Rotating hematite cube chains

Recently a two-dimensional chiral fluid was experimentally demonstrated. It was obtained from cubic-shaped hematite colloidal particles placed in a rotating magnetic field. Here we look at building blocks of that fluid by analyzing short hematite chain behavior in a rotating magnetic field. We find equilibrium structures of chains in static magnetic fields and observe chain dynamics in rotating magnetic fields. We find and experimentally verify that there are three planar motion regimes and one where the cube chain goes out of the plane of the rotating magnetic field. In this regime we observe interesting dynamics-the chain rotates slower than the rotating magnetic field. In order to catch up with the magnetic field, it rolls on an edge and through rotation in the third dimension catches up with the magnetic field. The same dynamics is also observable for a single cube when gravitational effects are explicitly taken into account.

Energetically favorable configurations of hematite cube chains

Hematite at room temperature is a weak ferromagnetic material. Its permanent magnetization is three orders smaller than for magnetite. Thus, hematite colloids allow us to explore a different physical range of particle interaction parameters compared to ordinary ferromagnetic particle colloids. In this paper we investigate a colloid consisting of hematite particles with cubic shape. We search for energetically favorable structures in an external magnetic field with analytical and numerical methods and molecular dynamics simulations and analyze whether it is possible to observe them in experiments. We find that energetically favorable configurations are observable only for short chains. Longer chains usually contain kinks which are formed in the process of chain formation due to the interplay of energy and thermal fluctuations as an individual cube can be in one of two alignments with an equal probability.

Equilibrium shapes and stability of magnetic filaments

Equilibrium shapes of magnetic rods and their stability under the action of an applied field are analyzed. The family of shapes is characterized by two magnetoelastic numbers due to the remanent magnetization and paramagnetic susceptibility of the rod. Since in experiments with flexible magnetic rods the ends are usually unfixed and unclamped, their stability is analyzed under these conditions. Solutions of the corresponding eigenvalue problems for particular cases show that under these conditions the equilibrium shapes are unstable.

Instability caused swimming of ferromagnetic filaments in pulsed field

Magnetic filaments driven by external magnetic field are an interesting topic of research in-terms of the possible bio-medical applications. In this paper, we investigated the applicability of using ferromagnetic filaments as micro swimmers both experimentally and numerically. It was found that applying a pulse wave field profile with a duty cycle of 30[Formula: see text] induced experimentally observable swimming, which is similar to the breast stroke of micro algae. Good agreement with numerical simulations was found. Moreover, for stable continuous swimming, an initial filament shape is required to avoid transition to the structurally preferred non-swimming S-like mode.

Magnetic field tuning of mechanical properties of ultrasoft PDMS-based magnetorheological elastomers for biological applications

We report tuning of the moduli and surface roughness of magnetorheological elastomers (MREs) by varying applied magnetic field. Ultrasoft MREs are fabricated using a physiologically relevant commercial polymer, Sylgard™ 527, and carbonyl iron powder (CIP). We found that the shear storage modulus, Young's modulus, and root-mean-square surface roughness are increased by ~41×, ~11×, and ~11×, respectively, when subjected to a magnetic field strength of 95.5 kA m-1. Single fit parameter equations are presented that capture the tunability of the moduli and surface roughness as a function of CIP volume fraction and magnetic field strength. These magnetic field-induced changes in the mechanical moduli and surface roughness of MREs are key parameters for biological applications.

Thermodiffusion anisotropy under a magnetic field in ionic liquid-based ferrofluids

Ferrofluids based on maghemite nanoparticles (NPs), typically 10 nm in diameter, are dispersed in an ionic liquid (1-ethyl 3-methylimidazolium bistriflimide - EMIM-TFSI). The average interparticle interaction is found to be repulsive by small angle scattering of X-rays and of neutrons, with a second virial coefficient A2 = 7.3. A moderately concentrated sample at Φ = 5.95 vol% is probed by forced Rayleigh scattering under an applied magnetic field (up to H = 100 kA m-1) from room temperature up to T = 460 K. Irrespective of the values of H and T, the NPs in this study are always found to migrate towards the cold region. The in-field anisotropy of the mass diffusion coefficient Dm and that of the (always positive) Soret coefficient ST are well described by the presented model in the whole range of H and T. The main origin of anisotropy is the spatial inhomogeneities of concentration in the ferrofluid along the direction of the applied field. Since this effect originates from the magnetic dipolar interparticle interaction, the anisotropy of thermodiffusion progressively vanishes when temperature and thermal motion increase.

