Controlling the coarsening dynamics of ferrogranular networks by means of a vertical magnetic field.

We are exploring in experiments the aggregation process in a shaken granular mixture of glass and magnetized steel beads, filled in a horizontal vessel, after the shaking amplitude is suddenly decreased. Then the magnetized beads form a transient network that coarsens in time into compact clusters, resembling a viscoelastic phase separation [Tanaka, J. Phys.: Condens. Matter 12, R207 (2000)0953-898410.1088/0953-8984/12/15/201], where attached beads represent the slow phase. Here we investigate how a homogeneous magnetic field oriented in vertical direction impedes the emergence and growth of the networks. With increasing field amplitude this phase is replaced by a fluctuating arrangement of repelling, isolated steel beads. The experimental results are compared with those of computer simulations. Coarse-grained molecular dynamics confirms the impact of an applied magnetic field on the structural transitions and allows us to investigate long-time regimes and magnetic response not yet accessible in the experiment. It turns out that an applied magnetic field has different impacts, depending on it strength. It can be used either to slow down the dynamics of the structural transitions without changing the type of the resulting phases and only affecting the amount and sizes of clusters, or to fully impede the formation of network-like and compact aggregates of steel beads.

Long-Term Stability, Biocompatibility, and Magnetization of Suspensions of Isolated Bacterial Magnetosomes.

Magnetosomes are magnetic nanoparticles biosynthesized by magnetotactic bacteria. Due to a genetically strictly controlled biomineralization process, the ensuing magnetosomes have been envisioned as agents for biomedical and clinical applications. In the present work, different stability parameters of magnetosomes isolated from Magnetospirillum gryphiswaldense upon storage in suspension (HEPES buffer, 4 °C, nitrogen atmosphere) for one year in the absence of antibiotics are examined. The magnetic potency, measured by the saturation magnetization of the particle suspension, drops to one-third of its starting value within this year-about ten times slower than at ambient air and room temperature. The particle size distribution, the integrity of the surrounding magnetosome membrane, the colloidal stability, and the biocompatibility turn out to be not severely affected by long-term storage.

Precise Assembly of Genetically Functionalized Magnetosomes and Tobacco Mosaic Virus Particles Generates a Magnetic Biocomposite.

Magnetosomes represent magnetic nanoparticles with unprecedented characteristics. Both their crystal morphology and the composition of the enveloping membrane can be manipulated by genetic means, allowing the display of functional moieties on the particle surface. In this study, we explore the generation of a new biomaterial assembly by coupling magnetosomes with tobacco mosaic virus (TMV) particles, both functionalized with complementary recognition sites. TMV consists of single-stranded RNA encapsidated by more than 2100 coat proteins, which enable chemical modification via functional groups. Incubation of EmGFP- or biotin-decorated TMV particles with magnetosomes genetically functionalized with GFP-binding nanobodies or streptavidin, respectively, results in the formation of magnetic, mesoscopic, strand-like biocomposites. TMV facilitates the agglomeration of magnetosomes by providing a scaffold. The size of the TMV-magnetosome mesostrands can be adjusted by varying the TMV-magnetosome particle ratios. The versatility of this novel material combination is furthermore demonstrated by coupling magnetosomes and terminal, 5'-functionalized TMV particles with high molecular precision, which results in "drumstick"-like TMV-magnetosome complexes. In summary, our approaches provide promising strategies for the generation of new biomaterial assemblies that could be used as scaffold for the introduction of further functionalities, and we foresee a broad application potential in the biomedical and biotechnological field.

Coarsening dynamics of ferromagnetic granular networks-experimental results and simulations.

