Table of contents

The 11th International Conference on Magnetic Fluids (ICMF 11) was held in Košice, Slovakia between 23–27 July 2007. Attendance at the conference was high and its motivation was in line with the ten previous ICMF conferences organized in Udine, Orlando, Bangor, Sendai-Tokyo, Riga, Paris, Bhavnagar, Timisoara, Bremen and Guarujá. The conference in Slovakia reflected the scientific community's enthusiasm and worldwide support, with 256 participants, from 30 countries attending.The main objective of ICMF 11 was to promote progress and knowledge in the field of magnetic fluids regarding their chemistry, physical and magnetic properties, heat and mass transfer, surface phenomena, as well as their technological and biomedical applications. As research on magnetic fluids is essentially interdisciplinary, experts from related areas were invited to present their contributions with a view to increasing knowledge in the field and highlighting new trends. Submitted communications were refereed by members of the Scientific Organizing Committee and abstracts were assembled in a book of abstracts. Participants presented 180 posters in two poster sessions and 56 oral presentations. All presentations contributed to a greater understanding of the area, and helped to bridge the gap between physics, chemistry, technology, biology and medical sciences. Contributions to this conference are presented in 115 scientific papers, with some published in Journal of Physics: Condensed Matter and the rest in Magnetohydrodynamics.

The organization of the conference was made possible by generous support from the Institute of Experimental Physics and Institute of Geotechnics of the Slovak Academy of Sciences, the University of Pavol Jozef Šafárik and the Slovak Physical Society. Financial support from Ferrotec, Cryosoft Ltd, Mikrochem, Liquids Research Ltd, Askony and US Steel Košice, is also gratefully acknowledged.

Stable suspensions of superparamagnetic iron oxide nanoparticles in water (water-based ferrofluids) were prepared using citric acid (CA) as a surfactant. The influences of different factors on the amount of nanoparticles in a stable suspension were systematically studied. These factors, including the temperature, the pH value and the concentration of CA applied during the adsorption of the CA onto the nanoparticles and during their suspension in water, were evaluated. The highest content of nanoparticles in a stable suspension was obtained when the CA was absorbed at pH values of around 5.2, where two carboxyl groups are dissociated, and when the nanoparticles were suspended at a pH of around 10, where all three carboxyl groups of the CA are in a dissociated state.

This work deals with the partitioning of the cationic amphiphilic drug, propranolol, in the coating of so-called magnetoliposomes (MLs), which consist of nanometre-sized, magnetizable iron oxide cores covered with a phospholipid bilayer. MLs of two types were used: either the ML coat consisted entirely of anionic dimyristoylphosphatidylglycerol, or it was mixed with zwitterionic dimyristoylphosphatidylcholine in a 5/95 molar ratio. To separate sorbed from non-sorbed propranolol, high-gradient magnetophoresis was used. The sorption profiles clearly show that electrostatic interactions play a key role in the sorption process as drug incorporation in the ML coat was favoured by increasing the anionic character of the ML envelope and by reducing the salt concentration of the medium. Also, upon drug binding some phospholipid molecules were expelled from the ML coat. The observations may be of relevance in the biomedical field, i.e. in the development of ML-based, intracellular theranostics.

Magnetite nanoparticles were coated with surfactant double layers in order to prepare water based magnetic fluids (MFs). The effects of head group (sulfonate, carboxylate) and alkyl chain length (11–17 C atoms) and the combination of surfactants were studied. Adsorption, dynamic light scattering (DLS) and electrophoretic mobility measurements were performed. The quantity of surfactant varied between 0.3 and 0.5 g, i.e. their specific amount ranges over 1.5–2 mmol g⁻¹ magnetite in MFs. The adsorption isotherm of Na oleate on magnetite proved the double layer formation with 2 mmol g⁻¹ saturation value in good harmony with the empirical doses. The effect of diluting MFs, pH and salt concentration was studied. The pH-dependent stability and the salt tolerance of MFs were different owing to the dissociation of the outermost hydrophilic groups and the hydrophobic interactions scaling with the alkyl chain length of surfactant. The hydrophobic interactions are favored only for oleic and myristic acid double layers. In these MFs, aggregation cannot be observed even in fairly dilute systems up to the physiological salt concentration around neutral pH 6–8 favored in biomedical application. The stable oleic and myristic acid double layers can hinder effectively the aggregation of magnetite particles due to the combined steric and electrostatic stabilization.

This paper describes an electrochemical method which permits us to transform solid metals (cobalt, iron or nickel) into nanoparticles. An electrolysis cell is made, the anode being a metal bar and the cathode a mercury layer. Magnetic nanoparticles are obtained in one step by electroreduction of mercury. Electrolysis is performed in an aqueous medium at pH above 6 in order to avoid the reduction of protons. The magnetic nanoparticles obtained are kept in mercury and can be recovered in an organic solvent.

In this paper, the synthesis of core–shell particles (i.e. temperature-sensitive ferrite (TSF) covered with silica) has been investigated. At first, TSF (mean diameter of 10 nm) was prepared by the coprecipitation method in an alkaline solution. Then, silica coating on the TSF surface was carried out by the controlled hydrolysis and condensation of tetraethyl orthosilicate (TEOS). The core–shell particles were formed by a surface precipitation procedure using TSF nanoparticles as a core material. The particles of silica were formed and these particles were then absorbed on the TSF nanoparticles. The coating procedure was described and explained by calculating the potential energies of interaction between the TSF and SiO₂ nanoparticles, according to the Derjarguin–Landau–Verwey–Overbeck (DLVO) theory. The coating process was found to be influenced by the pH and concentration of the TEOS precursor. The thickness of the silica layer on TSF cores was observed by means of transmission electron microscopy (TEM). The results showed that the optimum thickness of the SiO₂ layer on TSF core particles was obtained at pH 7.5, while the TEOS concentration was kept at 9 mM.

