1,754 research outputs found
Plastic flow in polycrystal states in a binary mixture
Using molecular dynamics simulation we examine dynamics in sheared
polycrystal states in a binary mixture containing 10% larger particles in two
dimensions. We find large stress fluctuations arising from sliding motions of
the particles at the grain boundaries, which occur cooperatively to release the
elastic energy stored. These dynamic processes are visualized with the aid of a
sixfold angle representing the local crystal orientation and a
disorder variable representing a deviation from the hexagonal order
for particle .Comment: 3 pages, 3 figure
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
. In this self-organized region, the temperature deviation
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)).Comment: 8 pages, 7 figure
Quantifying Nonequilibrium Behavior with Varying Cooling Rates
We investigate nonequilibrium behavior in (1+1)-dimensional stochastic field
theories in the context of Ginzburg-Landau models at varying cooling rates. We
argue that a reliable measure of the departure from thermal equilibrium can be
obtained from the absolute value of the rate of change of the
momentum-integrated structure function, . We show that the
peak of scales with the cooling, or quench, time-scale,
, in agreement with the prediction by Laguna and Zurek for the scaling
of freeze-out time in both over and under-damped regimes. Furthermore, we show
that the amplitude of the peak scales as independent of the
viscosity.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
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.Comment: 9 pagers, 9 figures, to be published in J. Chem. Phy
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.Comment: 24 pages, 10 figures, to appear in Journal of Physics: Condensed
Matte
Thermoacoustic effects in supercritical fluids near the critical point: Resonance, piston effect, and acoustic emission and reflection
We present a general theory of thermoacoustic phenomena in supercritical
fluids near the critical point in a one-dimensional cell. We take into account
the effects of the heat conduction in the boundary walls and the bulk viscosity
near the critical point. We introduce a coefficient characterizing
reflection of sound with frequency at the boundary. As applications,
we examine the acoustic eigenmodes in the cell, the response to time-dependent
perturbations, sound emission and reflection at the boundary. Resonance and
rapid adiabatic changes are noteworthy. In these processes, the role of the
thermal diffusion layers is enhanced near the critical point because of the
strong critical divergence of the thermal expansion.Comment: 15 pages, 7 figure
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.Comment: 17 pages, 15 figures, to appear in Physical Review
Molecular Dynamics Simulation of Heat-Conducting 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 liquid-like (entropy-poor)
clusters move toward the warmer boundary and gas-like (entropy-rich) regions
move toward the cooler boundary in a temperature gradient. This counterflow
results in critical enhancement of the thermal conductivity
Surface tension of electrolytes: Hydrophilic and hydrophobic ions near an interface
We calculate the ion distributions around an interface in fluid mixtures of
highly polar and less polar fluids (water and oil) for two and three ion
species. We take into account the solvation and image interactions between ions
and solvent. We show that hydrophilic and hydrophobic ions tend to undergo a
microphase separation at an interface, giving rise to an enlarged electric
double layer. We also derive a general expression for the surface tension of
electrolyte systems, which contains a negative electrostatic contribution
proportional to the square root of the bulk salt density. The amplitude of this
square-root term is small for hydrophilic ion pairs, but is much increased for
hydrophilic and hydrophobic ion pairs. For three ion species including
hydrophilic and hydrophobic ions, we calculate the ion distributions to explain
those obtained by x-ray reflectivity measurements.Comment: 8 figure
Intermediate states at structural phase transition: Model with a one-component order parameter coupled to strains
We study a Ginzburg-Landau model of structural phase transition in two
dimensions, in which a single order parameter is coupled to the tetragonal and
dilational strains. Such elastic coupling terms in the free energy much affect
the phase transition behavior particularly near the tricriticality. A
characteristic feature is appearance of intermediate states, where the ordered
and disordered regions coexist on mesoscopic scales in nearly steady states in
a temperature window. The window width increases with increasing the strength
of the dilational coupling. It arises from freezing of phase ordering in
inhomogeneous strains. No impurity mechanism is involved. We present a simple
theory of the intermediate states to produce phase diagrams consistent with
simulation results.Comment: 16 pages, 14 figure
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