29 research outputs found
Comment on "Magnetoviscosity and relaxation in ferrofluids"
It is shown and discussed how the conventional system of hydrodynamic
equations for ferrofluids was derived. The set consists of the equation of
fluid motion, the Maxwell equations, and the magnetization equation. The latter
was recently revised by Felderhof [Phys. Rev. E, v.62, p.3848 (2000)]. His
phenomenological magnetization equation looks rather like corresponding
Shliomis' equation, but leads to wrong consequences for the dependence of
ferrofluid viscosity and magnetization relaxation time on magnetic field.Comment: 6 pages, 1 figure, Submitted to Phys. Rev.
Dissipation in ferrofluids: Mesoscopic versus hydrodynamic theory
Part of the field dependent dissipation in ferrofluids occurs due to the
rotational motion of the ferromagnetic grains relative to the viscous flow of
the carrier fluid. The classical theoretical description due to Shliomis uses a
mesoscopic treatment of the particle motion to derive a relaxation equation for
the non-equilibrium part of the magnetization. Complementary, the hydrodynamic
approach of Liu involves only macroscopic quantities and results in dissipative
Maxwell equations for the magnetic fields in the ferrofluid. Different stress
tensors and constitutive equations lead to deviating theoretical predictions in
those situations, where the magnetic relaxation processes cannot be considered
instantaneous on the hydrodynamic time scale. We quantify these differences for
two situations of experimental relevance namely a resting fluid in an
oscillating oblique field and the damping of parametrically excited surface
waves. The possibilities of an experimental differentiation between the two
theoretical approaches is discussed.Comment: 14 pages, 2 figures, to appear in PR
Study of solid 4He in two dimensions. The issue of zero-point defects and study of confined crystal
Defects are believed to play a fundamental role in the supersolid state of
4He. We report on studies by exact Quantum Monte Carlo (QMC) simulations at
zero temperature of the properties of solid 4He in presence of many vacancies,
up to 30 in two dimensions (2D). In all studied cases the crystalline order is
stable at least as long as the concentration of vacancies is below 2.5%. In the
2D system for a small number, n_v, of vacancies such defects can be identified
in the crystalline lattice and are strongly correlated with an attractive
interaction. On the contrary when n_v~10 vacancies in the relaxed system
disappear and in their place one finds dislocations and a revival of the
Bose-Einstein condensation. Thus, should zero-point motion defects be present
in solid 4He, such defects would be dislocations and not vacancies, at least in
2D. In order to avoid using periodic boundary conditions we have studied the
exact ground state of solid 4He confined in a circular region by an external
potential. We find that defects tend to be localized in an interfacial region
of width of about 15 A. Our computation allows to put as upper bound limit to
zero--point defects the concentration 0.003 in the 2D system close to melting
density.Comment: 17 pages, accepted for publication in J. Low Temp. Phys., Special
Issue on Supersolid
Jamming at Zero Temperature and Zero Applied Stress: the Epitome of Disorder
We have studied how 2- and 3- dimensional systems made up of particles
interacting with finite range, repulsive potentials jam (i.e., develop a yield
stress in a disordered state) at zero temperature and applied stress. For each
configuration, there is a unique jamming threshold, , at which
particles can no longer avoid each other and the bulk and shear moduli
simultaneously become non-zero. The distribution of values becomes
narrower as the system size increases, so that essentially all configurations
jam at the same in the thermodynamic limit. This packing fraction
corresponds to the previously measured value for random close-packing. In fact,
our results provide a well-defined meaning for "random close-packing" in terms
of the fraction of all phase space with inherent structures that jam. The
jamming threshold, Point J, occurring at zero temperature and applied stress
and at the random close-packing density, has properties reminiscent of an
ordinary critical point. As Point J is approached from higher packing
fractions, power-law scaling is found for many quantities. Moreover, near Point
J, certain quantities no longer self-average, suggesting the existence of a
length scale that diverges at J. However, Point J also differs from an ordinary
critical point: the scaling exponents do not depend on dimension but do depend
on the interparticle potential. Finally, as Point J is approached from high
packing fractions, the density of vibrational states develops a large excess of
low-frequency modes. All of these results suggest that Point J may control
behavior in its vicinity-perhaps even at the glass transition.Comment: 21 pages, 20 figure
On the Raman spectrum of argon dimers
The pressure broadening of the rotational Raman transitions of argon dimers is calculated, using the semiclassical method. Simple models to compute the line broadening of noble gas dimers are put forward. The predictions of these simple models are compared with the results of the full semiclassical calculation. The optimal experimental conditions to resolve the Ar2 Raman spectrum are discussed
Evidence for an orientationally ordered two dimensional fluid phase from molecular dynamics calculations
Results of molecular-dynamics calculations on melting in a two-dimensional (2D) Lennard-Jones system are presented. We find that this system loses its resistance to shear at a temperature T1 (≈0.36). However, long-range "orientational" order persists up to a higher temperature T2 (≈0.57). These observations are compatible with the existence of a liquid-crystal-like phase with sixfold anisotropy separated from both the solid and the isotropic-fluid phase by second-order phase transitions. Such two-stage melting behavior in 2D has been predicted by Halperin and Nelson
On the Raman spectrum of argon dimers
The pressure broadening of the rotational Raman transitions of argon dimers is calculated, using the semiclassical method. Simple models to compute the line broadening of noble gas dimers are put forward. The predictions of these simple models are compared with the results of the full semiclassical calculation. The optimal experimental conditions to resolve the Ar2 Raman spectrum are discussed