Evaluation of Physicochemical Properties of Amphiphilic 1,4-Dihydropyridines and Preparation of Magnetoliposomes

This study was focused on the estimation of the targeted modification of 1,4-DHP core with (1) different alkyl chain lengths at 3,5-ester moieties of 1,4-DHP (C₁₂, C₁₄ and C₁₆); (2) N-substituent at position 1 of 1,4-DHP (N-H or N-CH₃); (3) substituents of pyridinium moieties at positions 2 and 6 of 1,4-DHP (H, 4-CN and 3-Ph); (4) substituent at position 4 of 1,4-DHP (phenyl and napthyl) on physicochemical properties of the entire molecules and on the characteristics of the obtained magnetoliposomes formed by them. It was shown that thermal behavior of the tested 1,4-DHP amphiphiles was related to the alkyl chains length, the elongation of which decreased their transition temperatures. The properties of 1,4-DHP amphiphile monolayers and their polar head areas were determined. The packing parameters of amphiphiles were in the 0.43–0.55 range. It was demonstrated that the structure of 1,4-DHPs affected the physicochemical properties of compounds. “Empty” liposomes and magnetoliposomes were prepared from selected 1,4-DHP amphiphiles. It was shown that the variation of alkyl chains length or the change of substituents at positions 4 of 1,4-DHP did not show a significant influence on properties of liposomes.

3D motion of flexible ferromagnetic filaments under a rotating magnetic field

Ferromagnetic filaments in a rotating magnetic field are studied both numerically and experimentally. The filaments are made from micron-sized ferromagnetic particles linked with DNA strands. It is found that at low frequencies of the rotating field, a filament rotates synchronously with the field and beyond a critical frequency, it undergoes a transition to a three dimensional regime. In this regime the tips of the filament rotate synchronously with the field on circular trajectories in the plane parallel to the plane of the rotating field. The characteristics of this motion found numerically match the experimental data and allow us to obtain the physical properties of such filaments. We also discuss the differences in behaviour between magnetic rods and filaments and the applicability of filaments in mixing.

Magnetically enhancing the Seebeck coefficient in ferrofluids

The influence of the magnetic field on the Seebeck coefficient (Se) was investigated in dilute magnetic nanofluids (ferrofluids) composed of maghemite magnetic nanoparticles dispersed in dimethyl-sulfoxide (DMSO). A 25% increase in the Se value was found when the external magnetic field was applied perpendicularly to the temperature gradient, reminiscent of an increase in the Soret coefficient (S _T, concentration gradient) observed in the same fluids. In-depth analysis of experimental data, however, revealed that different mechanisms are responsible for the observed magneto-thermoelectric and -thermodiffusive phenomena. Possible physical and physico-chemical origins leading to the enhancement of the fluids' Seebeck coefficient are discussed.

Rotating-field-driven ensembles of magnetic particles

Vortex patterns in ensembles of magnetic particles driven by a rotating field are studied. The driving arises due to the lubrication forces between the rotating particles acting in the direction perpendicular to the radius vector between the particles. Since the lubrication forces cannot be equilibrated by the radial forces due to the dipolar attraction and steric repulsion, the ensemble is in a nonequilibrium state. Different regimes are found for the dynamics of the driven ensembles-solid-body rotation at low frequency of the rotating field and stick-slip motion of the external layers of the aggregate with respect to the internal structure as the frequency is increased. The relation obtained for describing the angular velocity of the solid-body rotation is in good agreement with existing experimental data.