We investigate the phase separation of a shaken mixture of glass and magnetised steel spheres after a sudden quench of the shaker amplitude. After quenching, transient networks of steel spheres emerge in the experiment. For the developing network clusters we estimate the number of spheres in them, and the characteristic path lengths. We find that both quantities follow a log-normal distribution function. Moreover, we study the temporal evolution of the networks. In the sequence of snapshots we observe an initial regime, where the network incubates, followed by a temporal regime where network structures are elongated and broken, and finally a regime where the structures have relaxed to compact clusters of rounded shapes. This phaenomenology resembles the initial, elastic and hydrodynamic regimes observed by H. Tanaka [J. Phys.: Condens. Matter, 2000, 12, R207] during the viscoelastic phase separation for dynamically asymmetric mixtures of polymers. In order to discriminate the three regimes we investigate in the experiment order parameters like the mean number of neighbors and the efficiency of the networks. In order to capture the origin for a viscoelastic phase separation in our granular mixture, we use a simple simulation approach. Not aiming at a quantitative description of the experimental results, we rather use the simulations to define the key interactions in the experimental system. This way, we discover that along with dipolar and steric interactions, there is an effective central attraction between the magnetised spheres that is responsible for the coarsening dynamics. Our simulations show as well three regimes in the evolution of characteristic order parameters.

Pace and patterns of magnetic swimmers in a billiard pool.

We experimentally investigate magnetic surface swimmers on water. These objects self-assemble from ferromagnetic microparticles and a nonmagnetic disk. They are floating on the liquid surface due to interface tension and move under the influence of a harmonically oscillating homogeneous magnetic field oriented vertically, which is distinguished by its amplitude and frequency. The speed of the surface swimmers strongly depends on these parameters. The functional dependencies between speed and amplitude and between speed and frequency are investigated by independently varying both control parameters. In the first case, the data obtained are in good agreement with the predicted scaling while there are some deviations in the latter case. Moreover, due to the interplay between the surface bound swimmers and the ascending liquid meniscus at the edge of the experimental vessel, different dynamics can be realized. We observe periodic and quasiperiodic trajectories in a circular vessel and aperiodic trajectories in a vessel shaped like a Bunimovich stadium.

Retarding the growth of the Rosensweig instability unveils a new scaling regime.

Using a highly viscous magnetic fluid, the dynamics in the aftermath of the Rosensweig instability can be slowed down by more than 2000 times. In this way we expand the regime where the growth rate is predicted to scale linearly with the bifurcation parameter by six orders of magnitude, while this regime is tiny for standard ferrofluids and cannot be resolved experimentally there. We measure the growth of the pattern by means of a two-dimensional imaging technique, and find that the slopes of the growth and decay rates are not the same-a qualitative discrepancy with respect to the theoretical predictions. We solve this discrepancy by taking into account a viscosity which is assumed to be different for the growth and decay. This may be a consequence of the measured shear thinning of the ferrofluid.

Comment on "Self-assembly of magnetic balls: From chains to tubes".

The paper states that magnetic balls preferably assemble in a tube geometry if the number of particles exceeds N≥14. We find that for substantially higher particle counts, such as N>1300, a round cluster of densely packed magnetic balls with an fcc lattice can outmatch the described tube structure.

Spherical sample holders to improve the susceptibility measurement of superparamagnetic materials.

The design of two custom sample holders with a spherical cavity for commercial vibrating sample magnetometer systems is described. For such cavities, the magnetization M[over ->] and the internal magnetic field H(i)[over ->] of a sample are both homogeneous. Consequently, the material parameter M(H(i)) of a sample can be determined even for liquids and powders with a high magnetic susceptibility.

Thermoreversible hydroferrogels with tunable mechanical properties utilizing block copolymer mesophases as template.

