arXiv subjects

Akira Onuki

Publications and source records attributed to Akira Onuki.

At least 37 records · Page 2Linked to original sources

arXiv2015

Hydrodynamics in bridging and aggregation of two colloidal particles in a near-critical binary mixture

We investigate bridging and aggregation of two colloidal particles in a near-critical binary mixture when the fluid far from the particles is outside the coexistence (CX) curve and is rich in the component disfavored by the colloid surfaces. In such situations, the adsorption-induced interaction is enhanced, leading to bridging and aggregation of the particles. We realize bridging firstly by changing the temperature with a fixed interparticle separation and secondly by letting the two particles aggregate. The interparticle attractive force dramatically increases upon bridging. The dynamics is governed by hydrodynamic flow around the colloid surfaces. In aggregation, the adsorption layers move with the particles and squeezing occurs at narrow separation. We take into account the renormalization effect due to the critical fluctuations using the recent local functional theory [J. Chem. Phys. 136, 114704 (2012)].

cond-mat.soft↗

arXiv2015

Molecular Dynamics Simulation of Water between Metal Walls under Electric Field: Dielectric Response and Dynamics after Field Reversal

We study water between parallel metal walls under applied electric field accounting for the image effect at $T=298$ K. The electric field due to the surface charges serves to attract and orient nearby water molecules, while it tends to a constant determined by the mean surface charge density away from the walls. We find Stern boundary layers with thickness about $5$ $\rm Å$ and a homogeneously polarized bulk region. The molecules in the layers more sensitively respond to the applied field than in the bulk. As a result, the potential drop in the layers is larger than that in the bulk unless the cell length exceeds 10 nm. We also examine the hydrogen bonds, which tend to make small angles with respect to the walls in the layers even without applied field. The average local field considerably deviates from the classical Lorentz field and the local field fluctuations are very large in the bulk. If we suppose a nanometer-size sphere around each molecule, the local field contribution from its exterior is nearly equal to that from the continuum electrostatics and that from its interior yields the deviation from the classical Lorentz field. As a nonequilibrium problem, we investigate the dynamics after a reversal of applied field, where the relaxation is mostly caused by large-angle rotational jumps after 1 ps due to the presence of the hydrogen bond network. The molecules undergoing these jumps themselves form hydrogen-bonded clusters heterogeneously distributed in space.

cond-mat.soft↗

arXiv2013

Orientational glass in mixtures of elliptic and circular particles: Structural heterogeneities, rotational dynamics, and rheology

Using molecular dynamics simulation with an angle-dependent Lennard-Jones potential, we study orientational glass with quadrupolar symmetry in mixtures of elliptic particles and circular impurities in two dimensions. With a mild aspect ratio ($=1.2$) and a mild size ratio ($=1.2$), we realize a plastic crystal at relatively high temperature $T$. With further lowering $T$, we find a structural phase transition for very small impurity concentration $c$ and pinned disordered orientations for not small $c$. The ellipses are anchored by the impurities in the planar alignment. With increasing $c$, the orientation domains composed of isosceles triangles gradually become smaller, resulting in orientational glass with crystal order. In our simulation, the impurity distribution becomes heterogeneous during quenching from liquid, which then produces rotational dynamic heterogeneities. We also examine rheology in orientational glass to predict a shape memory effect and a superelasticity effect, where a large fraction of the strain is due to collective orientation changes.

cond-mat.soft↗

arXiv2013

Formation of double glass in binary mixtures of anisotropic particles: Dynamic heterogeneities in rotations and displacements

We study glass behavior in a mixture of elliptic and circular particles in two dimensions at low temperatures using an orientation-dependent Lennard-Jones potential. The ellipses have a mild aspect ratio ($\sim 1.2$) and tend to align at low temperatures, while the circular particles play the role of impurities disturbing the ellipse orientations at a concentration of 20%. These impurities have a size smaller than that of the ellipses and attract them in the homeotropic alignment. As a result, the coordination number around each impurity is mostly five or four in glassy states. We realize double glass, where both the orientations and the positions are disordered but still hold mesoscopic order. We find a strong heterogeneity in the flip motions of the ellipses, which sensitively depends on the impurity clustering. The average rotational diffusion constant is determined by rapidly flipping ellipses, which still remain even at low temperatures in our model. In contrast, the non-flip rotations (with angle changes not close to $\pm π$) are mainly caused by the cooperative configuration changes involving many particles. Then, there arises a long-time heterogeneity in the non-flip rotations closely correlated with the dynamic heterogeneity in displacements.