Thermodiffusion of citrate-coated γ-Fe2O3 nanoparticles in aqueous dispersions with tuned counter-ions - anisotropy of the Soret coefficient under a magnetic field

Under a temperature gradient, the direction of thermodiffusion of charged γ-Fe2O3 nanoparticles (NPs) depends on the nature of the counter-ions present in the dispersion, resulting in either a positive or negative Soret coefficient. Various counter-ions are probed in finely tuned and well characterized dispersions of citrate-coated NPs at comparable concentrations of free ionic species. The Soret coefficient ST is measured in stationary conditions together with the mass-diffusion coefficient Dm using a forced Rayleigh scattering method. The strong interparticle repulsion, determined by SAXS, is also attested by the increase of Dm with NP volume fraction Φ. The Φ-dependence of ST is analyzed in terms of thermophoretic and thermoelectric contributions of the various ionic species. The obtained single-particle thermophoretic contribution of the NPs (the Eastman entropy of transfer ŝNP) varies linearly with the entropy of transfer of the counter-ions. This is understood in terms of electrostatic contribution and of hydration of the ionic shell surrounding the NPs. Two aqueous dispersions, respectively, with ST > 0 and with ST < 0 are then probed under an applied field H[combining right harpoon above], and an anisotropy of Dm and of ST is induced while the in-field system remains monophasic. Whatever the H[combining right harpoon above]-direction (parallel or perpendicular to the gradients and ), the Soret coefficient is modulated keeping the same sign as in zero applied field. In-field experimental determinations are well described using a mean field model of the interparticle magnetic interaction.

Salmon fibrinogen and chitosan scaffold for tissue engineering: in vitro and in vivo evaluation

3D fibrous scaffolds have received much recent attention in regenerative medicine. Use of fibrous scaffolds has shown promising results in tissue engineering and wound healing. Here we report the development and properties of a novel fibrous scaffold that is useful for promoting wound healing. A scaffold made of salmon fibrinogen and chitosan is produced by electrospinning, resulting in a biocompatible material mimicking the structure of the native extracellular matrix (ECM) with suitable biochemical and mechanical properties. The scaffold is produced without the need for enzymes, in particular thrombin, but is fully compatible with their addition if needed. Human dermal fibroblasts cultured on this scaffold showed progressive proliferation for 14 days. Split-thickness experimental skin wounds treated and untreated were compared in a 10-day follow-up period. Wound healing was more effective using the fibrinogen-chitosan scaffold than in untreated wounds. This scaffold could be applicable in various medical purposes including surgery, tissue regeneration, burns, traumatic injuries, and 3D cell culture platforms.

Gravity effects on mixing with magnetic micro-convection in microfluidics

Mixing remains an important problem for the development of successful microfluidic and lab-on-a-chip devices, where simple and predictable systems are particularly interesting. One is magnetic micro-convection, an instability happening on the interface of miscible magnetic and non-magnetic fluids in a Hele-Shaw cell under applied field. Previous work proved that the Brinkman model quantitatively explains the experiments. However, a gravity-caused convective motion complicated the tests. Here we first improve the experimental system to exclude the parasitic convection. Afterwards, we experimentally observe the magnetic micro-convection, by finding and quantifying how gravity and laminar flow stabilizes the perturbations that create it. Accordingly, we improve our theoretical model for a zero-flow condition and perform a linear analysis. Two dimensionless quantities --magnetic and gravitational Rayleigh numbers-- are used to compare the experimental observations and theoretical predictions for the critical field of instability and the characteristic size of the emerging pattern. Finally, we discuss the conditions at which gravity plays an important role in microfluidic systems.

Dynamics of a flexible ferromagnetic filament in a rotating magnetic field

Flexible magnetic filaments have garnered considerable attention as prospective materials for the creation of different microdevices. We describe a theoretical model of a ferromagnetic filament and derive its equations of motion by variational techniques. The numerical algorithm used to solve the filament dynamics in magnetic fields of different configurations is described. It is found that in a rotating field the filament transitions between synchronous and asynchronous regimes with respect to the rotating field, similarly to a rigid magnetic dipole. The mean angular velocity of the filament is well described by a relation valid for a rigid magnetic dipole with quantitative differences attributable to the flexibility of the filament.