Hydrogels have the potential for providing drug delivery systems with long release rates. The polymerization kinetics and the physical entrapment capacity of photo-cross-linked hydroxyethyl methacrylate hydroxyethylstarch hydrogels are investigated with a non-destructive method. For this purpose, superparamagnetic nanoparticles as replacements for biomolecules are used as probes. By analyzing their magnetic relaxation behavior, the amounts of physically entrapped and mobile nanoparticles can be determined. The hydrogels were loaded with five different concentrations of nanoparticles. Different methods of analysis of the relaxation curves and the influence of the microviscosity are discussed. This investigation allows one to optimize the UV light irradiation time and to determine the amount of physically entrapped nanoparticles in the hydrogel network. It was found that the polymerization kinetics is faster for decreasing nanoparticle concentration but not all nanoparticles can be physically entrapped in the network.

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.

The after-effect function, b(t), describes how the magnetization of a dissipative magnetic fluid decreases with time when a polarizing field, H, is suddenly removed. It is shown that with increasing H, the rate of decay of b(t) increases and also that the area, , under each decay curve decreases. Here we investigate the significance of this and by means of a simple model, show that the normalized function, B/b(0), is in fact equal to the Debye relaxation time τ_D. The results of applying the model to theoretically generated data and also to data obtained from a magnetic fluid sample are presented.

We observe the dynamics of waves propagating on the surface of a ferrofluid under the influence of a spatially and temporally modulated field. In particular, we excite plane waves by applying a traveling lamellar modulation of the magnetization. By means of this external driving, both the wavelength and the propagation velocity of the waves can be controlled. The amplitude of the excited waves exhibits a resonance phenomenon similar to that of a forced harmonic oscillator. Its analysis reveals the dispersion relation of the free surface waves, from which the critical magnetic field for the onset of the Rosensweig instability can be extrapolated.

The dielectric properties (permittivity, loss factor, dielectric breakdown strength) of magnetic liquids were investigated. The magnetic liquids were composed of magnetite particles coated with oleic acid as surfactant and dispersed in transformer oil. To determine their dielectric properties they were subjected to a uniform magnetic field at high alternating electric fields up to 14 MV m⁻¹. Nearly constant permittivity of magnetic liquid with particle volume concentration Φ = 0.0019 as a function of electric field was observed. Magnetic liquids with concentrations Φ = 0.019 and 0.032 showed significant changes of permittivity and loss factor dependent on electric and magnetic fields. The best concentration of magnetic fluid was found at which partial current impulse magnitudes were the lowest. The breakdown strength distribution of the magnetic liquid with Φ = 0.0025 was fitted with the Duxbury–Leath, Weibull and Gauss distribution functions.

The influence of polydispersity on the magnetization of two-dimensional dipolar discs with short-range repulsive interaction is studied by means of Monte Carlo simulations and a high field approximation perturbation theory. Within the framework of perturbation theory an analytical expression is derived for the magnetization of monodisperse and polydisperse systems. The theoretical predictions are in good agreement with the corresponding Monte Carlo simulation data.

The heat capacities of ferrofluids are investigated using a thermodynamic perturbation theory approach and the NVT and NpT Monte Carlo simulation methods. The systems studied are considered as one-, three-, and five-component dipolar mixtures modeled by the Stockmayer interaction potential. The isochoric and isobaric heat capacities are calculated and compared with the data determined for a monodisperse equivalent of the system.

Theories and simulations have demonstrated that field-induced dipolar chains affect the static magnetic properties of ferrofluids. Experimental verification, however, has been complicated by the high polydispersity of the available ferrofluids, and the morphology of the dipolar chains was left to the imagination. We now present the concentration- and field-dependent magnetization of particularly well-defined ferrofluids, with a low polydispersity, three different average particle sizes, and with dipolar chains that were imaged with and without magnetic field using cryogenic transmission electron microscopy. At low concentrations, the magnetization curves obey the Langevin equation for noninteracting dipoles. Magnetization curves for the largest particles strongly deviate from the Langevin equation but quantitatively agree with a recently developed mean-field model that incorporates the field-dependent formation and alignment of flexible dipolar chains. The combination of magnetic results and in situ electron microscopy images provides original new evidence for the effect of dipolar chains on the field-dependent magnetization of ferrofluids.

The effects of the intrinsic dipolar magnetic field on the energy levels and charge density distribution of a spherical magnetic semiconductor nanoparticle have been investigated in the framework of quantum mechanics using the finite element method. It was found that the dipolar magnetic field not only removes the degeneracy of the energy levels, resulting in a redistribution of carriers, but also directly changes the charge density distribution, leading to a modification of the surface charge density with a strong influence upon the colloidal stability. These effects strongly depend on both the nanoparticle magnetization value and the nanoparticle size. The bigger the nanoparticles, the larger the effects of the intrinsic dipolar magnetic field upon the charge density distribution.

Mn_xZn_1−xFe₂O₄-based magnetic fluids with x = 0.1–0.9 are synthesized by coprecipitation. The samples are heated in a radio frequency (rf) magnetic field using an rf generator at different powers, and the temperature is measured as function of time using an optical thermometer. The heating effect of the dispersed magnetic nanoparticles is proportional to the imaginary part of the dynamic magnetic susceptibility of the ferrofluid, a quantity that depends on the temperature through the magnetization of the ferrite nanoparticles and the Néel or Brownian relaxation times, respectively. We propose an extrapolation method to actuate the Curie temperatures of the dispersed magnetic nanoparticles. By means of appropriate fitting functions for (dT/dt) versus T for both the heating and the cooling process, we deduce the Curie temperature of the samples under investigation. For Mn_xZn_1−xFe₂O₄-based magnetic nanoparticles the Curie temperatures decrease with increasing Zn content. They turn out to be lower than the literature values for bulk Mn_xZn_1−xFe₂O₄, a phenomenon which is generally observed for phase transitions of nanocrystalline materials.

