the average composition where the solvent would be in one-phase states without ions. The volume fraction of such water domains is decreased to zero as the interaction parameter $χ$ (dependent on the temperature) is decreased toward a critical value for each average composition.

cond-mat.soft↗

arXiv2009

Dynamics of Binary Mixtures with Ions: Dynamic Structure Factor and Mesophase Formation

Dynamic equations are presented for polar binary mixtures containing ions in the presence of the preferential solvation. In one-phase states, we calculate the dynamic structure factor of the composition accounting for the ion motions. Microphase separation can take place for sufficiently large solvation asymmetry of the cations and the anions. We show two-dimensional simulation results of the mesophase formation with an antagonistic salt, where the cations are hydrophilic and the anions are hydrophobic. The structure factor S(q) in the resultant mesophase has a sharp peak at an intermediate wave number on the order of the Debye-Huckel wave number. As the quench depth is increased, the surface tension nearly vanishes in mesophases due to an electric double layer.

cond-mat.soft↗

arXiv2009

Ion distribution around a charged rod in one and two component solvents: Preferential solvation and first order ionization phase transition

In one and two component solvents, we calculate the counterion distribution around a charged rod treating the degree of ionization $α$ as an annealed variable dependent on its local environment. In the one component case, $α$ is determined under various conditions without and with salt. In the two component case, we take into account the preferential solvation of the counterions and the ionized monomers and the short-range interaction between the rod and the solvent without salt. It then follows a composition-dependent mass action law. Mesoscopic variations of the composition and the counterions are produced around a chraged rod, which sensitively depend on various parameters of the molecular interactions. Furthermore, we predict a first order phase transition of weak-to-strong dissociation for strong preferential solvation. It can occur in expanded states of a polymer chain. This transition line starts from a point on the solvent coexistence curve and ends at a critical point in the plane of the temperature and the solvent composition. The composition change around a charged rod is long-ranged near the solvent critical point.

cond-mat.soft↗

arXiv2009

Defect structures in nematic liquid crystals around charged particles

We numerically study the orientation deformations in nematic liquid crystals around charged particles. We set up a Ginzburg-Landau theory with inhomogeneous electric field. If the dielectric anisotropy varepsilon_1 is positive, Saturn ring defects are formed around the particles. For varepsilon_1<0, novel "ansa" defects appear, which are disclination lines with their ends on the particle surface. We find unique defect structures around two charged particles. To lower the free energy, oppositely charged particle pairs tend to be aligned in the parallel direction for varepsilon_1>0 and in the perpendicular plane for varepsilon_1<0 with respect to the background director . For identically charged pairs the preferred directions for varepsilon_1>0 and varepsilon_1<0 are exchanged. We also examie competition between the charge-induced anchoring and the short-range anchoring. If the short-range anchoring is sufficiently strong, it can be effective in the vicinity of the surface, while the director orientation is governed by the long-range electrostatic interaction far from the surface.

cond-mat.stat-mech↗

arXiv2009

Structural and dynamical heterogeneities in two-dimensional melting

Using molecular dynamics simulation, we study structural and dynamical heterogeneities at melting in two-dimensional one-component systems with 36000 particles. Between crystal and liquid we find intermediate hexatic states, where the density fluctuations are enhanced at small wave number k as well as those of the six-fold orientational order parameter. Their structure factors both grow up to the smallest wave number equal to the inverse system length. The intermediate scattering function of the density S(k,t) is found to relax exponentially with decay rate Gamma_k ~ k^z with z~2.6 at small k in the hexatic phase.

cond-mat.stat-mech↗

arXiv2009

Bubble and droplet motion in binary mixtures: Evaporation-condensation mechanism and Marangoni effect