Synchronized rotation in swarms of magnetotactic bacteria

Self-organizing behavior has been widely reported in both natural and artificial systems, typically distinguishing between temporal organization (synchronization) and spatial organization (swarming). Swarming has been experimentally observed in systems of magnetotactic bacteria under the action of external magnetic fields. Here we present a model of ensembles of magnetotactic bacteria in which hydrodynamic interactions lead to temporal synchronization in addition to the swarming. After a period of stabilization during which the bacteria form a quasiregular hexagonal lattice structure, the entire swarm begins to rotate in a direction opposite to the direction of the rotation of the magnetic field. We thus illustrate an emergent mechanism of macroscopic motion arising from the synchronized microscopic rotations of hydrodynamically interacting bacteria, reminiscent of the recently proposed concept of swarmalators.

Hydrodynamics with spin in bacterial suspensions

We describe a kind of self-propelling motion of bacteria based on the cooperative action of rotating flagella on the surface of bacteria. Describing the ensemble of rotating flagella in the framework of the hydrodynamics with spin, the reciprocal theorem of Stokesian hydrodynamics is generalized accordingly. The velocity of the self-propulsion is expressed in terms of the characteristics of the vector field of flagella orientation and it is shown that the unusually high velocities of Thiovulum majus bacteria may be explained by the cooperative action of the rotating flagella. The expressions obtained enable us to estimate the torque created by the rotary motors of the bacterium and show quantitative agreement with the existing experimental data.

Magnetic microrods as a tool for microrheology

Dynamics of superparamagnetic rods in crossed constant and alternating magnetic fields as a function of field frequency are studied and it is shown that above the critical value of the amplitude of the alternating field the rod oscillates around the direction of the alternating field. The fit of the experimentally measured time dependence of the mean orientation angle of the rod allows one to determine the ratio of magnetic and viscous torques which act on the rod. The protocol of microrheological measurements consists of recording the dynamics of the orientation of the rod when the magnetic field is applied at an angle to the rod and observing its relaxation due to the accumulated elastic energy after the field is switched off. The microrheological data obtained are in reasonable agreement with the macrorheological measurements.

Poiseuille flow of a Quincke suspension

The controversy of models of dielectric particle suspensions with antisymmetric stress, which predict a nonphysical cusp of the velocity profile in plane Poiseuille flow under the action of the electrical field, is resolved. In the mean-field approximation, the nonlinear kinetic equation is derived for coupled due to the flow translational and rotational motion of the particles. By its numerical solution, it is shown that the velocity profile is smeared due to the translational diffusion of the particles with opposite directions of rotation. The obtained results for the velocity profiles and flow rates as a function of the electric field strength are in qualitative agreement with the existing experimental results.

Relaxation of polar order in suspensions with Quincke effect

The Quincke effect--spontaneous rotation of dielectric particles in a liquid with low conductivity under the action of an electric field--is considered. The distribution functions for the orientation of particle rotation planes are introduced and a set of nonlinear kinetic equations is derived in the mean field approximation considering the dynamics of their orientation in the flow induced by rotating particles. As a result the nonequilibrium phase transition to the polar order, if the concentration of the particles is sufficiently high, is predicted and the condition of the synchronization of particle rotations is established. Two cases are considered: the layer of the Quincke suspension with one free boundary and the ensemble of the particles rolling on the solid wall under the action of a torque in an electric field. It is shown that in both cases the synchronization of particle rotations occurs due to the hydrodynamic interactions. In the limit of small spatial nonhomogeneity a set of nonlinear partial differential equations for the macroscopic variables--the concentration and the director of the polar order--is derived from the kinetic equation. Its properties are analyzed and compared with available recent experimental results.