cond-mat.soft↗

arXiv2013

Applying electric field to charged and polar particles between metallic plates: Extension of the Ewald method

We develop an efficient Ewald method of molecular dynamics simulation for calculating the electrostatic interactions among charged and polar particles between parallel metallic plates, where we may apply an electric field with an arbitrary size. We use the fact that the potential from the surface charges is equivalent to the sum of those from image charges and dipoles located outside the cell. We present simulation results on boundary effects of charged and polar fluids, formation of ionic crystals, and formation of dipole chains, where the applied field and the image interaction are crucial. For polar fluids, we find a large deviation of the classical Lorentz-field relation between the local field and the applied field due to pair correlations along the applied field. As general aspects, we clarify the difference between the potential-fixed and the charge-fixed boundary conditions and examine the relationship between the discrete particle description and the continuum electrostatics.

cond-mat.soft↗

arXiv2013

Attractive interaction and bridging transition between neutral colloidal particles due to preferential adsorption in a near-critical binary mixture

We examine the solvent-mediated interaction between two neutral colloidal particles due to preferential adsorption in a near-critical binary mixture. We take into account the renormalization effect due to the critical fluctuations using the recent local functional theory $[$J. Chem. Phys. {\bf 136}, 114704 (2012)$]$. We calculate the free energy and the force between two colloidal particles as functions of the temperature $T$, the composition far from the colloidal particles $c_\infty$, and the colloid separation $\ell$. The interaction is much enhanced when the component favored by the colloid surfaces is poor in the reservoir. For such off-critical compositions, we find a surface of a first-order bridging transition $\ell= \ell_{\rm cx}(T,c_\infty)$ in the $T$-$c_\infty$-$\ell$ space in a universal, scaled form, across which a discontinuous change occurs between separated and bridged states. This surface starts from the bulk coexistence surface (CX) and ends at a bridging critical line $\ell= \ell_{c}(T)$. On approaching the critical line, the discontinuity vanishes and the derivatives of the force with respect to $T$ and $\ell$ both diverge. Furthermore, bridged states continuously change into separated states if $c_\infty$ (or $T$) is varied from a value on CX to value far from CX with $\ell$ kept smaller than $\ell_c(T)$.

cond-mat.soft↗

arXiv2013

Formation of double glass in binary mixtures of anisotropic particles

We study glass transitions in mixtures of elliptic and circular particles in two dimensions using an orientation-dependent Lennard-Jones potential. Changing anisotropic parameters of the potential, the size ratio, and the concentration, we realize double glass, where both the particle positions and orientations are disordered but still hold mesoscopic order. The ellipses are anchored around the circular impurities in the homeotropic or planar directions. We examine slowing-down of rotational and translational time-correlation functions. Turnover motions of the ellipses are activated more frequently than the configuration changes, where the latter cause the structural relaxation.

cond-mat.soft↗

arXiv2013

Phase separation in a binary mixture confined between symmetric parallel plates: Capillary condensation transition near the bulk critical point

We investigate phase separation of near-critical binary mixtures between parallel symmetric walls in the strong adsorption regime. We take into account the renormalization effect due to the critical fluctuations using the recent local functional theory [J. Chem. Phys. 136, 114704 (2012)]. In statics, a van der Waals loop is obtained in the relation between the average order parameter $ $ in the film and the chemical potential when the temperature $T$ is lower than the film critical temperature $T_c^{ca}$ (in the case of an upper critical solution temperature). In dynamics, we lower $T$ below the capillary condensation line from above $T_c^{ca}$. We calculate the subsequent time-development assuming no mass exchange between the film and the reservoir. In the early stage, the order parameter $psi$ changes only in the direction perpendicular to the walls. For sufficiently deep quenching, such one-dimensional profiles become unstable with respect to the fluctuations varying in the lateral directions. The late-stage coarsening is then accelerated by the hydrodynamic interaction. A pancake domain of the phase disfavored by the walls finally appears in the middle of the film.

cond-mat.soft↗

arXiv2012

Slow relaxations and stringlike jump motions in fragile glass-forming liquids: Breakdown of the Stokes-Einstein relation

