Biomedical subjects

Akira Onuki

Publications and source records attributed to Akira Onuki.

13 recordsLinked to original sources

PubMed2006

Charged inclusion in nematic liquid crystals.

We present a general theory of liquid crystals under inhomogeneous electric field in a Ginzburg-Landau scheme. The molecular orientation can be deformed by electric field when the dielectric tensor is orientation dependent. We then investigate the influence of a charged particle on the orientation order in a nematic state. The director is aligned either along or perpendicular to the local electric field around the charge, depending on the sign of the dielectric anisotropy. The deformation becomes stronger with increasing the ratio Ze/R, where Ze is the charge and R is the radius of the particle. Numerical analysis shows the presence of defects around the particle for large Ze/R. They are nanometer-scale defects for microscopic ions. If the dielectric anisotropy is positive, a Saturn ring defect appears. If it is negative, a pair of point defects appear apart from the particle surface, each being connected to the surface by a disclination line segment.

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PubMed2006

Transitions among crystal, glass, and liquid in a binary mixture with changing particle-size ratio and temperature.

Using molecular dynamics simulation we examine changeovers among crystal, glass, and liquid at high density in a two dimensional binary mixture. We change the ratio between the diameters of the two components and the temperature. The transitions from crystal to glass or liquid occur with proliferation of defects. We visualize the defects in terms of a disorder variable Dj(t) representing a deviation from the hexagonal order for particle j. The defect structures are heterogeneous and are particularly extended in polycrystal states. They look similar at the crystal-glass crossover and at the melting. Taking the average of Dj(t) over the particles, we define a disorder parameter D(t), which measures the degree of overall disorder. The relaxation of D(t) after quenching becomes slow at low temperature in the presence of size dispersity. Its steady state average is small in crystal and large in glass and liquid. We also find that grain boundaries tend to be pinned with increasing size dispersity in polycrystal states.

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PubMed2006

High-speed observation of the piston effect near the gas-liquid critical point.

We investigated adiabatic changes in a near-critical fluid on acoustic time scales using an ultrasensitive interferometer. A sound emitted by very weak continuous heating caused a stepwise adiabatic change at its front with a density change of order 10(-7) g/cm(3) and a temperature change of order 10(-5) K. Very small heat inputs at a heater produced short acoustic pulses with width of order 10 micros, which were broadened as they moved through the cell and interacted with the boundaries. The pulse broadening became enhanced near the critical point. We also examined theoretically how sounds are emitted from a heater and how applied heat is transformed into mechanical work. Our predictions agree well with our data.

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PubMed2006

Ginzburg-Landau theory of solvation in polar fluids: Ion distribution around an interface.

We present a Ginzburg-Landau theory of solvation of ions in polar binary mixtures. The solvation free energy arising from the ion-dipole interaction can strongly depend on the composition and the ion species. Most crucial in phase separation is then the difference in the solvation free energy between the two phases, which is the origin of the Galvani potential difference known in electrochemistry. We also take into account an image potential acting on each ion, which arises from inhomogeneity in the dielectric constant and is important close to an interface at very small ion densities. Including these solvation and image interactions, we calculate the ion distributions and the electric potential around an interface with finite thickness. In particular, on approaching the critical point, the ion density difference between the two phases becomes milder. The critical temperature itself is much shifted even by a small amount of ions. We examine the surface tension in the presence of ions in various cases.

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PubMed2005

Droplet motion with phase change in a temperature gradient.

We examine the droplet motion in one-component fluids in a small temperature gradient by solving linearized hydrodynamic equations supplemented with appropriate surface boundary conditions. We show that the velocity field and the temperature around the droplet are strongly influenced by first-order phase transition taking place at the interface. Latent heat released or absorbed at the interface drastically changes the hydrodynamic flow around the droplet. As a result, the temperature becomes almost homogeneous inside the droplet and the Marangoni effect arising from the surface tension gradient is much suppressed. The droplet velocity is also much decelerated.

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PubMed2005

Ion-induced nucleation in polar one-component fluids.

We present a Ginzburg-Landau theory of ion-induced nucleation in a gas phase of polar one-component fluids, where a liquid droplet grows with an ion at its center. By calculating the density profile around an ion, we show that the solvation free energy is larger in gas than in liquid at the same temperature on the coexistence curve. This difference much reduces the nucleation barrier in a metastable gas.

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PubMed2005

Dynamic van der waals theory of two-phase fluids in heat flow.

We present a dynamic van der Waals theory. It is useful to study phase separation when the temperature varies in space. We show that, if heat flow is applied to liquid suspending a gas droplet at zero gravity, a convective flow occurs such that the temperature gradient within the droplet nearly vanishes. As the heat flux is increased, the droplet becomes attached to the heated wall that is wetted by liquid in equilibrium. In one case corresponding to partial wetting by gas, an apparent contact angle can be defined. In the other case with larger heat flux, the droplet completely wets the heated wall expelling liquid.