Polyelectrolyte properties of filamentous biopolymers and their consequences in biological fluids

Anionic polyelectrolyte filaments are common in biological cells. DNA, RNA, the cytoskeletal filaments F-actin, microtubules, and intermediate filaments, and polysaccharides such as hyaluronan that form the pericellular matrix all have large net negative charge densities distributed over their surfaces. Several filamentous viruses with diameters and stiffnesses similar to those of cytoskeletal polymers also have similar negative charge densities. Extracellular protein filaments such collagen, fibrin and elastin, in contrast, have notably smaller charge densities and do not behave as highly charged polyelectrolytes in solution. This review summarizes data that demonstrate generic counterion-mediated effects on four structurally unrelated biopolymers of similar charge density: F-actin, vimentin, Pf1 virus, and DNA, and explores the possible biological and pathophysiological consequences of the polyelectrolyte properties of biological filaments.

Three-dimensional dynamics of a particle with a finite energy of magnetic anisotropy in a rotating magnetic field

A model of a single ferromagnetic particle with a finite coupling energy of the magnetic moment with the body of the particle is formulated, and regimes of its motion in a rotating magnetic field are investigated. Regimes are possible that are synchronous and asynchronous with the field. In a synchronous regime the easy axis of the particle is in the plane of the rotating magnetic field at low frequencies (a planar regime) and on the cone at high frequencies (a precession regime). The stability of these regimes is investigated, and it is shown that the precession regime is stable for field strengths below the critical value. In a particular range of field strength value, irreversible jumps of the magnetic moment take place in the asynchronous planar regime. The stability of this regime is investigated, and it is shown that it is stable for field strengths above the critical value, which depends on the frequency. The implications of these results for the energy dissipation in a rotating field are analyzed, and it is shown that the maximum of the heat production near the transition to the synchronous regime is flattened out by the transition to the precession regime.

Dynamics of anisotropic superparamagnetic particles in a precessing magnetic field

A bifurcation diagram for anisotropic magnetic particles in a precessing magnetic field is analyzed. It is found that a synchronous regime in the case of a prolate particle exists for all precession angles of the magnetic field if the frequency of field rotation is below some critical value. An oblate particle has a synchronous regime in a limited range of precession angle. To understand the flow of suspensions of these particles in precessing fields, it is essential to take into account the differing dynamics of prolate and oblate particles.

Parametric excitation of bending deformations of a rod by periodic twist

A model of a semiflexible magnetic filament with magnetization frozen in the direction perpendicular to the tangent of its center line is formulated. It is shown that if the rod is magnetized at its ends in opposite directions, an AC magnetic field causes parametric excitation of bending deformations. Neutral curves of parametric excitation are calculated both analytically and numerically. The shapes arising upon parametric excitation of bending deformations are chiral. Periodic rotation of the chiral filament due to nonhomogeneous twist in a nonhomogeneous AC field causes its unidirectional motion.

Bilayer properties of giant magnetic liposomes formed by cationic pyridine amphiphile and probed by active deformation under magnetic forces

We synthesize giant magnetic liposomes by a reverse-phase evaporation method (REV) using a new self-assembling Cationic Pyridine Amphiphile (CPA) derived from 1,4-dihydropyridine as liposome-forming agent and a magnetic ferrofluid based on γ-Fe(2)O(3) nanoparticles. Having in view the potential interest of CPA in targeted transport by magnetic forces, the mechanical elastic properties of such bilayers are here directly investigated in vesicles loaded with magnetic nanoparticles. Bending elastic modulus K(b) ∼ 0.2 to 5k(B)T and pre-stress τ ∼ 3.2 to 12.10(-6) erg/cm(2) are deduced from the under-field deformations of the giant magnetic liposomes. The obtained K(b) values are discussed in terms of A. Wurgers's theory.

Coupled stochastic dynamics of magnetic moment and anisotropy axis of a magnetic nanoparticle

An algorithm is developed for numerical simulation of coupled stochastic dynamics of magnetic moment and magnetic anisotropy axis of a nanoparticle. Time-correlation functions of the magnetic moment and its components longitudinal and transverse to the magnetic anisotropy axis are calculated by averaging along the stochastic trajectory. The longitudinal and transverse relaxation times are found by fitting the time correlation functions. Existing theoretical relations derived by the effective field approach in the limit of small fields are confirmed. The time-correlation functions of magnetic moments of nanoparticles in dependence on their properties are calculated numerically for arbitrary large magnetic fields and it is shown that they may be approximated by a sum of several exponentials. These results are applied for the calculation of relaxivity parameters of nuclear magnetic resonance (NMR) imaging in dependence on the field strength.