We perform molecular dynamics simulation on a glass-forming liquid binary mixture with the soft-core potential in three dimensions. We investigate crossover of the configuration changes caused by stringlike jump motions. With lowering the temperature $T$, the motions of the particles composing strings become larger in sizes and displacements, while those of the particles surrounding strings become smaller. Then, the contribution of the latter to time-correlation functions tends to be long-lived as $T$ is lowered. As a result, the relaxation time $τ_α$ and the viscosity $η$ grow more steeply than the inverse diffusion constant $D^{-1}$ at low $T$, leading to breakdown of the Stokes-Einstein relation. At low $T$, the diffusion occurs as activation processes and may well be described by short-time analysis of rare jump motions with broken bonds and large displacements. Some characteristic features of the van Hove self-correlation function arise from escape jumps over high potential barriers.

cond-mat.soft↗

arXiv2012

Relationship between bond-breakage correlations and four-point correlations in heterogeneous glassy dynamics: Configuration changes and vibration modes

We investigate the dynamic heterogeneities of glassy particle systems in the theoretical schemes of bond breakage and four-point correlation functions. In the bond-breakage scheme, we introduce the structure factor S_b(q,t) and the susceptibility chi_b(t) to detect the spatial correlations of configuration changes. Here chi_b(t) attains a maximum at t =t_b^max as a function of time t, where the fraction of the particles with broken bonds phi_b(t)$ is about 1/2. In the four-point scheme, treating the structure factor S_4(q,t) and the susceptibility chi_4(t), we detect superpositions of the heterogeneity of bond breakage and that of thermal low-frequency vibration modes. While the former grows slowly, the latter emerges quickly to exhibit complex space-time behavior. In two dimensions, the vibration modes extending over the system yield significant contributions to the four-point correlations, which depend on the system size logarithmically. A maximum of chi_4(t) is attained at t= t_4^max, where these two contributions become of the same order. As a result, t_4^max is considerably shorter than t_b^max.

cond-mat.soft↗

arXiv2012

Molecular dynamics simulation of orientational glass formation in anisotropic particle systems in three dimensions

We propose a simple microscopic model of molecular dynamics simulation to study orientational glass in three dimensions. We present simulation results for mixtures of mildly anisotropic particles and spherical impurities. We realize fcc solids without orientational order in a rotator phase. As the temperature $T$ is lowered, the disordered matrix is gradually replaced by four kinds of orientationally ordered, rhombohedral domains. Two-phase coexistence is realized in a temperature window. The impurities serve to anchor the orientations of the surrounding anisotropic particles, resulting in finely divided domains or medium long-range orientational order. We examine the rotational dynamics of the molecular orientations which is slowed down at low $T$. We predict the shape memory effect under a stretching cycle due to inter-variant transformation.

cond-mat.soft↗

arXiv2012

Structural phase transition and orientation-strain glass formation in anisotropic particle systems with impurities in two dimensions

Using a modified Lennard-Jones model for elliptic particles and spherical impurities, we present results of molecular dynamics simulation in two dimensions. In one-component systems of elliptic particles, we find an orientation phase transition on a hexagonal lattice as the temperature $T$ is lowered. It is also a structural one because of spontaneous strain. At low $T$, there arise three martensitic variants due to the underlying lattice, leading to a shape memory effect without dislocation formation. Thermal hysteresis, a minimum of the shear modulus, and a maximum of the specific heat are also found with varying $T$. With increasing the composition $c$ of impurities, the three kinds of orientation domains are finely divided, yielding orientation-strain glass with mesoscopically ordered regions still surviving. If the impurities are large and repulsive, planar anchoring of the elliptic particles occurs around the impurity surfaces. If they are small and attractive, homeotropic anchoring occurs. Clustering of impurities is conspicuous. With increasing the anchoring power and/or the composition of the impurities, positional disorder can also be enhanced. We also investigate the rotational dynamics of the molecular orientations.

cond-mat.soft↗

arXiv2012

Casimir amplitudes and capillary condensation of near-critical fluids between parallel plates: Renormalized local functional theory