Thermoreversible hydroferrogels (FGs) have been prepared via gelation of aqueous maghemite ferrofluids (FFs) using the triblock copolymer Pluronic P123 as gelator. In the investigated concentration range of 28-42 wt % P123, long-term stable homogeneous FGs can be prepared from FFs with a maximum maghemite content of 14 wt %. For higher FF concentrations up to 29 wt %, however, homogeneous FGs were formed only for gelator contents up to ca. 33 wt %. A combination of rheology and µ-DSC was applied as an alternative method to construct the P123 phase diagram, without the need for visual methods or scattering techniques. Using this procedure, we could show that maghemite nanoparticles can be effectively templated by the cubic and hexagonal P123 mesophases in a concentration range of 33-38 wt % P123 and FF concentrations up to 14 wt %, respectively. Most importantly, the phase behavior and the corresponding phase-transition temperatures of P123 were not significantly altered. As a result, the FGs show a reversible temperature-triggered transition from a cubic hard gel to a hexagonal gel, which is linked with a softening of the gel. Furthermore, this concept can be applied to template cobalt ferrite nanoparticle effectively, too. Magnetization experiments revealed that the superparamagnetic behavior of the maghemite nanoparticles, which show a Néel type relaxation, is not altered in the corresponding FGs. In contrast, FGs based on blocked cobalt ferrite nanoparticles show a hysteretic behavior, which indicates a strong mechanical coupling between the P123 mesophase and the magnetic nanoparticles.

Magnetic traveling-stripe forcing: Enhanced transport in the advent of the Rosensweig instability.

A contactless pumping mechanism is realized in a layer of ferrofluid via a spatiotemporally modulated magnetic field. The resulting pressure gradient leads to a liquid ramp, which is measured by means of x-rays. The transport mechanism works best if a resonance of the surface waves with the driving is achieved. The behavior can be understood by considering the magnetically influenced dispersion relation of the fluid.

Alignment of tellurium nanorods via a magnetization-alignment-demagnetization ("MAD") process assisted by an external magnetic field.

Tellurium (Te) nanorods have been successfully aligned on a solid substrate via a magnetization-alignment-demagnetization ("MAD") process in the presence of an external magnetic field. Te nanorods carrying a poly(tert-butyl methacrylate) shell were first converted into magnetic nanocylinders by assembling magnetite nanoparticles on their surface via a hydrophobic interaction in THF. We demonstrate that, below a critical concentration of the nanoparticles, this assembly process is able to quantitatively tune the magnetite nanoparticles' density on the nanorods in terms of their stoichiometric ratio. Due to the polymer and surfactant on their surface, the formed magnetic nanocylinders are soluble in THF and aligned when dried on a solid substrate in the presence of an external magnetic field. The demagnetization of the prealigned nanocylinders was achieved via an acid-etching process, leaving Te nanorods in an aligned state. This MAD process can be extended as a general procedure for other nonmagnetic 1-D nanostructures. Additionally, the nonetched magnetic nanocylinders can be potentially applied in field of magnetorheology.

Measuring the deformation of a ferrogel sphere in a homogeneous magnetic field.

A sphere of a ferrogel is exposed to a homogeneous magnetic field. In accordance to theoretical predictions, it gets elongated along the field lines. The time dependence of the elastic shear modulus causes the elongation to increase with time, similar to mechanic creep experiments, and the rapid excitation causes the sphere to vibrate. Both phenomena can be well described by a damped harmonic oscillator model. By comparing the elongation along the field to the contraction perpendicular to it, we can calculate Poisson's ratio of the gel. The magnitude of the elongation is compared to the theoretical predictions for elastic spheres in homogeneous fields.

Reorientation of a hexagonal pattern under broken symmetry: the hexagon flip.

An unexpected pattern transition has been found experimentally in the transformation from hexagons to stripes caused by an applied anisotropy effect. The particular system studied is the surface instability of a horizontal layer of magnetic liquid in a tilted magnetic field. Two orthogonal Helmholtz pairs of coils provide a vertical and a tangential magnetic field. Whereas the vertical component destabilizes the flat layer, the tangential one preserves its stability. The ensuing surface patterns comprise regular hexagons, anisotropic hexagons, and stripelike ridges. The phase diagram for the tilted field instability is measured using a radioscopic technique. The investigation reveals an interesting effect: the flip from one hexagonal pattern to another under an increasing tangential field component, which is explained in terms of amplitude equations as a saddle-node bifurcation.