Magnetically induced elongation of magnetic nanocomposite micelles is observed microscopically. The superparamagnetic particles of double-surfacted water based ferrofluid are incorporated in spherical micelles of cetyltrimethyl ammonium bromide (CTABr) mixed with sodium salicylate salt (NaSal). Under the application of an external magnetic field these spherical magnetic micelles deformed to ellipsoids. The shape distortion occurs instantaneously and disappears when the external field is removed. This magnetodeformational effect is analyzed using linear magnetization and Hookean elasticity.

The ultrasonic propagation velocity and attenuation in a magnetic fluid subjected to magnetic field are measured precisely. Various characteristic properties of ultrasonic propagation in magnetic fluid such as hysteresis and anisotropy are observed. These results show that the ultrasonic propagation velocity and attenuation are dependent upon the intensity and the length of time for which the magnetic field is applied. When the magnetic field is applied, some of the magnetic particles in the magnetic fluid form clustering structures that influence ultrasonic propagation in a magnetic fluid. Our results indicate that the inner structure of a magnetic fluid can be analysed experimentally and we discuss the application of this non-contact inspection of the clustering structures in a magnetic fluid by ultrasonic techniques.

In this paper the influence of temperature on the electrical conductivity of a ferromagnetic gel is investigated. The material used was poly(dimethyl siloxane) (PDMS) gel which contained randomly distributed magnetite nanosized particles. The electrical conductivity was measured by means of the two-point dc method. During the heating of the PDMS in the temperature range of 295–460 K the electrical conductivity increased from about 2 × 10⁻¹² to 2 × 10⁻⁸ S m⁻¹. A study of the current–temperature dependence obtained during subsequent heating runs revealed two subranges of temperature characterized by different activation energies. The presence of these subranges could be explained either by the liberation of two different types of charge carrier or by the increase in the degree of polymer cross-linking. In the upper temperature subrange (420–460 K) both types of charge carrier probably contribute to the electrical conductivity of PDMS ferromagnetic gel.

Interest in molecular magnets continues to grow, offering a link between the atomic and nanoscale properties. The classical Heisenberg model has been effective in modelling exchange interactions in such systems. In this, the magnetization and susceptibility are calculated through the partition function, where the Hamiltonian contains both Zeeman and exchange energy. For an ensemble of N spins, this requires integrals in 2N dimensions. For two, three and four spin nearest-neighbour chains these integrals reduce to sums of known functions. For the case of the three and four spin chains, the sums are equivalent to results of Joyce. Expanding these sums, the effect of the exchange on the linear susceptibility appears as Langevin functions with exchange term arguments. These expressions are generalized here to describe an N spin nearest-neighbour chain, where the exchange between each pair of nearest neighbours is different and arbitrary. For a common exchange constant, this reduces to the result of Fisher. The high-temperature expansion of the Langevin functions for the different exchange constants leads to agreement with the appropriate high-temperature quantum formula of Schmidt et al, when the spin number is large. Simulations are presented for open linear chains of three, four and five spins with up to four different exchange constants, illustrating how the exchange constants can be retrieved successfully.

A theory describing magneto-orientational properties of suspensions containing antiferromagnetic nanoparticles is developed. Due to their small size, these particles possess, apart from an anisotropic magnetic susceptibility pertinent to antiferromagnets, a spontaneous magnetic moment caused by sublattice decompensation. In a colloid subjected to a DC field of increasing strength an orientational crossover takes place: the particle magnetic moments, initially aligned along the field, turn to the transverse orientation. This behavior considerably changes the observable characteristics of the system: the spectrum of linear dynamic susceptibility and the integral time of magnetic relaxation under a pulse field.

Ferroelastic composites are smart materials with unique properties including large magnetodeformational effects, strong field enhancement of the elastic modulus and magnetic shape memory. On the basis of mechanical tests, direct microscopy observations and magnetic measurements we conclude that all these effects are caused by reversible motion of the magnetic particles inside the polymeric matrix in response to an applied field. The basic points of a model accounting for particle structuring in a magnetoactive elastomer under an external field are presented.

A second-order Taylor series expansion of the free energy functional provides analytical expressions for the magnetic field dependence of the free energy and of the magnetization of ferrofluids, here modeled by dipolar Yukawa interaction potentials. The corresponding hard core dipolar Yukawa reference fluid is studied within the framework of the mean spherical approximation. Our findings for the magnetic and phase equilibrium properties are in quantitative agreement with previously published and new Monte Carlo simulation data.

In this work we describe the observations of structural transitions in ferronematics based on the thermotropic nematics 6CHBT (4-trans-4'-n-hexyl-cyclohexyl-isothiocyanato-benzene). The ferronematic droplets were observed in solutions of nematogenic 6CHBT dissolved in phenyl isocyanate and doped with fine magnetic particles. The phase diagram of the transitions from the isotropic phase to the nematic phase via a droplet state was found. Magneto-dielectric measurements of various structural transitions in this new system enabled us to estimate the type of anchoring of the nematic molecules on the magnetic particle surfaces in the droplets.

Repulsive magnetic fluids show a dynamical freezing above a volume fraction Φ^*, which depends on the physico-chemistry of the system. Φ^* is here determined by a magneto-optical technique. The out-of-equilibrium dynamics of a glass-forming magnetic fluid (Φ = 1.2Φ^*) is studied by x-ray photon correlation spectroscopy and analyzed in terms of intensity auto-correlation functions. The relaxation is age dependent and follows a compressed exponential law with a characteristic time scaling as the inverse of the scattering vector Q. The dynamical susceptibility χ is then deduced from a time resolved correlation analysis at an intermediate Q and for ages larger than 10⁴ s.