Magnetic dipole with a flexible tail as a self-propelling microdevice

By numerical simulations, it is illustrated that a magnetic dipole with a flexible tail behaves as a swimmer in AC magnetic fields. The behavior of the swimmer on long time scales is analyzed and it is shown that due to the flexibility of the tail two kinds of torques arise, the first is responsible for the orientation of the swimmer perpendicularly to the AC field and the second drags the filament in the direction of the rotating field. Due to this, circular trajectories of the swimmer are possible; however, these are unstable. The self-propulsion velocity of this swimmer is higher than the velocities of other magnetic microdevices for comparable values of the magnetoelastic number.

Dynamics of superparamagnetic filaments with finite magnetic relaxation time

In this paper we formulate a model of superparamagnetic filaments with internal dissipative torques due to the action of a rotating magnetic field. It is shown that spirals are formed at both ends of the filament due to the action of the internal torques. These spirals propagate to the center of the filament and collide, forming a compact cluster that rotates in accordance with the rotating magnetic field. These results are in agreement with recent experiments with chains of superparamagnetic beads in a rotating magnetic field.

Ferromagnetic microswimmer

The self-propelling motion of the flexible ferromagnetic swimmer is described. Necessary symmetry breaking is achieved by the buckling instability at field inversion. The characteristics of self-propulsion are in good agreement with the numerical calculations of the Floquet multipliers for the ferromagnetic filament under the action of ac magnetic field. In the low frequency range the power stroke of self-propelling motion is similar to that used by the unicellular green algae chlamydomonas and in the high frequency region the self-propulsion is due to the undulation waves propagating from the free ends perpendicularly to ac magnetic field.

Overdamped dynamics of paramagnetic ellipsoids in a precessing magnetic field

We report on the dynamical behavior of paramagnetic ellipsoidal particles dispersed in water and floating above a flat plane when subjected to an external precessing magnetic field. When the magnetic field and the long axis of the particles are on the same plane, two clear regimes are distinguished in which the particles follow the magnetic modulation synchronously or asynchronously. Both regimes are also observed when the field precesses at an angle theta<90 degrees with respect to the normal to the confining plane, while the transition frequency increases with decreasing precession angle. We combine experimental observations with a theoretical model to characterize the particle dynamics. The possibility to control and/or reorient microscopic elongated particles by changing the frequency or strength of the applied field makes them suitable in microfluidic devices such as microgates for microchannels or active fluid mixers when placed close to channel junctions.

Electrostatic contribution to the surface pressure of charged monolayers containing polyphosphoinositides

Structural and functional studies of lateral heterogeneity in biological membranes have underlined the importance of membrane organization in biological function. Most inquiries have focused on steric determinants of membrane organization, such as headgroup size and acyl-chain saturation. This manuscript reports a combination of theory and experiment that shows significant electrostatic contributions to surface pressures in monolayers of phospholipids where the charge spacing is smaller than the Bjerrum length. For molecules with steric cross sections typical of phospholipids in the cell membrane (approximately 50 A(2)), only polyphosphoinositides achieve this threshold. The most abundant such lipid is phosphatidylinositol bisphosphate, which has between three and four charged groups at physiological conditions. Theory and experiment show that surface pressure increases linearly with phosphatidylinositol bisphosphate net charge and reveal crossing of high and low ionic strength pressure-area isotherms, due to opposing effects of ionic strength in compressed and expanded monolayers. Theory and experiment show that electrostatic effects are negligible for monolayers of univalent lipids, emphasizing the unique importance of electrostatic effects for lateral organization of polyphosphoinositides. Quantitative differences between theory and experiment suggest that attractive interactions between polyphosphoinositides, possibly mediated by hydrogen bonding, can lessen the effect of electrostatic repulsions.

Elastic properties of DNA linked flexible magnetic filaments

Elastic properties of magnetic filaments linked by DNA in solutions of univalent and bivalent salts with different pH values are investigated through their deformation in an external field. A strong dependence of the bending modulus in bivalent salt solution on the pH is shown. Experimental results are interpreted on the basis of the magnetic elastica.

