127 research outputs found
Comment on "Novel Convective Instabilities in a Magnetic Fluid"
Comment on the paper "Novel Convective Instabilities in a Magnetic Fluid" by
W. Luo, T. Du, and J. Huang, Phys. Rev. Lett., v.82, p.4134 (1999).Comment: 1 page, 1 figure, To appear in Phys. Rev. Lett. (2001
Magnetization of rotating ferrofluids: the effect of polydispersity
The influence of polydispersity on the magnetization is analyzed in a
nonequilibrium situation where a cylindrical ferrofluid column is enforced to
rotate with constant frequency like a rigid body in a homogeneous magnetic
field that is applied perpendicular to the cylinder axis. Then, the
magnetization and the internal magnetic field are not longer parallel to each
other and their directions differ from that of the applied magnetic field.
Experimental results on the transverse magnetization component perpendicular to
the applied field are compared and analyzed as functions of rotation frequency
and field strength with different polydisperse Debye models that take into
account the polydispersity in different ways and to a varying degree.Comment: 11 pages, 7 figures, to be published in Journal of Physics
Magnetic Soret effect: Application of the ferrofluid dynamics theory
The ferrofluid dynamics theory is applied to thermodiffusive problems in
magnetic fluids in the presence of magnetic fields. The analytical form for the
magnetic part of the chemical potential and the most general expression of the
mass flux are given. By employing these results to experiments, global Soret
coefficients in agreement with measurements are determined. Also an estimate
for a hitherto unknown transport coefficient is made.Comment: 7 pages, 2 figure
Axisymmetric solitary waves on the surface of a ferrofluid
We report the first observation of axisymmetric solitary waves on the surface
of a cylindrical magnetic fluid layer surrounding a current-carrying metallic
tube. According to the ratio between the magnetic and capillary forces, both
elevation and depression solitary waves are observed with profiles in good
agreement with theoretical predictions based on the magnetic analogue of the
Korteweg-deVries equation. We also report the first measurements of the
velocity and the dispersion relation of axisymmetric linear waves propagating
on the cylindrical ferrofluid layer that are found in good agreement with
theoretical predictions.Comment: to be published in Phys. Rev. Let
Transport properties of an ionic magnetic colloid: experimental study of increasing the ionic strength
Abstract The phase separation of an ionic magnetic colloid in zero magnetic field by increasing the ionic strength is well known [Universite Paris VI, France, 1987; J. Colloid Interface Sci. 132 (1989) 1]. The present work deals with investigation of an ionic ferrofluid by increasing the ionic strength in the range of 0-0.14 mol/l, being safely below the threshold value at which the effect of phase separation occurs. By the optical grid setup [Fourth International conference PAMIR, France, 2000], the main transport properties of a ferrofluid, i.e. translational mass diffusion and thermal diffusion (Soret) coefficients, are measured. The obtained results show a strong dependence of mass diffusion coefficient and no dependence of the Soret coefficient upon increasing the ionic strength. It is possible to explain both these effects theoretically; nevertheless, there is the lack of an exact theoretical model due to its complexity.
Invalidation of the Kelvin Force in Ferrofluids
Direct and unambiguous experimental evidence for the magnetic force density
being of the form in a certain geometry - rather than being the
Kelvin force - is provided for the first time. (M is the
magnetization, H the field, and B the flux density.)Comment: 4 pages, 4 figure
Wave turbulence on the surface of a ferrofluid submitted to a magnetic field
We report the observation of wave turbulence on the surface of a ferrofluid
mechanically forced and submitted to a static normal magnetic field. We show
that magnetic surface waves arise only above a critical field. The power
spectrum of their amplitudes displays a frequency-power law leading to the
observation of a magnetic wave turbulence regime which is experimentally shown
to involve a 4-wave interaction process. The existence of the regimes of
gravity, magnetic and capillary wave turbulence is reported in the phase space
parameters as well as a triple point of coexistence of these three regimes.
Most of these features are understood using dimensional analysis or the
dispersion relation of the ferrohydrodynamics surface waves
Ferrohydrodynamics: testing a new magnetization equation
A new magnetization equation recently derived from irreversible
thermodynamics is employed to the calculation of an increase of ferrofluid
viscosity in a magnetic field. Results of the calculations are compared with
those obtained on the basis of two well-known magnetization equations. One of
the two was obtained phenomenologically, another one was derived
microscopically from the Fokker-Planck equation. It is shown that the new
magnetization equation yields a quite satisfactory description of
magnetiviscosity in the entire region of magnetic field strength and the flow
vorticity. This equation turns out to be valid -- like the microscopically
derived equation but unlike the former phenomenological equation -- even far
from equilibrium, and so it should be recommended for further applications.Comment: 4 pages, 3 figures, Submitted to Phys. Rev.
Instability of the origami of a ferrofluid drop in a magnetic field
Capillary origami is the wrapping of an usual fluid drop by a planar elastic
membrane due to the interplay between capillary and elastic forces. Here, we
use a drop of magnetic fluid whose shape is known to strongly depend on an
applied magnetic field. We study the quasi-static and dynamical behaviors of
such a magnetic capillary origami. We report the observation of an overturning
instability that the origami undergoes at a critical magnetic field. This
instability is triggered by an interplay between magnetic and gravitational
energies in agreement with the theory presented here. Additional effects of
elasticity and capillarity on this instability are also discussed.Comment: in press in PRL (2011
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