97 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
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Diffusion-jump model for the combined Brownian and Neel relaxation dynamics of ferrofluids in the presence of external fields and flow
Relaxation of suspended magnetic nanoparticles occurs via Brownian rotational diffusion of the particle as well as internal magnetization dynamics. The latter is often modeled by the stochastic Landau-Lifshitz equation, but its numerical treatment becomes prohibitively expensive in many practical applications due to a time-scale separation between fast, Larmor-type precession and slow, barrier-crossing dynamics. Here, a diffusion-jump model is proposed to take advantage of the time-scale separation and to approximate barrier-crossings as thermally activated jump processes that occur alongside rotational diffusion. The predictions of our diffusion-jump model are compared to reference results obtained by solving the stochastic Landau-Lifshitz equation coupled to rotational Brownian motion. Good agreement is found in the regime of high energy barriers where Neel relaxation can be considered a thermally activated rare event. While many works in the field have neglected N\'eel relaxation altogether, our approach opens the possibility to efficiently include Neel relaxation also into interacting many-particle models
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.
Magnetic properties of colloidal suspensions of interacting magnetic particles
We review equilibrium thermodynamic properties of systems of magnetic
particles like ferrofluids in which dipolar interactions play an important
role. The review is focussed on two subjects: ({\em i}) the magnetization with
the initial magnetic susceptibility as a special case and ({\em ii}) the phase
transition behavior. Here the condensation ("gas/liquid") transition in the
subsystem of the suspended particles is treated as well as the
isotropic/ferromagnetic transition to a state with spontaneously generated
long--range magnetic order.Comment: Review. 62 pages, 4 figure
Theoretical study of the magnetization dynamics of non-dilute ferrofluids
The paper is devoted to the theoretical investigation of the magnetodipolar
interparticle interaction effect on remagnetization dynamics in moderately
concentrated ferrofluids. We consider a homogeneous (without particle
aggregates) ferrofluid consisting of identical spherical particles and employ a
rigid dipole model, where magnetic moment of a particle is fixed with respect
to the particle itself. In particular, for the magnetization relaxation after
the external field is instantly switched off, we show that the magnetodipolar
interaction leads to the increase of the initial magnetization relaxation time.
For the complex ac-susceptibility we find that the this interaction leads to an
overall increase of the imaginary susceptibility part and shifts the peak on
its frequency dependence towards lower frequencies. Comparing results obtained
with our analytical approach (second order virial expansion) to numerical
simulation data (Langevin dynamics method), we demonstrate that the employed
virial expansion approximation gives a good qualitative description of the
ferrofluid magnetization dynamics and provides a satisfactory quantitative
agreement with numerical simulations for the dc magnetization relaxation - up
to the particle volume fraction c ~ 10% and for the ac-susceptibility - up to c
~ 5 %.Comment: 12 pages, 6 figures, submitted to PR
Climate and Dispersal: Black-Winged Stilts Disperse Further in Dry Springs
Climate affects the abundance and distribution of many species of wildlife. Nevertheless, the potential effects of climate on dispersive behaviour remain unstudied. Here, I combine data from (i) a long-term Black-winged Stilt (Himantopus himantopus) monitoring program, (ii) a capture-recapture marking program in Doñana, and (iii) reports from the Rare Birds Committee in the United Kingdom to analyse at different geographical scales the relationship between climate, survival, philopatry, and dispersive behaviour. Black-winged Stilt populations varied in size in consonance with changes in both the North Atlantic Oscillation (NAO) and local rainfall during the breeding season. Changes in population size are related to changes in philopatry and increases in dispersal beyond the traditional range of the species. The results indicate that climatic conditions influence the dispersive behaviour of individual birds, explaining rapid changes in the local population of this species breeding in unstable Mediterranean wetlands
Rolling ferrofluid drop on the surface of a liquid
We report on the controlled transport of drops of magnetic liquid, which are
swimming on top of a non-magnetic liquid layer. A magnetic field which is
rotating in a vertical plane creates a torque on the drop. Due to surface
stresses within the immiscible liquid beneath, the drop is propelled forward.
We measure the drop speed for different field amplitudes, field frequencies and
drop volumes. Simplifying theoretical models describe the drop either as a
solid sphere with a Navier slip boundary condition, or as a liquid half-sphere.
An analytical expression for the drop speed is obtained which is free of any
fitting parameters and is well in accordance with the experimental
measurements. Possible microfluidic applications of the rolling drop are also
discussed
Logging Affects Fledgling Sex Ratios and Baseline Corticosterone in a Forest Songbird
Silviculture (logging) creates a disturbance to forested environments. The degree to which forests are modified depends on the logging prescription and forest stand characteristics. In this study we compared the effects of two methods of group-selection (“moderate” and “heavy”) silviculture (GSS) and undisturbed reference stands on stress and offspring sex ratios of a forest interior species, the Ovenbird (Seiurus aurocapilla), in Algonquin Provincial Park, Canada. Blood samples were taken from nestlings for corticosterone and molecular sexing. We found that logging creates a disturbance that is stressful for nestling Ovenbirds, as illustrated by elevated baseline corticosterone in cut sites. Ovenbirds nesting in undisturbed reference forest produce fewer male offspring per brood (proportion male = 30%) while logging with progressively greater forest disturbance, shifted the offspring sex ratio towards males (proportion male: moderate = 50%, heavy = 70%). If Ovenbirds in undisturbed forests usually produce female-biased broods, then the production of males as a result of logging may disrupt population viability. We recommend a broad examination of nestling sex ratios in response to anthropogenic disturbance to determine the generality of our findings
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