In order to investigate the peculiarities of the aggregation processes in ferrofluids in a quasi-2D geometry, a combination of density functional theory (DFT) and molecular dynamics (MD) simulations is presented. The microstructure formation in monodisperse ferrofluid monolayers is studied thoroughly through a comparison of the theoretical and computational results. Theoretical and simulation results show similar trends which renders the theoretical approach a useful tool for getting insight into the microstructure formation in monolayers.

A consistent continuum model of a soft magnetic elastomer (SME) is presented and developed for the case of finite strain. The numeric algorithm enabling one to find the field-induced shape changes of an SME body is described. The reliability of the method is illustrated by several examples revealing specifics of the magnetostriction effect in SME samples of various geometries.

Self-assembly has for the large part focused on the assembly of molecules without guidance or management from an outside source. However, self-assembly is in principle by no means limited to molecules or the nanoscale. A particularly interesting method to the self-assembly of micro- to millimetre sized components is the use of the 'magnetic hole' effect. In this method, nonmagnetic particles can be manipulated by external magnetic fields by immersing them in a dispersion of colloidal, magnetic nanoparticles, denoted ferrofluids. Nonmagnetic particles in magnetized ferrofluids are in many ways ideal model systems to test various forms of particle self-assembly and dynamics. When microspheres are confined to a monolayer between two parallel plates and subjected to static or oscillating magnetic fields they show a variety of dynamical behaviours and assemblages, depending on the frequency and direction of the external fields. A single pair of magnetic holes oscillating in a ferrofluid layer may be used to measure the viscosity of tiny volumes of the fluid. We have also observed ordering of dilute dispersions of macromolecules and nanoparticles in magnetized ferrofluids. The self-assembly at this length scale results from structural correlations between these nanostructures and ferrofluid particles rather than from the macroscopic magnetostatic effect for the magnetic holes.

In this paper, we present experimental results on heat transfer from a nonmagnetic cylinder to a magnetic fluid under the influence of transverse laminar free convection. Measurements are performed in the presence of uniform and nonuniform external magnetic fields; both the fields and their main gradients are directed transversely to the cylinder axis. In zero field the measurement results for both the unsteady and the stationary heat transfer (Ra>10⁶) agree well with the dependences found in the framework of a boundary layer approximation. If a nonuniform magnetic field is applied, the theoretically predicted additive action of gravitation and magnetic convection on the heat transfer intensity is confirmed. In the presence of a uniform field, the changes of heat transfer intensity are insignificant despite the action of strong internal field gradients induced by the nonmagnetic cylinders themselves.

In this work we investigate the pair interaction of magnetic particles in a dilute polydisperse sedimenting suspension. The suspension is composed of magnetic spherical forms of different radii and densities immersed in a Newtonian fluid, settling due to the gravity. When in close contact, the particles may exert on each other a magnetic force due to a permanent magnetization. We restrict our attention to dispersions of micromagnetic composite with negligible Brownian motion. The calculations of the relative particle trajectories are based on direct computations of the hydrodynamic interactions among rigid spheres in the regime of low particle Reynolds number. Depending on the relative importance of the interparticle forces and gravity, the collisions may result in aggregation or simply in a breaking of the particle relative trajectory time reversibility. After summing over all possible encounters, the transverse self-diffusion and down-gradient diffusion coefficients that describe the cross-flow migration of the particles are calculated. Our calculation shows first evidence and the significance of the diffusion process arising from magnetic interactions in dilute non-Brownian suspensions.

The subject of the paper is to investigate the coupling phenomena of magnetic and non-uniform temperature fields in ferrofluids. The coupling creates a special kind of mass transfer and an inhomogeneous concentration of ferrofluid arises especially near bodies, where higher field gradients are present. Particular attention is paid to the oriented mass transfer, i.e. the magnitude and direction of ferrofluid flux with respect to the temperature gradient and magnetic field. Quantitatively, oriented phoretic transport can be characterized by the magnetic Soret coefficient and osmotic pressure difference. The problem is solved using two-dimensional (2D) numerical simulations for the periodic structure of the bodies. Special attention is paid to the magnetic bulk force as the driving force.

Magnetic fluids (MFs) with a similar narrow size distribution of the iron oxide core were stabilized with lauric acid (MF 1), oleate (MF 2) or, after dialysis in the presence of liposomes, with phospholipid molecules (MF 3 and MF 4, respectively). The hydrodynamic sizes of the MF 1 and MF 3 were half those found for MF 2 and MF 4. The MFs were exposed to inductive heating in an alternating magnetic field at a frequency of 200 kHz and a maximum magnetic field strength of 3.8 kA m⁻¹. Specific absorption rates (SAR) of 294 ± 42 (MF 1), 214 ± 16 (MF 2), 297 ± 13 (MF 3) and 213  ± 6 W g⁻¹ Fe (MF 4) were obtained. The data for MF 2 and MF 4 were identical to those found for the commercially available ferucarbotran. The biomedical relevance of the phospholipid-coated MFs is briefly discussed.

We report a rheological study of suspensions of non-Brownian chain-like magnetic particles in the presence of magnetic fields. These particles have been synthesized using spherical iron particles by linking them with a polymer and are called polymerized chains. We have shown that, in oscillatory squeeze mode, the suspensions of such chain-like particles develop yield stress several times higher than that of conventional magnetorheological fluids based on spherical iron particles. This is explained in terms of solid friction between polymerized chains, which form entangled aggregates in the presence of a magnetic field. For the suspension of spherical particles, the squeezing force increases with the magnetic field intensity at low magnetic fields, but decreases dramatically at higher fields because of cavitation or air entrainment. Such a decrease in transmitted force does not take place in suspensions of polymerized chains, at least for fields smaller than 30 kA m⁻¹, which could make these suspensions preferable for application in squeeze-film dampers.

