arXiv subjects

Akira Onuki

Publications and source records attributed to Akira Onuki.

85 records · Page 5Linked to original sources

arXiv2004

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 non-bonded beads (those excluding the adjacent ones in the same chain). The interaction energy of each bead with the non-bonded beads is averaged over a time interval $τ$ much longer than microscopic times but shorter than the onset time of tube constraints $τ_{\rm e}$. 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 entangle motion. With increasing strain the chains take zigzag shapes and a 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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arXiv2003

Electric Field Effects Near Critical Points

We present a general Ginzburg-Landau theory of electrostatic interactions and electric field effects for the order parameter, the polarization, and the charge density. Electric field effects are then investigated in near-critical fluids and liquid crystals. Some new predictions are given on the liquid-liquid phase transition in polar binary mixtures with a small fraction of ions and the deformation of the liquid crystal order around a charged particle.

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arXiv2003

Liquid Crystals in Electric Field

We present a general theory of electric field effects in liquid crystals where the dielectric tensor depends on the orientation order. As applications, we examine (i) the director fluctuations in nematic states in electric field for arbitrary strength of the dielectric anisotropy and (ii) deformation of the nematic order around a charged particle. Some predictions are made for these effects.

cond-mat.soft↗

arXiv2003

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 $\bi u$ and the lattice velocity ${\bi v}=\p {\bi u}/\p t$. 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 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 $\pm π/4$ 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 metastable, structurally disordered states. 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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arXiv2003

Nonlinear Strain Theory of Plastic Flow in Solids

We present a phenomenological time-dependent Ginzburg-Landau theory of nonlinear plastic deformations in solids. Because the problem is very complex, we first give models in one and two dimensions without vacancies and interstitials, where large strains produce densely distributed slips but the mass density deviations remain small except near the tips of slips. Next we set up a two-dimensional model including a vacancy field (or local free-volume fraction), where relevant is the sensitive dependence of the elastic shear modulus on the vacancy density. In our simulation, if strains are applied to nearly defectless solids but in the presence of such elastic inhomogeneity, the vacancy density and the mass density can become considerably heterogeneous for large strains on spatial scales much longer than the atomic size. These strain-induced disordered states are metastable or long-lived once they are created.

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arXiv2002

Dynamics and Rheology of a Supercooled Polymer Melt in Shear Flow

Using molecular dynamics simulations, we study dynamics of a model polymer melt composed of short chains with bead number N=10 in supercooled states. In quiescent conditions, the stress relaxation function $G(t)$ is calculated, which exhibits a stretched exponential relaxation on the time scale of the $α$ relaxation time $τ_α$ and ultimately follows the Rouse dynamics characterized by the time $τ_{\rm R} \sim N^2 τ_α$. After application of shear $\gdot$, transient stress growth $σ_{xy}(t)/\gdot$ first obeys the linear growth $\int_0^t dt'G(t')$ for strain less than 0.1 but saturates into a non-Newtonian viscosity for larger strain. In steady states, shear-thinning and elongation of chains into ellipsoidal shapes take place for shear $\gdot$ larger than $τ_{\rm R}^{-1}$. In such strong shear, we find that the chains undergo random tumbling motion taking stretched and compact shapes alternatively. We examine the validity of the stress-optical relation between the anisotropic parts of the stress tensor and the dielectric tensor, which are violated in transient states due to the presence of a large glassy component of the stress. We furthermore introduce time-correlation functions in shear to calculate the shear-dependent relaxation times, $τ_α(T,\gdot)$ and $τ_{\rm R} (T,\gdot)$, which decrease nonlinearly as functions of $\gdot$ in the shear-thinning regime.

cond-mat.soft↗

arXiv2002

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, overall temperature changes induced by plume arrivals at the boundary walls are amplified, giving rise to overshoot behavior in transient states and significant noises of 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.

cond-mat.soft↗

arXiv2000

Entanglements in a Quiescent and Sheared Polymer Melt

Molecular dynamics simulations were performed for a polymer melt. In quiescent states, the inter-chain interaction energy supported by each particle takes relatively large values persistently for long times if the particle is close to an entanglement. Thus, if the interaction is averaged over appropriate time intervals, we can detect active spots on the chains produced by entanglement constrains. If rapid shearing is applied, these active regions subsequently become bended in eventual zig-zag shapes of the chains. We also demonstrate that stress overshoot occurs with onset of disentanglement.

