973 research outputs found
Percolation and orientational ordering in systems of magnetic nanorods
Based on Monte Carlo (MC) computer simulations we study the structure
formation of a system of magnetic nanorods. Our model particles consist of
fused spheres with permanent magnetic dipole moments, as inspired by recent
experiments. The resulting system behaves significantly different from a system
of hard (non-magnetic) rods or magnetic rods with a single longitudinal dipole.
In particular, we observe for the magnetic nanorods a significant decrease of
the percolation threshold (as compared to non-magnetic rods) at low densities,
and a stabilization of the high-density nematic phase. Moreover, the
percolation threshold is tunable by an external magnetic field.Comment: 11 pages, 12 figure
Feedback-controlled transport in an interacting colloidal system
Based on dynamical density functional theory (DDFT) we consider a
non-equilibrium system of interacting colloidal particles driven by a constant
tilting force through a periodic, symmetric "washboard" potential. We
demonstrate that, despite of pronounced spatio-temporal correlations, the
particle current can be reversed by adding suitable feedback control terms to
the DDFT equation of motion. We explore two distinct control protocols with
time delay, focussing on either the particle positions or the density profile.
Our study shows that the DDFT is an appropriate framework to implement
time-delayed feedback control strategies widely used in other fields of
nonlinear physicsComment: 6 pages, 5 figure
Translational and rotational dynamics in suspensions of magnetic nanorods
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Using computer simulations we investigate the translational and rotational diffusion of dilute suspensions of magnetic nanorods with and without a (homogeneous) external magnetic field. The magnetic rods are represented as spherocylinders with a longitudinal point dipole at their center and length-to-breadth ratios L/D = 3 or L/D = 9. In the absence of a field, the rods tend to form compact clusters with antiparallel ordering and thus behave very differently to dipolar spheres (L/D = 0), which tend to form head-to-tail chains. Furthermore, for rod-like particles the external field tends to destabilize rather than to support cluster formation. We show that these differences in the aggregation behavior have profound consequences not only in static material properties such as the field-induced magnetization and the zero-frequency susceptibility, but also in the dynamics. In particular, for magnetic rods the translational diffusion constant parallel to the field is larger than the perpendicular one, in contrast to the behavior observed for magnetic spheres. Moreover, the rod-like character greatly affects the shape and the density dependence of the single-particle and collective dipole–dipole time correlation functions and their counterparts in the frequency domain
Surface-charge-induced freezing of colloidal suspensions
Using grand-canonical Monte Carlo simulations we investigate the impact of
charged walls on the crystallization properties of charged colloidal
suspensions confined between these walls. The investigations are based on an
effective model focussing on the colloids alone. Our results demonstrate that
the fluid-wall interaction stemming from charged walls has a crucial impact on
the fluid's high-density behavior as compared to the case of uncharged walls.
In particular, based on an analysis of in-plane bond order parameters we find
surface-charge-induced freezing and melting transitions
Crystal structures and freezing of dipolar fluids
We investigate the crystal structure of classical systems of spherical
particles with an embedded point dipole at T=0. The ferroelectric ground state
energy is calculated using generalizations of the Ewald summation technique.
Due to the reduced symmetry compared to the nonpolar case the crystals are
never strictly cubic. For the Stockmayer (i.e., Lennard-Jones plus dipolar)
interaction three phases are found upon increasing the dipole moment:
hexagonal, body-centered orthorhombic, and body-centered tetragonal. An even
richer phase diagram arises for dipolar soft spheres with a purely repulsive
inverse power law potential . A crossover between qualitatively
different sequences of phases occurs near the exponent . The results are
applicable to electro- and magnetorheological fluids. In addition to the exact
ground state analysis we study freezing of the Stockmayer fluid by
density-functional theory.Comment: submitted to Phys. Rev.
Nanotribology of biopolymer brushes in aqueous solution using dissipative particle dynamics simulations: an application to PEG covered liposomes in theta solvent
We undertake the investigation of sheared polymer chains grafted on flat
surfaces to model liposomes covered with polyethylene glycol brushes as a case
study for the mechanisms of efficient drug delivery in biologically relevant
situations, for example, as carriers for topical treatments of illnesses in the
human vasculature. For these applications, specific rheological properties are
required, such as low viscosity at high shear rate to improve the transport of
the liposomes. Therefore non - equilibrium, DPD simulations of polymer brushes
of various length and shear rates are performed to obtain the average viscosity
and friction coefficient of the system as functions of the shear rate and
polymerization degree under theta solvent conditions, and find that the brushes
experience shear thinning at large shear rates.The viscosity and the friction
coefficient are shown to obey scaling laws at high shear rate in theta solvent,
irrespective of the brushes degree of polymerization. These results confirm
recent scaling predictions and reproduce very well trends in measurements of
the viscosity at high shear of red blood cells in a liposome containing medium.Comment: 32 pages, 8 figure
Density functional formalism in the canonical ensemble
Density functional theory, when applied to systems with , is based
on the grand canonical extension of the Hohenberg-Kohn-Sham theorem due to
Mermin (HKSM theorem). While a straightforward canonical ensemble
generalization fails, work in nanopore systems could certainly benefit from
such extension. We show that, if the asymptotic behaviour of the canonical
distribution functions is taken into account, the HKSM theorem can be extended
to the canonical ensemble. We generate -modified correlation and
distribution functions hierarchies and prove that, if they are employed, either
a modified external field or the density profiles can be indistinctly used as
independent variables. We also write down the % -modified free energy
functional and prove that its minimum is reached when the equilibrium values of
the new hierarchy are used. This completes the extension of the HKSM theorem.Comment: revtex, to be submitted to Phys. Rev. Let
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