By combining magnetic properties with nanosized biocompatible materials, superparamagnetic nanoparticles may serve as colloidal heating mediators for cancer therapy. This unique potential has attracted attention for designing new magnetic nanoparticles with high efficiency heating properties. Their heating power under high frequency magnetic field is governed by the mechanisms of magnetic energy dissipation for single-domain particles due both to internal Néel fluctuations of the particle magnetic moment and to the external Brownian fluctuations. These mechanisms are highly sensitive to the crystal size, the particle material, and the solvent properties. Here we explore the heating properties of maghemite particles with large particle sizes, in the range 15–50 nm, synthesized through a new procedure which includes a hydrothermal process. Particle shape and size distribution, hydrodynamic volume, and magnetic anisotropy are characterized, respectively, by transmission electron microscopy, dynamic magnetically induced birefringence, and ferromagnetic resonance. Together with our previous data on low diameter particles (Fortin J P et al 2007 J. Am. Chem. Soc129 2628–35), this study provides the whole size dependence of heating efficiency in the range 5–50 nm and assesses the balance between Néel and Brownian contributions to thermal losses. In agreement with theoretical predictions, the heating efficiency shows a maximum for an optimal size of about 15 nm.

The main objective of this paper is the numerical investigation of the process of thermomagnetic convection of a special temperature sensitive ferrofluid. The fluid is studied in a cylindrical domain, with constant temperatures on the top and bottom ends and adiabatic boundary conditions on the sidewalls. The thermomagnetic convection is generated by a non-uniform constant magnetic field of a solenoid, which is placed in a hollow area inside the domain. It has been found that the efficiency of convective heat transfer in such a set-up can be increased up to sevenfold by magnetic field within the studied range of parameters.

Previous theoretical investigations on thermal flow in a horizontal fluid layer have shown that the critical temperature difference, where heat transfer changes from diffusion to convective flow, depends on the frequency of a time-modulated driving force. The driving force of thermal convection is the buoyancy force resulting from the interaction of gravity and the density gradient provided by a temperature difference in the vertical direction of a horizontal fluid layer. An experimental investigation of such phenomena fails because of technical problems arising if buoyancy is to be changed by altering the temperature difference or gravitational acceleration.

The possibility of influencing convective flow in a horizontal magnetic fluid layer by magnetic forces might provide us with a means to solve the problem of a time-modulated magnetic driving force. An experimental setup to investigate the dependence of the critical temperature difference on the frequency of the driving force has been designed and implemented. First results show that the time modulation of the driving force has significant influence on the strength of the convective flow. In particular a pronounced minimum in the strength of convection has been found for a particular frequency.

Magnetorheological elastomers are smart materials made by aligning magnetic microparticles inside a liquid polymer before the curing process has started. Once cured, the composite presents new properties such as a large change of elasticity when applying a magnetic field. We analyze here another specific property of these materials which is the piezoresistivity. Two cases are studied: one where the particles inside the matrix are not in contact and the other where they are in contact. We show that in the first case we observe an exponential dependence of the resistivity versus pressure and in the second case a power law dependence. These behaviors are explained with the help of a conductivity model based on the dependence of the tunnel effect on the area of contact.

The appearance of field- and shear-dependent changes of viscosity—the magnetoviscous effect—is correlated to the formation of chains and structures of magnetic nanoparticles. Moreover, the formation of these structures leads to the appearance of viscoelastic effects or other non-Newtonian features in ferrofluids in the presence of a magnetic field. In order to describe these phenomena, different theoretical approaches have been developed which explain the mechanism of these effects with different assumptions. One point in which these models differ, and which has to be clarified, is the appearance of yield stress and its dependence on magnetic field strength. With this aim, a stress controlled rheometer has been designed to prove the existence of this very small field-dependent yield stress for ferrofluids. The results presented here show a dependence of the yield stress on the magnetic field strength as well as on the interparticle interaction and particle size distribution. Finally, yield stress experiments have been performed for different geometries of the shear cell in order to get more information about the microstructure formed by the magnetic particles.

We present results of a theoretical study of the magnetorheological viscosity η of a suspension versus the applied magnetic field H and shear rate . It is supposed that the macroscopic rheological effects are provided by linear chain-like aggregates. Unlike in traditional models, the natural statistical distribution of the chains over the number of particles in them is taken into account. The results obtained explain important features of the rheological η versus law, which has been detected in experiments but qualitatively contradicts known theories of rheological properties of magnetic suspensions.

Experiments performed for different ferrofluids under shear flow have shown that an increase of the magnetic field strength applied to the sample yields an increase of the fluid's viscosity, the so called magnetoviscous effect. It has been shown that the magnitude of the effect is strongly related to the modification of the microstructure of ferrofluids and can be influenced by varying both the dipole–dipole interaction between the particles and the concentration of large particles within the fluid. This result has been further used to synthesize new ferrofluids which, on one hand, are more compatible for technical applications but, on the other hand, led to difficulties for the experimenters in measuring the viscous behavior in the presence of a magnetic field. To overcome this problem, a specially designed ferrofluid-compatible capillary viscometer has been developed. Within this paper, the experimental setup as well as experimental results concerning the investigation of the magnetoviscous effect in both diluted and concentrated cobalt-based ferrofluids are presented.

A magnetic liquid in a horizontal Hele–Shaw cell is subjected to a vertical magnetic field. The width of the magnetic fluid finger is measured as a function of applied field and compared to a theoretical model. The theoretical model uses an energy minimization procedure and predicts a double energy minimum, hysteresis, and discontinuous transitions between a circle and a finger. The experimental data set agrees very well with the theory for a well-defined magnetic fluid finger. Near the transitions, the experiments show hysteresis and support for a double energy minimum; however, the agreement is not quite so good. The discrepancy between theory and experiment near the transition region is likely due to the simplified finger model used in the theory.