cond-mat.soft↗

arXiv1999

Rheology of a Supercooled Polymer Melt

Molecular dynamics simulations are performed for a polymer melt composed of short chains in quiescent and sheared conditions. The stress relaxation function $G(t)$ exhibits a stretched exponential form in a relatively early stage and ultimately follows the Rouse function in quiescent supercooled state. Transient stress evolution after application of shear obeys the linear growth $\int_0^t dt'G(t')$ for strain less than 0.1 and then saturates into a non-Newtonian viscosity. In steady states, strong shear-thinning and elongation of chains into ellipsoidal shapes are found at extremely small shear. A glassy component of the stress is much enhanced in these examples.

cond-mat.soft↗

arXiv1998

Dynamics of Highly Supercooled Liquids

The diffusivity of tagged particles is demonstrated to be heterogeneous on time scales comparable to or less than the structural relaxation time %taking place at the interparticle distance in a highly supercooled liquid via 3D molecular dynamics simulation. The particle motions in the relatively active regions dominantly contribute to the mean square displacement, giving rise to a diffusion constant systematically larger than the Stokes-Einstein value. The van Hove self-correlation function $G_s(r,t)$ is shown to have a large $r$ tail which can be scaled in terms of $r/t^{1/2}$ for $t \ls 3 τ_α$, where $τ_α\cong$ the stress relaxation time. Its presence indicates heterogeneous diffusion in the active regions. However, the diffusion process eventually becomes homogeneous on time scales longer than the life time of the heterogeneity structure ($\sim 3 τ_α$).

cond-mat.soft↗

arXiv1998

Heterogeneous Diffusion in Highly Supercooled Liquids

The diffusivity of tagged particles is demonstrated to be very heterogeneous on time scales comparable to or shorter than the $α$ relaxation time $τ_α$ ($\cong$ the stress relaxation time) in a highly supercooled liquid via 3D molecular dynamics simulation. The particle motions in the relatively active regions dominantly contribute to the mean square displacement, giving rise to a diffusion constant systematically larger than the Einstein-Stokes value. The van Hove self-correlation function $G_s(r,t)$ is shown to have a long distance tail which can be scaled in terms of $r/t^{1/2}$ for $t \ls 3τ_α$. Its presence indicates heterogeneous diffusion in the active regions. However, the diffusion process eventually becomes homogeneous on time scales longer than the life time of the heterogeneity structure ($\sim 3 τ_α$).

cond-mat.soft↗

arXiv1998

Kinetic Heterogeneities in a Highly Supercooled Liquid

We study a highly supercooled two-dimensional fluid mixture via molecular dynamics simulation. We follow bond breakage events among particle pairs, which occur on the scale of the $α$ relaxation time $τ_α$. Large scale heterogeneities analogous to the critical fluctuations in Ising systems are found in the spatial distribution of bonds which are broken in a time interval with a width of order $0.05τ_α$. The structure factor of the broken bond density is well approximated by the Ornstein-Zernike form. The correlation length is of order $100 σ_1$ at the lowest temperature studied, $σ_1$ being the particle size. The weakly bonded regions thus identified evolve in time with strong spatial correlations.

cond-mat.soft↗

arXiv1998

Dynamics of Highly Supercooled Liquids:Heterogeneity, Rheology, and Diffusion

Highly supercooled liquids with soft-core potentials are studied via molecular dynamics simulations in two and three dimensions in quiescent and sheared conditions.We may define bonds between neighboring particle pairs unambiguously owing to the sharpness of the first peak of the pair correlation functions. Upon structural rearrangements, they break collectively in the form of clusters whose sizes grow with lowering the temperature $T$. The bond life time $τ_b$, which depends on $T$ and the shear rate $\gdot$, is on the order of the usual structural or $α$ relaxation time $τ_α$ in weak shear $\gdot τ_α \ll 1$, while it decreases as $1/\gdot$ in strong shear $\gdotτ_α \gg 1$ due to shear-induced cage breakage. Accumulated broken bonds in a time interval ($\sim 0.05τ_b$) closely resemble the critical fluctuations of Ising spin systems. For example, their structure factor is well fitted to the Ornstein-Zernike form, which yields the correlation length $ξ$ representing the maximum size of the clusters composed of broken bonds. We also find a dynamical scaling relation, $τ_b \sim ξ^{z}$, valid for any $T$ and $\gdot$ with $z=4$ in two dimensions and $z=2$ in three dimensions. The viscosity is of order $τ_b$ for any $T$ and $\gdot$, so marked shear-thinning behavior emerges. The shear stress is close to a limiting stress in a wide shear region. We also examine motion of tagged particles in shear in three dimensions. The diffusion constant is found to be of order $τ_b^{-ν}$ with $ν=0.75 \sim 0.8$ for any $T$ and $\gdot$, so it is much enhanced in strong shear compared with its value at zero shear. This indicates breakdown of the Einstein-Stokes relation in accord with experiments. Some possible experiments are also proposed.

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