We investigate the critical behavior of a near-critical fluid confined between two parallel plates in contact with a reservoir by calculating the order parameter profile and the Casimir amplitudes (for the force density and for the grand potential). Our results are applicable to one-component fluids and binary mixtures. We assume that the walls absorb one of the fluid components selectively for binary mixtures. We propose a renormalized local functional theory accounting for the fluctuation effects. Analysis is performed in the plane of the temperature T and the order parameter in the reservoir ψ_{\infty} . Our theory is universal if the physical quantities are scaled appropriately. If the component favored by the walls is slightly poor in the reservoir, there appears a line of first-order phase transition of capillary condensation outside the bulk coexistence curve. The excess adsorp- tion changes discontinuously between condensed and noncondensed states at the transition. With increasing T, the transition line ends at a capillary critical point T = T_c^{ca} slightly lower than the bulk critical temperature T_c. The Casimir amplitudes are larger than their critical-point values by 10-100 times between the transition line and the bulk coexistence curve and slightly above the capillary critical point.

cond-mat.stat-mech↗

arXiv2012

Selective solvation in aqueous mixtures: Interface deformations and instability

We briefly review the effects of selective solvation of ions in aqueous mixtures, where the ion densities and the composition fluctuations are strongly coupled. We then examine the surface tension γof a liquid-liquid interface in the presence of ions. We show that γcan be decreased drastically due to the electrostatic and solvation interactions near the interface. We calculate how the free energy is changed due to small surface undulations in the presence of an electric double layer. A surface instability occurs for negative γ, which can easily be realized for antagonistic ion pairs near the solvent criticality. Three-dimensional simulation shows how the surface instability is induced.

cond-mat.soft↗

arXiv2011

Structural phase transition of anisotropic particles and formation of orientation-strain glass with addition of impurities

Using a modified Lennard-Jones model for anisotropic particles, we present results of molecular dynamics simulation in two dimensions. In one-component systems, we find crystallization, a Berezinskii-Kosterlitz-Thouless phase, and a structural phase transition, as the temperatures is lowered. In the lowest temperature range, the crystal is composed of three martensitic variants on a hexagonal lattice, exhibiting the shape memory effect. With addition of larger spherical particles (impurities), these domains are finely divided, yielding glass with slow time evolution. With increasing the impurity size, the structural or translational disorder is also proliferated.

cond-mat.soft↗

arXiv2011

Visualization of stacking faults in fcc crystals in plastic deformations

Using molecular dynamics simulation, we investigate the dynamics of stacking faults in fcc crystals in uniaxial stretching in a Lennard-Jones binary mixture composed of 4096 particles in three dimensions. We visualize stacking faults using a disorder variable $D_j(t)$ for each particle $j$ constructed from local bond order parameters based on spherical harmonics (Steinhardt order parameters). Also introducing a method of bond breakage, we examine how stacking faults are formed and removed by collective particle motions. These processes are relevant in plasticity of fcc crystals.

cond-mat.soft↗

arXiv2011

Charged colloids in an aqueous mixture with a salt

We calculate the ion and composition distributions around colloid particles in an aqueous mixture, accounting for preferential adsorption, electrostatic interaction, selective solvation among ions and polar molecules, and composition-dependent ionization. On the colloid surface, we predict a precipitation transition induced by strong preference of hydrophilic ions to water and a prewetting transition between weak and strong adsorption and ionization. These transition lines extend far from the solvent coexistence curve in the plane of the interaction parameter χ(or the temperature) and the average solvent composition. The colloid interaction is drastically altered by these phase transitions on the surface. In particular, the interaction is much amplified upon bridging of wetting layers formed above the precipitation line. Such wetting layers can either completely or partially cover the colloid surface depending on the average solvent composition.

cond-mat.soft↗

arXiv2011

Pre-dewetting transition on a hydrophobic wall: Statics and dynamics

For one-component fluids, we predict a pre-dewetting phase transition between a thin and thick low-density layer in liquid on a wall repelling the fluid. This is the case of a hydrophobic wall for water. A pre-dewetting line starts from the coexistence curve and ends at a surface critical point in the phase diagram. We calculate this line numerically using the van der Waals model and analytically using the free energy expansion up to the quartic order. We also examine the pre-dewetting dynamics of a layer created on a hydrophobic spot on a heterogeneous wall. It is from a thin to thick layer during decompression and from a thick to thin layer during compression. Upon the transition, a liquid region above the film is cooled for decompression and heated for compression due to latent heat convection and a small pressure pulse is emitted from the film into the liquid.

cond-mat.soft↗