A new experimental/numerical technique of classification of flow regimes (flow patterns) in air–magnetic fluid two-phase flow is proposed in the present paper. The proposed technique utilizes the electromagnetic induction to obtain time-series signals of the electromotive force, allowing us to make a non-contact measurement. Firstly, an experiment is carried out to obtain the time-series signals in a vertical upward air–magnetic fluid two-phase flow. The signals obtained are first treated using two kinds of wavelet transforms. The data sets treated are then used as input vectors for an artificial neural network (ANN) with supervised training. In the present study, flow regimes are classified into bubbly, slug, churn and annular flows, which are generally the main flow regimes. To validate the flow regimes, a visualization experiment is also performed with a glycerin solution that has roughly the same physical properties, i.e., kinetic viscosity and surface tension, as a magnetic fluid used in the present study. The flow regimes from the visualization are used as targets in an ANN and also used in the estimation of the accuracy of the present method. As a result, ANNs using radial basis functions are shown to be the most appropriate for the present classification of flow regimes, leading to small classification errors.

A magnetic fluid microdevice using Diptera insect wings is proposed and constructed. The magnetic fluid device is composed of insect wings, a small permanent magnet, coil, and kerosene-based magnetic fluid. First, the structural properties of insect wings are studied through measurements of certain morphological parameters. Secondly, the novel type of microwind energy converter is constructed. Thirdly, the power generation characteristics of the magnetic fluid microdevice using insect wings are examined. It is found that the output power is roughly proportional to the cube of the airflow velocity.

We develop, test and apply a volume of fluid (VOF) type code for the direct numerical simulation of two-fluid configurations of magnetic fluids with dynamic interfaces. Equilibrium magnetization and linear magnetic material are assumed and uniform imposed magnetic fields are considered, although extensions to nonlinear materials and to fields with spatio-temporal variability are possible. Models are computed for configurations of bubbles of non-magnetic fluid rising in ferrofluid and droplets of ferrofluid falling through non-magnetic fluid. Bubbles and droplets exhibit similar changes of shape in the presence of vertical fields, due to a combination of elongation along the field lines and the fluid dynamics of ordinary rising or falling at small Bond number. Bubbles become more prolate than droplets under the same parameters and are accordingly found to break up more readily than droplets in stronger fields. Indirect effects are observed, such as the change in rise time and the consequent changes in the flow due to increased Reynolds number.

Control of the size and spatial distribution of materials at multiple length scales is one of the most compelling issues in nanotechnology research. We report a multiple-length-scale patterning of pure magnetic particles as well as biocompatible magnetic particles based on a printing technique named micro-injection molding in capillaries. The magnetic particles were prepared by a technique of co-precipitation of ferric and ferrous salts in an alkali medium. We demonstrate that the morphology and the size of the patterning nanoparticles can be controlled by simply controlling the concentration of the solution. Our method exploits the self-organization of the nanoparticles in a solution confined between a stamp and the surfaces of a substrate, exploiting confinement and competing interactions between the adsorbate and the substrate. Our approach represents a remarkable example of an integrated top-down/bottom-up process.

A magnetic fluid seal enables mechanical contact-free rotation of a shaft without frictional heat and material wear and hence has excellent durability. However, the durability of a magnetic fluid seal decreases in liquid. The life of a seal applied to a rotary blood pump is not known. We have developed a magnetic fluid seal that has a shield mechanism minimizing the influence of the rotary pump on the magnetic fluid. The developed magnetic fluid seal worked for over 286 days in a continuous flow condition, for 24 days (on-going) in a pulsatile flow condition and for 24 h (electively terminated) in blood flow. The magnetic fluid seal is promising as a shaft seal for rotary blood pumps.

Characteristics of a tuned magnetic fluid damper are examined. The optimal depth of a magnetic fluid in a cylindrical container is calculated using a linear analysis of a magnetic fluid sloshing. In order to avoid swirling in lower depth fluids, several experiments using greater fluid depths are carried out and a good damping range is obtained.

This work validates a method for increasing the radial restoring force on the voice coil in audio speakers containing ferrofluid. In addition, a study is made of factors influencing splash loss of the ferrofluid due to shock. Ferrohydrodynamic analysis is employed throughout to model behavior, and predictions are compared to experimental data.

To provide a new composite material having a high electrical sensitivity in the fields of robotics and sensing, a magnetic rubber having network-like magnetic clusters was developed by utilizing a magnetic compound fluid (MCF). MCF rubber with small deformations can provide an effective sensor. In this paper, we report many experiments in which changes of the MCF rubber's resistance were observed when the rubber was compressed and a deformation was generated; we then made a trial haptic sensor using the MCF conductive rubber and performed many experiments to observe changes of the electrical resistance of the sensor. The results of experiments showed that the proposed sensor made with MCF conductive rubber is useful for sensing small amounts of pressure or small deformations.

In severe nutriment conditions, the social amoeba Dictyostelium discoideum enters a particular life cycle where it forms multicellular patterns to achieve aggregation. Extensively observed from an initial dispersed state, its developmental program can usefully be studied from a confined population to implement theoretical developments regarding biological self-organization. The challenge is then to form a cell assembly of well-defined geometrical dimensions without hindering cell behavior. To achieve this goal, we imposed transient constraints by applying temporary external magnetic gradients to trap magnetically labeled cells. Deposits of various numbers of cells were geometrically characterized for different magnetic exposure conditions. We demonstrated that the cell deposit was organized as a three-dimensional (3D) structure by both stacking layers of cells and extending these layers in the substrate plane. This structure evolves during the aggregation phase, forming periodic aggregative centers along the linear initial pattern.