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PubMed2005

Molecular dynamics simulation of heat conduction in near-critical fluids.

Using molecular dynamics simulations we study supercritical fluids near the gas-liquid critical point under heat flow in two dimensions. We calculate the steady-state temperature and density profiles. The resultant thermal conductivity exhibits critical singularity in agreement with the mode-coupling theory in two dimensions. We also calculate distributions of the momentum and heat fluxes at fixed density. They indicate that liquidlike (entropy-poor) clusters move toward the warmer boundary and gaslike (entropy-rich) regions move toward the cooler boundary in a temperature gradient. This counterflow results in critical enhancement of the thermal conductivity.

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PubMed2004

Viscoelastic phase separation in shear flow.

We numerically investigate viscoelastic phase separation in polymer solutions under shear using a time-dependent Ginzburg-Landau model. The gross variables in our model are the polymer volume fraction and a conformation tensor. The latter represents chain deformations and relaxes slowly on the rheological time giving rise to a large viscoelastic stress. The polymer and the solvent obey two-fluid dynamics in which the viscoelastic stress acts asymmetrically on the polymer and, as a result, the stress and the diffusion are dynamically coupled. Below the coexistence curve, interfaces appear with increasing the quench depth and the solvent regions act as a lubricant. In these cases the composition heterogeneity causes more enhanced viscoelastic heterogeneity and the macroscopic stress is decreased at fixed applied shear rate. We find steady two-phase states composed of the polymer-rich and solvent-rich regions, where the characteristic domain size is inversely proportional to the average shear stress for various shear rates. The deviatoric stress components exhibit large temporal fluctuations. The normal stress difference can take negative values transiently at weak shear.

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PubMed2004

Entanglements in quiescent and sheared polymer melts.

We visualize entanglements in polymer melts using molecular dynamics simulation. A bead at an entanglement interacts persistently for long times with the nonbonded beads (those excluding the adjacent ones in the same chain). The interaction energy of each bead with the nonbonded beads is averaged over a time interval tau much longer than microscopic times but shorter than the onset time of tube constraints tau e. Entanglements can then be detected as hot spots consisting of several beads with relatively large values of the time-averaged interaction energy. We next apply a shear flow with rate much faster than the disengagement motion of entangled chains. With increasing strain the chains take zigzag shapes and one-half of the hot spots become bent. The chains are first stretched as a network but, as the bends approach the chain ends, disentanglements subsequently occur, leading to stress overshoot observed experimentally.

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PubMed2004

Solvation effects in near-critical binary mixtures.

A Ginzburg-Landau theory is presented to investigate solvation effects in near-critical polar fluid binary mixtures. Concentration dependence of the dielectric constant gives rise to a shell region around a charged particle within which solvation occurs preferentially. As the critical point is approached, the concentration has a long-range Ornstein-Zernike tail representing strong critical electrostriction. If salt is added, strong coupling arises among the critical fluctuations and the ions. The structure factors of the critical fluctuations and the charge density are calculated and the phase transition behavior is discussed.

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PubMed2003

Plastic flow in two-dimensional solids.

A time-dependent Ginzburg-Landau model of plastic deformation in two-dimensional solids is presented. The fundamental dynamic variables are the displacement field u and the lattice velocity v=delta(u)/delta(t). Damping is assumed to arise from the shear viscosity in the momentum equation. The elastic energy density is a periodic function of the shear and tetragonal strains, which enables the formation of slips at large strains. In this work we neglect defects such as vacancies, interstitials, or grain boundaries. The simplest slip consists of two edge dislocations with opposite Burgers vectors. The formation energy of a slip is minimized if its orientation is parallel or perpendicular to the flow in simple shear deformation and if it makes angles of +/-pi/4 with respect to the stretched direction in uniaxial stretching. High-density dislocations produced in plastic flow do not disappear even if the flow is stopped. Thus large applied strains give rise to structurally disordered states, which are metastable due to the Peierls potential. We divide the elastic energy into an elastic part due to affine deformation and a defect part. The latter represents degree of disorder and is nearly constant in plastic flow under cyclic straining.

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PubMed2002

Convective heat transport in compressible fluids.

We present hydrodynamic equations of compressible fluids in gravity as a generalization of those in the Boussinesq approximation used for nearly incompressible fluids. They account for adiabatic processes taking place throughout the cell (the piston effect) and those taking place within plumes (the adiabatic temperature gradient effect). Performing two-dimensional numerical analysis, we reveal some unique features of plume generation and convection in transient and steady states of compressible fluids. As the critical point is approached, the overall temperature changes induced by plume arrivals at the boundary walls are amplified, giving rise to overshoot behavior in transient states and significant noise in the temperature in steady states. The velocity field is suggested to assume a logarithmic profile within boundary layers. Random reversal of macroscopic shear flow is examined in a cell with unit aspect ratio. We also present a simple scaling theory for moderate Rayleigh numbers.

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