Magnetic elastica

Magnetic elastica with spontaneous magnetization and superparamagnetic properties are considered. Obtained solutions illustrate the characteristic transformations of their shapes as spontaneous magnetization increases. Solutions are selected on the basis of the stability analysis and results of numerical simulations. A different mechanism of the magnetic relaxation in suspension of ferromagnetic filaments by migration of a loop with antiparallel to the field magnetization is predicted.

Dynamics of magnetotactic bacteria in a rotating magnetic field

The dynamics of the motile magnetotactic bacterium Magnetospirillum gryphiswaldense in a rotating magnetic field is investigated experimentally and analyzed by a theoretical model. These elongated bacteria are propelled by single flagella at each bacterial end and contain a magnetic filament formed by a linear assembly of approximately 40 ferromagnetic nanoparticles. The movements of the bacteria in suspension are analyzed by consideration of the orientation of their magnetic dipoles in the field, the hydrodynamic resistance of the bacteria, and the propulsive force of the flagella. Several novel features found in experiments include a velocity reversal during motion in the rotating field and an interesting diffusive wandering of the trajectory curvature centers. A new method to measure the magnetic moment of an individual bacterium is proposed based on the theory developed.

Counterion-mediated attraction and kinks on loops of semiflexible polyelectrolyte bundles

The formation of kinks in a loop of bundled polyelectrolyte filaments is analyzed in terms of the thermal fluctuations of charge density due to polyvalent counterions adsorbed on the polyelectrolyte filaments. It is found that the counterion-mediated attraction energy of filaments depends on their bending. By consideration of curvature elasticity energy and counterion-mediated attraction between polyelectrolyte filaments, the characteristic width of the kink and the number of kinks per loop is found to be in reasonable agreement with existing experimental data for rings of bundled actin filaments.

Nonlinear dynamics of semiflexible magnetic filaments in an ac magnetic field

Flexible spontaneously magnetized filaments exist in the living world (magnetotactic bacteria) and arise in magnetic colloids with large magnetodipolar interaction parameter. We demonstrate that these filaments possess variety of novel nonlinear phenomena in an ac magnetic field: orientation of the filament in the direction perpendicular to the field and the development of the oscillating U-like shapes, which presumably can lead to the formation of rings of magnetic filaments. It is found that these phenomena are determined by the development of the localized boundary modes of the filament deformation. We have illustrated by qualitative estimates that the phenomena found may be useful for insight into the complex pattern formation phenomena in ensembles of magnetic particles under the action of an ac magnetic field.

Nucleation and collapse of the superconducting phase in type-I superconducting films

The phase transition between the intermediate and normal states in type-I superconducting films is investigated using magneto-optical imaging. Magnetic hysteresis with different transition fields for collapse and nucleation of superconducting domains is found. This is accompanied by topological hysteresis characterized by the collapse of circular domains and the appearance of lamellar domains. Magnetic hysteresis is shown to arise from supercooled and superheated states. Domain-shape instability resulting from long-range magnetic interaction accounts well for topological hysteresis.

Dynamics of an active magnetic particle in a rotating magnetic field

The motion of an active (self-propelling) particle with a permanent magnetic moment under the action of a rotating magnetic field is considered. We show that below a critical frequency of the external field the trajectory of a particle is a circle. For frequencies slightly above the critical point the particle moves on an approximately circular trajectory and from time to time jumps to another region of space. Symmetry of the particle trajectory depends on the commensurability of the field period and the period of the orientational motion of the particle. We also show how our results can be used to study the properties of naturally occurring active magnetic particles, so-called magnetotactic bacteria.

Dynamic fluctuations of dipolar semiflexible filaments

On the basis of the model of a flexible magnetic filament, the characteristics of their thermal fluctuations are considered. The crossover of the time dependence of the mean quadratic displacement from t(3/4) to t(1/2) at the magnetic field increase is found. Two characteristic mechanisms of the magnetization relaxation time distribution--straightening of the thermal undulations and excitation of the bending modes of the free ends under the action of an ac magnetic field--are described. In both cases, the characteristic scaling law omega(-3/4) of the magnetic susceptibility in a high-frequency range is found.

