1,660 research outputs found
Structural and dynamical heterogeneities in two-dimensional melting
Using molecular dynamics simulation, we study structural and dynamical
heterogeneities at melting in two-dimensional one-component systems with 36000
particles. Between crystal and liquid we find intermediate hexatic states,
where the density fluctuations are enhanced at small wave number k as well as
those of the six-fold orientational order parameter. Their structure factors
both grow up to the smallest wave number equal to the inverse system length.
The intermediate scattering function of the density S(k,t) is found to relax
exponentially with decay rate Gamma_k ~ k^z with z~2.6 at small k in the
hexatic phase.Comment: 6 pages, 8 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
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
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
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
Hydrodynamic fluctuations and the minimum shear viscosity of the dilute Fermi gas at unitarity
We study hydrodynamic fluctuations in a non-relativistic fluid. We show that
in three dimensions fluctuations lead to a minimum in the shear viscosity to
entropy density ratio as a function of the temperature. The minimum
provides a bound on which is independent of the conjectured bound in
string theory, , where is the entropy
density. For the dilute Fermi gas at unitarity we find \eta/s\gsim 0.2\hbar.
This bound is not universal -- it depends on thermodynamic properties of the
unitary Fermi gas, and on empirical information about the range of validity of
hydrodynamics. We also find that the viscous relaxation time of a hydrodynamic
mode with frequency diverges as , and that the shear
viscosity in two dimensions diverges as .Comment: 26 pages, 5 figures; final version to appear in Phys Rev
Ordered magnetic and quadrupolar states under hydrostatic pressure in orthorhombic PrCu2
We report magnetic susceptibility and electrical resistivity measurements on
single-crystalline PrCu2 under hydrostatic pressure, up to 2 GPa, which
pressure range covers the pressure-induced Van Vleck
paramagnet-to-antiferromagnet transition at 1.2 GPa. The measured anisotropy in
the susceptibility shows that in the pressure-induced magnetic state the
ordered 4f-moments lie in the ac-plane. We propose that remarkable pressure
effects on the susceptibility and resistivity are due to changes in the
quadrupolar state of O22 and/or O20 under pressure. We present a simple
analysis in terms of the singlet-singlet model.Comment: 14 pages, 9 figures submitted to Phys. Rev.
Phase separation transition in liquids and polymers induced by electric field gradients
Spatially uniform electric fields have been used to induce instabilities in
liquids and polymers, and to orient and deform ordered phases of
block-copolymers. Here we discuss the demixing phase transition occurring in
liquid mixtures when they are subject to spatially nonuniform fields. Above the
critical value of potential, a phase-separation transition occurs, and two
coexisting phases appear separated by a sharp interface. Analytical and
numerical composition profiles are given, and the interface location as a
function of charge or voltage is found. The possible influence of demixing on
the stability of suspensions and on inter-colloid interaction is discussed.Comment: 7 pages, 3 figures. Special issue of the J. Phys. Soc. Ja
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