Growth of surface undulations at the Rosensweig instability.

We investigate the growth of a pattern of liquid crests emerging in a layer of magnetic liquid when subjected to a magnetic field oriented normally to the fluid surface. After a steplike increase of the magnetic field, the temporal evolution of the pattern amplitude is measured by means of a Hall-sensor array. The extracted growth rate is compared with predictions from linear stability analysis by taking into account the proper nonlinear magnetization curve M(H) . The remaining discrepancy can be resolved by numerical calculations via the finite-element method. By starting with a finite surface perturbation, it can reproduce the temporal evolution of the pattern amplitude and the growth rate. The investigations are performed for two magnetic liquids, one with low and one with high viscosity.

Viscoelasticity of mono- and polydisperse inverse ferrofluids.

We report on measurements of a magnetorheological model fluid created by dispersing nonmagnetic microparticles of polystyrene in a commercial ferrofluid. The linear viscoelastic properties as a function of magnetic field strength, particle size, and particle size distribution are studied by oscillatory measurements. We compare the results with a magnetostatic theory proposed by De Gans et al. [Phys. Rev. E 60, 4518 (1999)] for the case of gap spanning chains of particles. We observe these chain structures via a long distance microscope. For monodisperse particles we find good agreement of the measured storage modulus with theory, even for an extended range, where the linear magnetization law is no longer strictly valid. Moreover we compare for the first time results for mono- and polydisperse particles. For the latter, we observe an enhanced storage modulus in the linear regime of the magnetization.

Hexagons become the secondary pattern if symmetry is broken.

Pattern formation on the free surface of a magnetic fluid subjected to a magnetic field is investigated experimentally. By tilting the magnetic field, the symmetry can be broken in a controllable manner. When increasing the amplitude of the tilted field, the flat surface gives way to liquid ridges. A further increase results in a hysteretic transition to a pattern of stretched hexagons. The instabilities are detected by means of a linear array of magnetic Hall sensors and compared with theoretical predictions.

Two-dimensional solitons on the surface of magnetic fluids.

We report an observation of a stable solitonlike structure on the surface of a ferrofluid, generated by a local perturbation in the hysteretic regime of the Rosensweig instability. Unlike other pattern-forming systems with localized 2D structures, magnetic fluids are characterized by energy conservation; hence their mechanism of soliton stabilization is different from the previously discussed gain-loss balance mechanism. The radioscopic measurements of the soliton's surface profile suggest that locking on the wavelength defined by the nonmonotonic dispersion curve is instrumental in its stabilization.

Oscillatory decay at the Rosensweig instability: experiment and theory.

Transient patterns of the Rosensweig instability are accessed with a pulse sequence. The critical scaling behavior of the oscillation frequency and of the propagation velocity of these patterns is experimentally investigated by switching the magnetic induction to subcritical values. The experimental findings are in good agreement with the linear theory, if the low viscosity and the finite thickness of the magnetic liquid layer are taken into account. In this way we elucidate the subcritical branch of the underlying steady state bifurcation, which is situated in the immediate vicinity of a splitting type bifurcation.

Critical exponents of directed percolation measured in spatiotemporal intermittency.

An experimental system showing a transition to spatiotemporal intermittency is presented. It consists of a ring of hundred oscillating ferrofluidic spikes. Four of five of the measured critical exponents of the system agree with those obtained from a theoretical model of directed percolation.

Glasslike relaxation of labyrinthine domain patterns.

A spatial analysis of globally disordered (labyrinthine) stripe domain patterns in thin ferrimagnetic garnet films is applied to investigate the pattern evolution. After demagnetization of the sample we obtain a branched (fernlike) structure. By periodic modulation of the magnetic field the number of the branches diminishes and a labyrinthine pattern develops. We describe the evolution of the pattern by a measure extracted from the curvature of the border line of the magnetic domains. The relaxation of this measure is found to be nonexponential and can be described by the Kohlrausch-Williams-Watts law.