1,843 research outputs found
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
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
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
Magnetic properties of PrCu at high pressure
We report a study of the low-temperature high-pressure phase diagram of the
intermetallic compound PrCu, by means of molecular-field calculations and
Cu nuclear-quadrupole-resonance (NQR) measurements under pressure.
The pressure-induced magnetically-ordered phase can be accounted for by
considering the influence of the crystal electric field on the electron
orbitals of the Pr ions and by introducing a pressure-dependent exchange
interaction between the corresponding local magnetic moments. Our experimental
data suggest that the order in the induced antiferromagnetic phase is
incommensurate. The role of magnetic fluctuations both at high and low
pressures is also discussed.Comment: 7 pages, 6 figures, submitted to Eur. Phys. J.
Viscoelastic Effect on Hydrodynamic Relaxation in Polymer Solutions
The viscoelastic effect on the hydrodynamic relaxation in semidilute polymer
solutions is investigated. From the linearized two-fluid model equations, we
predict that the dynamical asymmetry coupling between the velocity fluctuations
and the viscoelastic stress influences on the hydrodynamic relaxation process,
resulting in a wave-number-dependent shear viscosity.Comment: 7pages; To be published in Journal of the Physical Society of
Japan,Vol 72,No2,(2003
Defect structures in nematic liquid crystals around charged particles
We numerically study the orientation deformations in nematic liquid crystals
around charged particles. We set up a Ginzburg-Landau theory with inhomogeneous
electric field. If the dielectric anisotropy varepsilon_1 is positive, Saturn
ring defects are formed around the particles. For varepsilon_1<0, novel "ansa"
defects appear, which are disclination lines with their ends on the particle
surface. We find unique defect structures around two charged particles. To
lower the free energy, oppositely charged particle pairs tend to be aligned in
the parallel direction for varepsilon_1>0 and in the perpendicular plane for
varepsilon_1<0 with respect to the background director . For identically
charged pairs the preferred directions for varepsilon_1>0 and varepsilon_1<0
are exchanged. We also examie competition between the charge-induced anchoring
and the short-range anchoring. If the short-range anchoring is sufficiently
strong, it can be effective in the vicinity of the surface, while the director
orientation is governed by the long-range electrostatic interaction far from
the surface.Comment: 10 papes, 12 figures, to appear in European Physical Journal
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 much longer than microscopic times but
shorter than the onset time of tube constraints . 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.Comment: 19 pages, 11 figure
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
Shear flow effects on phase separation of entangled polymer blends
We introduce an entanglement model mixing rule for stress relaxation in a polymer blend to a modified Cahn-Hilliard equation of motion for concentration fluctuations in the presence of shear flow. Such an approach predicts both shear-induced mixing and demixing, depending on the relative relaxation times and plateau moduli of the two components
Strong Coupling between Antiferromagnetic and Superconducting Order Parameters in CeRhIn Studied by In-NQR Spectroscopy
We report on a novel pressure ()-induced evolution of magnetism and
superconductivity (SC) in a helical magnet CeRhIn with an incommensurate
wave vector through the In nuclear quadrupole
resonance (NQR) measurements under . Systematic measurements of the
In-NQR spectrum reveal that the commensurate antiferromagnetism (AFM)
with is realized above 1.7 GPa. An
important finding is that the size of SC gap and increase as the
magnitude of the AFM moment decreases in the region, where SC uniformly
coexists with the commensurate AFM. This result provides evidence of strong
coupling between the commensurate AFM order parameter (OP) and SC OP.Comment: 5 pages, 5 figure
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