The aim of this work is to provide a quantitative method for analysis of the concentration of superparamagnetic iron oxide nanoparticles (SPION), determined by means of ferromagnetic resonance (FMR), with the nanoparticles coupled to a specific antibody (AC 133), and thus to express the antigenic labeling evidence for the stem cells CD 133⁺. The FMR efficiency and sensitivity were proven adequate for detecting and quantifying the low amounts of iron content in the CD 133⁺ cells (∼6.16 × 10⁵ pg in the volume of 2 µl containing 4.5 × 10¹¹ SPION). The quantitative method led to the result of 1.70 × 10⁻¹³ mol of Fe (9.5 pg), or 7.0 × 10⁶ nanoparticles per cell. For the quantification analysis via the FMR technique it was necessary to carry out a preliminary quantitative visualization of iron oxide-labeled cells in order to ensure that the nanoparticles coupled to the antibodies are indeed tied to the antigen at the stem cell surface and that the cellular morphology was conserved, as proof of the validity of this method. The quantitative analysis by means of FMR is necessary for determining the signal intensity for the study of molecular imaging by means of magnetic resonance imaging (MRI).

In this study, we have prepared PLGA (poly-D,L-lactide-co-glycolide) nanospheres loaded with biocompatible magnetic fluid and anticancer drug taxol by a modified nanoprecipitation technique and investigated their magnetic properties. A magnetic fluid, MF-PEG, with a biocompatible layer of polyethylene glycol (PEG), was chosen as a magnetic carrier. The PLGA, whose copolymer ratio of D,L-lactide to glycolide is 85:15, was utilized as a capsulation material. Taxol, as an important anticancer drug, was chosen for its significant role against a wide range of tumours. The morphology and particle size distributions of the prepared nanospheres were investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) and showed a spherical shape of prepared nanospheres with size 250 nm. Infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and thermogravimetry (TGA) analysis confirmed incorporation of magnetic particles and taxol into the PLGA polymer. The results showed good encapsulation with magnetite content 21.5 wt% and taxol 0.5 wt%. Magnetic properties of magnetic fluids and taxol within the PLGA polymer matrix were investigated by SQUID magnetometry from 4.2 to 300 K. The SQUID measurements showed superparamagnetism of prepared nanospheres with a blocking temperature of 160 K and saturation magnetization 1.4 mT.

In order to reduce the side effects generated by the most common cancer treatment therapies, chemo- and radiotherapy, two new approaches are being investigated. These new approaches are magnetic drug targeting (MDT) and magnetic hyperthermia, and are based on the use of magnetic nanoparticles. In the first one, these magnetic nanoparticles are used as drug carriers and the success of the treatment depends on the correct distribution of the drug within the tumour tissue. Computed tomography analysis has been performed on tumour tissue after MDT in order to find out the distribution of the nanoparticles. The measurements have been carried out in two different laboratories, one based on a synchrotron beamline and another one with a cone x-ray source. First results show that the drug carriers form clusters within the tumour tissue.

In recent years, techniques employing magnetizable solid-phase supports (MSPS) have found application in numerous biological fields. This magnetic separation procedure offers several advantages in terms of subjecting the analyte to very little mechanical stress compared to other methods. Secondly, these methods are non-laborious, cheap, and often highly scalable. The current paper details a genomic DNA isolation method optimized in our laboratory using magnetic nanoparticles as a solid-phase support. The quality and yields of the isolated DNA from all the samples using magnetic nanoparticles were higher or equivalent to the traditional DNA extraction procedures. Additionally, the magnetic method takes less than 15 min to extract polymerase chain reaction (PCR) ready genomic DNA as against several hours taken by traditional phenol–chloroform extraction protocols. Moreover, the isolated DNA was found to be compatible in PCR amplification and restriction endonuclease digestion. The developed procedure is quick, inexpensive, robust, and it does not require the use of organic solvents or sophisticated instruments, which makes it more amenable to automation and miniaturization.

Modulated differential scanning calorimetry measurements on bulk (Na₂O)_x(GeO₂)_1−x glasses show a sharp reversibility window in the 14%<x<19% soda range, which correlates well with a broad global minimum in molar volumes. Raman and IR reflectance transverse- and longitudinal-optic mode frequencies exhibit anomalies between x_c(1) = 14% (stress transition) and x_c(2) = 19% (rigidity transition), with optical elasticity power laws confirming the nature of the transitions. Birefringence measurements dramatize the macroscopically stress-free nature of the intermediate phase in the reversibility window.

Many phenomenological properties of reactive polymers like polyurethanes increase or decrease continuously in the course of the curing process before saturating at the end of the chemical reaction. This holds true for instance for the mass density, the refractive index, the chemical turnover and the hypersonic properties. The reason for this monotone behaviour is that the chemical reaction behaves like a continuous succession of irreversible phase transitions. These transitions are superposed by the sol–gel transition and possibly by the chemically induced glass transition, with the drawback that the latter two highlighted transitions are often hidden by the underlying curing process. In this work we propose generalized mode Grüneisen parameters as an alternative probe for elucidating the polymerization process itself and the closely related transition phenomena. As a model system we use polyurethane composed of a diisocyanate and varying ratios of difunctional and trifunctional alcohols.

Phase-change materials based on chalcogenide alloys have been widely used for optical data storage and are promising materials for nonvolatile electrical memory use. However, the mechanism behind the utilization is unclear as yet. Since the rewritable data storage involved an extremely fast laser melt-quenched process for chalcogenide alloys, the liquid structure of which is one key to investigating the mechanism of the fast reversible phase transition and hence rewritable data storage, here by means of ab initio molecular dynamics we have studied the local structure of liquid Ge₁Sb₂Te₄. The results show that the liquid structure gives a picture of most Sb atoms being octahedrally coordinated, and the coexistence of tetrahedral and fivefold coordination at octahedral sites for Ge atoms, while Te atoms are essentially fourfold and threefold coordinated at octahedral sites, as characterized by partial pair correlation functions and bond angle distributions. The local structure of liquid Ge₁Sb₂Te₄ generally resembles that of the crystalline form, except for the much lower coordination number. It may be this unique liquid structure that results in the fast and reversible phase transition between crystalline and amorphous states.

