arXiv subjects

Ryoichi Yamamoto

Publications and source records attributed to Ryoichi Yamamoto.

68 records · Page 4Linked to original sources

arXiv2003

Supercooled Liquids under Shear: Computational Approach

We have performed molecular dynamics simulations for a model two-dimensional soft-core mixture in a supercooled state. The mixture exhibits a slow structural relaxation in a quiescent state, however, the relaxation is much enhanced in sheared states. There observed surprisingly small anisotropy both in the coherent and incoherent density correlation functions even under extremely strong shear which is $10^3$ times faster than the structural relaxation rate. The present simulation results agree well with predictions of the recently developed mode-coupling theory in shear.

cond-mat.soft↗

arXiv2003

Supercooled liquids under shear: A mode-coupling theory approach

We generalize the mode-coupling theory of supercooled fluids to systems under stationary shear flow. Our starting point is the generalized fluctuating hydrodynamic equations with a convection term. The method is applied to a two dimensional colloidal suspension. The shear rate dependence of the intermediate scattering function and shear viscosity is analyzed. The results show a drastic reduction of the structural relaxation time due to shear and strong shear thinning behavior of the viscosity which are in qualitative agreement with recent simulations. The microscopic theory with minimal assumptions can explain the behavior far beyond the linear response regime.

cond-mat.soft↗

arXiv2003

A Smooth Interface Method for Simulating Liquid Crystal Colloid Dispersions

A new method is presented for mesoscopic simulations of particle dispersions in liquid crystal solvents. It allows efficient first-principle simulations of the dispersions involving many particles with many-body interactions mediated by the solvents. Demonstrations have been performed for the aggregation of colloid dispersions in two-dimensional nematic and smectic-C* solvents neglecting hydrodynamic effects, which will be taken into account in the near future.

cond-mat.soft↗

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↗

arXiv2001

Simulating Particle Dispersions in Nematic Liquid-Crystal Solvents

A new method is presented for mesoscopic simulations of particle dispersions in nematic liquid crystal solvents. It allows efficient first-principle simulations of the dispersions involving many particles with many-body interactions mediated by the solvents. A simple demonstration is shown for the aggregation process of a two dimentional dispersion.

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↗

arXiv2000

Equilibrating Glassy Systems with Parallel Tempering

We discuss the efficiency of the so-called parallel tempering method to equilibrate glassy systems also at low temperatures. The main focus is on two structural glass models, SiO_2 and a Lennard-Jones system, but we also investigate a fully connected 10 state Potts-glass. By calculating the mean squared displacement of a tagged particle and the spin-autocorrelation function, we find that for these three glass-formers the parallel tempering method is indeed able to generate, at low temperatures, new independent configurations at a rate which is O(100) times faster than more traditional algorithms, such as molecular dynamics and single spin flip Monte Carlo dynamics. In addition we find that this speedup increases with decreasing temperature. The reliability of the results is checked by calculating the distribution of the energy at various temperatures and by showing that these can be mapped onto each other by the reweighting technique.

cond-mat.stat-mech↗

arXiv2000

Replica-exchange molecular dynamics simulation for supercooled liquids

We investigate to what extend the replica-exchange Monte Carlo method is able to equilibrate a simple liquid in its supercooled state. We find that this method does indeed allow to generate accurately the canonical distribution function even at low temperatures and that its efficiency is about 10-100 times higher than the usual canonical molecular dynamics simulation.

cond-mat.soft↗

arXiv1999

Apparent finite-size effects in the dynamics of supercooled liquids

Molecular dynamics simulations are performed for a supercooled simple liquid with changing the system size from N=108 to $10^4$ to examine possible finite-size effects. Although almost no systematic deviation is detected in the static pair correlation functions, it is demonstrated that the structural $α$ relaxation in a small system becomes considerably slower than that in larger systems for temperatures below $T_c$ at which the size of the cooperative particle motions becomes comparable to the unit cell length of the small system. The discrepancy increases with decreasing temperature.

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.

cond-mat.soft↗