Bubble and droplet motion in binary mixtures is studied in weak heat and diffusion fluxes and in gravity by solving the linearized hydrodynamic equations supplemented with appropriate surface boundary conditions. Without gravity, the velocity field is induced by evaporation and condensation at the interface and by the Marangoni effect due to a surface tension gradient. In pure fluids, the latter nearly vanishes since the interface temperature tends to the coexistence temperature $T_{\rm cx}(p)$ even in heat flow. In binary mixtures, the velocity field can be much enhanced by the Marangoni effect above a crossover concentration $c^*$ inversely proportional to the radius $R$ of the bubble or droplet. Here $c^*$ is usually very small for large $R$ for non-azeotropic mixtures. The temperature and concentration deviations are also calculated.

cond-mat.soft↗

arXiv2009

Onion structures induced by hydrophilic and hydrophobic ions in a binary mixture

Phase transition is observed between one-phase disordered phase and an ordered phase with multi-lamellar (onion) structures in an off-critical mixture of D$_2$O and 3-methylpyridine (3MP) containing a salt at 85mM. The salt consists of hydrophilic cations and hydrophobic anions, which interact asymmetrically with the solvent composition fluctuations inducing mesophases. The structure factor of the composition distribution obtained from small-angle neutron scattering has a peak at an intermediate wave number in the disordered phase and multiple peaks in the ordered phase. Lamellar layers forming onions are composed of solvation-induced charged membranes swollen by D$_2$O. The onion phase is realized only for small volume fractions of 3MP (in D$_2$O-rich solvent).

cond-mat.soft↗

arXiv2009

Solvation and Dissociation in Weakly Ionized Polyelectrolytes

We present a Ginzburg-Landau theory of inhomogeneous polyelectrolytes with a polar solvent. First, we take into account the molecular (solvation) interaction among the ions, the charged monomers, the uncharged monomers, and the solvent molecules, together with the electrostatic interaction with a composition-dependent dielectric constant. Second, we treat the degree of ionization as a fluctuating variable dependent on the local electric potential. With these two ingredients included, our results are as follows. (i) We derive a mass reaction law and a general expression for the surface tension. (ii) We calculate the structure factor of the composition fluctuations as a function of various parameters of the molecular interactions, which provides a general criterion of the formation of mesophases. (iii) We numerically examine some typical examples of interfaces and mesophase structures, which strongly depend on the molecular interaction parameters.

cond-mat.soft↗

arXiv2009

Henry's law, surface tension, and surface adsorption in dilute binary mixtures

Equilibrium properties of dilute binary fluid mixtures are studied in two-phase states on the basis of a Helmholtz free energy including the gradient free energy. The solute partitioning between gas and liquid (Henry's law) and the surface tension change $Δγ$ are discussed. A derivation of the Gibbs law $Δγ=-TΓ$ is given with $Γ$ being the surface adsorption. Calculated quantities include the derivatives $d T_c/dX$ and $d p_c/dX$ of the critical temperature and pressure with respect to the solute molar fraction $X$ and the temperature-derivative $(\p γ/\p T)_{{\rm cx},p}$ of the surface tension at fixed pressure $p$ on the coexistence surface. Here $(\p γ/\p T)_{{\rm cx},p}$ can be both positive and negative, depending on the solute molecular size and the solute-solvent interaction,and diverges on the azeptropic line. Explicit expressions are given for all these quantities in the van der Waals model.

cond-mat.stat-mech↗

arXiv2008

Droplet evaporation in one-component fluids: Dynamic van der Waals theory

In a one-component fluid, we investigate evaporation of a small axysymmetric liquid droplet in the partial wetting condition on a heated wall at $T\sim 0.9 T_c$. In the dynamic van der Waals theory (Phys. Rev. E {\bf 75}, 036304 (2007)), we take into account the latent heat transport from liquid to gas upon evaporation. Along the gas-liquid interface, the temperature is nearly equal to the equilibrium coexisting temperature away from the substrate, but it rises sharply to the wall temperature close to the substrate. On an isothermal substrate, evaporation takes place mostly on a narrow interface region near the contact line in a late stage, which is a characteristic feature in one-component fluids.

cond-mat.soft↗