94 research outputs found
Nonequilibrium dynamics and magnetoviscosity of moderately concentrated magnetic liquids: A dynamic mean--field study
A mean-field Fokker-Planck equation approach to the dynamics of ferrofluids
in the presence of a magnetic field and velocity gradients is proposed that
incorporates magnetic dipole-dipole interactions of the colloidal particles.
The model allows to study the combined effect of a magnetic field and dipolar
interactions on the viscosity of the ferrofluid. It is found that dipolar
interactions lead to additional non-Newtonian contributions to the stress
tensor, which modify the behavior of the non-interacting system. The
predictions of the present model are in qualitative agreement with experimental
results, such as presence of normal stress differences, enhancement and
different anisotropy of magnetoviscous effect and the dependence of the
viscosity on the hydrodynamic volume fraction. A quantitative comparison of the
concentration dependence of the magnetoviscosity shows good agreement with
experimental results for low concentrations.Comment: 12 pages, 5 figure
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Entanglement dynamics at flat surfaces: investigations using multi-chain molecular dynamics and a single-chain slip-spring model
The dynamics of an entangled polymer melt confined in a channel by parallel plates is investigated by Molecular Dynamics (MD) simulations of a detailed, multi-chain model. A Primitive Path Analysis predicts that the density of entanglements remains approximately constant throughout the gap and drops to lower values only in the immediate vicinity of the surface. Based on these observations, we propose a coarse-grained, single-chain slip-spring model with a uniform density of slip-spring anchors and slip-links. The slip-spring model is compared to the Kremer-Grest MD bead-spring model via equilibrium correlation functions of chain orientations. Reasonably good agreement between the single-chain model and the detailed multi-chain model is obtained for chain relaxation dynamics, both away from the surface and for chains whose center of mass positions are at a distance from the surface that is less than the bulk chain radius of gyration, without introducing any additional model parameters. Our results suggest that there is no considerable drop in topological interactions for chains in the vicinity of a single flat surface. We infer from the slip-spring model that the experimental plateau modulus of a confined polymer melt may be different to a corresponding unconfined system even if there is no drop in topological interactions for the confined case
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Hydrodynamics with spin angular momentum from systematic coarse graining: a tutorial example
The derivation of time evolution equations for slow collective variables starting from a micro- scopic model system is demonstrated for the tutorial example of the classical, two-dimensional XY model. Projection operator techniques are used within a nonequilibrium thermodynamics framework together with molecular simulations in order to establish the building blocks of the hydrodynamics equations: Poisson brackets that determine the deterministic drift, the driving forces from the macroscopic free energy and the friction matrix. The approach is rather general and can be applied for deriving the equations of slow variables for a broad variety of systems
Magneto-Permeability Effect in Ferrofluid Flow through Porous Media studied via Multiparticle Collision Dynamics
As more and more promising applications of magnetic nanoparticles in
complicated environments are explored, their flow properties in porous media
are of increasing interest. We here propose a hybrid approach based on the
Multiparticle Collision Dynamics Method extended to porous media via friction
forces and coupled with Brownian Dynamics simulations of the rotational motion
of magnetic nanoparticles' magnetic moment. We simulate flow in planar channels
homogeneously filled with a porous medium and verify our implementation by
reproducing the analytical velocity profile of the Darcy-Brinkman model in the
non-magnetic case. In the presence of an externally applied magnetic field, the
non-equilibrium magnetization and friction forces lead to field-dependent
velocity profiles that result in effective, field-dependent permeabilities. We
provide a theoretical expression for this magneto-permeability effect in
analogy with the magneto-viscous effect. Finally, we study the flow through
planar channels, where only the walls are covered with a porous medium. We find
a smooth crossover from the Poiseuille profile in the center of the channel to
the Brinkman-Darcy flow in the porous layers. We propose a simple estimate of
the thickness of the porous layer based on the flow rate and maximum flow
velocity.Comment: 22 pages, 8 figure
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Equilibrium magnetization and magnetization relaxation of multicore magnetic nanoparticles
Multi-core magnetic nanoparticles show promising features for biomedical applications.
Their magnetic properties, however, are not well-understood to date, so that several ad hoc assumptions are often needed to interpret experimental results.
Here, we present a comprehensive computer simulation study on the effect of dipolar interactions and magnetic anisotropy on the equilibrium magnetization and magnetization relaxation dynamics of monodisperse multi-core magnetic nanoparticles in viscous solvents.
We include thermal fluctuations of the internal N\'eel relaxation via the stochastic Landau-Lifshitz-Gilbert equation coupled to rotational Brownian motion of the cluster.
We find that the effective magnetic moment of the cluster is reduced compared to the non--interacting case due to frustrated dipole-dipole interactions.
Furthermore, the magnetization relaxation is found to proceed in a two--step fashion with a fast initial decay being followed by a long-time relaxation.
For moderate dipolar interaction strengths, the latter can be approximated quite well by an exponential decay with rate given by the sum of the relaxation rates in the immobilized state and the Brownian rotation.
These findings can be helpful for a better interpretation of experimental data obtained from magnetization relaxation measurements
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