A detailed study of glass formation from aerodynamically levitated liquids in the (Y₂O₃)_x(Al₂O₃)_1−x system for the composition range 0.21≤x≤0.41 was undertaken by using pyrometric, optical imaging and x-ray diffraction methods. Homogeneous and clear single-phase glasses were produced over the composition range . For Y₂O₃-rich compositions (), cloudy materials were produced which contain inclusions of crystalline yttrium aluminium garnet (YAG) of diameter up to 40 µm in a glassy matrix. For Y₂O₃-poor compositions around x = 0.24, cloudy materials were also produced, but it was not possible to deduce whether this resulted from (i) sub-micron inclusions of a nano-crystalline or glassy material in a glassy matrix or (ii) a glass formed by spinodal decomposition. For x = 0.21, however, the sample cloudiness results from crystallization into at least two phases comprising yttrium aluminium perovskite and alumina. The associated pyrometric cooling curve shows slow recalescence events with a continuous and slow evolution of excess heat which contrasts with the sharp recalescence events observed for the crystallization of YAG at compositions near x = 0.375. The materials that are the most likely candidates for demonstrating homogeneous nucleation of a second liquid phase occur around x = 0.25, which corresponds to the limit for formation of a continuous random network of corner-shared AlO₄ tetrahedra.

The self-consistent generalized Langevin equation (SCGLE) theory of colloid dynamics is employed to describe the ergodic–non-ergodic transition in model mono-disperse colloidal dispersions whose particles interact through hard-sphere plus short-ranged attractive forces. The ergodic–non-ergodic phase diagram in the temperature–concentration state space is determined for the hard-sphere plus attractive Yukawa model within the mean spherical approximation for the static structure factor by solving a remarkably simple equation for the localization length of the colloidal particles. Finite real values of this property signals non-ergodicity and determines the non-ergodic parameters f(k) and f_s(k). The resulting phase diagram for this system, which involves the existence of reentrant (repulsive and attractive) glass states, is compared with the corresponding prediction of mode coupling theory. Although both theories coincide in the general features of this phase diagram, there are also clear qualitative differences. One of the most relevant is the SCGLE prediction that the ergodic–attractive glass transition does not preempt the gas–liquid phase transition, but always intersects the corresponding spinodal curve on its high-concentration side. We also calculate the ergodic–non-ergodic phase diagram for the sticky hard-sphere model to illustrate the dependence of the predicted SCGLE dynamic phase diagram on the choice of one important constituent element of the SCGLE theory.

Measurements of the total ion yield (TIY) x-ray absorption spectrum (XAS) of liquid water by Wilson et al (2002 J. Phys.: Condens. Matter14 L221 and 2001 J. Phys. Chem. B 105 3346) have been revisited in light of new experimental and theoretical efforts by our group. Previously, the TIY spectrum was interpreted as a distinct measure of the electronic structure of the liquid water surface. However, our new results indicate that the previously obtained spectrum may have suffered from as yet unidentified experimental artifacts. Although computational results indicate that the liquid water surface should exhibit a TIY-XAS that is fundamentally distinguishable from the bulk liquid XAS, the new experimental results suggest that the observable TIY-XAS is actually nearly identical in appearance to the total electron yield (TEY-)XAS, which is a bulk probe. This surprising similarity between the observed TIY-XAS and TEY-XAS likely results from large contributions from x-ray induced electron stimulated desorption of ions, and does not necessarily indicate that the electronic structure of the bulk liquid and liquid surface are identical.

The temperature dependence of the density correlation function at next-neighbour distances has been investigated for the liquid metal lead. This correlation function is a sensitive parameter for changes in the local environment and its Fourier transform was measured in a coherent inelastic neutron scattering experiment. The zero-frequency amplitude related to the long time decay of density fluctuations decreases in a nonlinear way and indicates a change in dynamics above the melting point. The derived generalized longitudinal viscosity shows a decrease near this temperature range. From these observations we suppose that solidification sets in on a microscopic level distinct above the melting point.

Neutron diffraction measurements for D₂O in SBA-15 silica of pore diameter 86 Å have been made in a temperature range from 300 to 100 K. The pore-filling factor for the liquid phase is 0.95, resulting in an 'almost-filled' sample. The nucleation and transformation of the ice phase were determined for cooling and warming cycles at two different rates. The primary nucleation event at 258 K leads to a defective form of ice-I with predominantly cubic ice features. For temperatures below the main nucleation event, the data indicate the formation of an interfacial layer of disordered water/ice that varies with temperature and is reversible. The main diffraction peak for the water phase shows similar features to those observed in earlier studies, indicating enhanced hydrogen bonding and network correlations for the confined phase as the temperature is decreased. A detailed profile analysis of the triplet peak is presented in the accompanying paper (Seyed-Yazdi et al 2008 J. Phys.: Condens. Matter 20 205108).

The diffraction results for the formation of ice in 86 Å diameter pores of a SBA-15 silica sample are analysed to provide information on the characteristics of the ice created in the pores. The asymmetric triplet at ∼1.7 Å⁻¹, which involves several overlapping peaks, is particularly relevant to the different ice phases and contains a number of components that can be individually identified. The use of a set of three peaks with an asymmetric profile to represent the possibility of facetted growth in the pores was found to give an unsatisfactory fit to the data. The alternative method involving the introduction of additional peaks with a normal symmetric profile was found to give excellent fits with five components and was the preferred analytic procedure. Three peaks could be directly linked to the positions for the triplet of hexagonal ice, I_h, and one of the other two broad peaks could be associated with a form of amorphous ice. The variation of the peak intensity (and position) was systematic with temperature for both cooling and heating runs. The results indicate that a disordered state of ice is formed as a component with the defective crystalline ices. The position of a broad diffraction peak is intermediate between that of high-density and low-density amorphous ice. The remaining component peak is less broad but does not relate directly to any of the known ice phases and cannot be assigned to any specific structural feature at the present time.