Bending of flexible magnetic rods

The flexible inextensible magnetic rod model is applied for the study of bending and buckling deformations of the paramagnetic particle chains linked by polymer molecules. It is shown that the existing experimental results can be reasonably well described by this model which takes into account the normal magnetic forces arising at chain bending deformation. By matching the experimentally observed shapes with our numerical simulation results different physical properties of the linked paramagnetic particle chains are determined.

Bidirectional random motion driven by globally coupled noisy active elements in an electric field

The assembly of the insulating Brownian particles globally coupled due to the macroscopic flow of the liquid with low conductivity has transitions between the states of random motion and random bidirectional and unidirectional motion. The threshold values of the parameters for the transition to random bidirectional motion is found by the effective field method and correspond to those found by Brownian dynamics. The behavior of the assembly is similar to the behavior of different active multistable systems.

Dynamics of a flexible magnetic chain in a rotating magnetic field

The model of an elastic magnetic rod is applied for a study of a behavior of the flexible magnetic particle chain in a rotating magnetic field. By numerical simulation it is shown that behavior of a flexible magnetic chain is characterized by the existence of a critical frequency beyond which the dynamics of the rod is periodic with subsequent stages of bending and straightening. The value of the critical frequency found is explained by a simple model. Below the critical frequency the chain is bent and rotates synchronously with a field. It is illustrated that in particular cases the considered model reproduces phenomena observed experimentally and numerically for the magnetic particle chains in magnetorheological suspensions. It is emphasized that the present approach gives the general framework for the description of different phenomena in magnetorheological suspensions.

Bistability and "negative" viscosity for a suspension of insulating particles in an electric field

It is shown that a suspension of insulating particles in a liquid with low conductivity possesses bistability and has a "negative" effective viscosity effect in the electric field due to internal rotations. By Brownian dynamics simulation it has been found that thermal fluctuations of the angular velocity of particles in this bistable system can have a large effect on the viscosity of the suspension.

Dynamics of an elongated magnetic droplet in a rotating field

A model is proposed for the dynamics of an elongated droplet under the action of a low frequency rotating magnetic field. This model determines a set of critical frequencies at which the transitions to more complex bent shapes take place. These transitions occur through propagation of jumps of the droplet's axial tangent angle described by a nonlinear singularly perturbed partial differential equation with the intrinsic viscosity of the droplet playing the regularizing role.

Magnetic-field-induced anisotropic curvature elasticity of a vesicle membrane containing magnetic polyions

Interaction between a charged membrane and the electrolyte solution containing magnetic polyions is considered. A self-magnetic field, which arises due to the nonhomogeneous magnetic particle distribution near a charged membrane increases the effective charge screening length for the parts of a membrane normal to a magnetic field. The anisotropy of elastic properties of a membrane depending on the screening length is calculated on the basis of the curvature expansion. It is shown that due to diminishing of the spontaneous curvature for the parts of a membrane normal to a magnetic field there are two competing mechanisms of the ferrovesicle shape transformation under the influence of a magnetic field-the formation of a prolate shape orientated along a field due to the diminishing action of the demagnetizing field energy and the deformation to a oblate shape due to the decrease in the spontaneous curvature of the parts of a membrane normal to a field.

Electrohydrodynamic instabilities and orientation of dielectric ellipsoids in low-conducting fluids

We study the dynamics of an ellipsoidal particle in a weakly conducting dielectric liquid when submitted to a dc electric field. At low field intensities, the particle long axis is aligned in the field direction. When the field strength is increased, we show that, depending on the initial orientation of the particle, there exist two stable orientations: the one with the long axis parallel to the field direction remains possible while a spinning state with the long axis perpendicular to the field appears. This last striking orientation is due to the finite Maxwell-Wagner polarization relaxation time. For sufficiently high field intensities, each state loses its stability and the particle dynamics becomes chaotic. Those conclusions from the theoretical model are supported by experimental observations.