Magnetite (Fe3O4) forms the basis of most dispersions studied in the field of magnetic fluids and magnetic colloids. Despite extensive theory and simulations on chain formation in dipolar fluids in zero field, such structures have not yet been imaged in laboratory-made magnetite dispersions. Here, we present the first direct observation of dipolar chain formation in zero field in a ferrofluid containing the largest synthetic single-domain magnetite particles studied so far. To our knowledge, this is the only ferrofluid system available at present that allows quantifying chain length and ring-size distributions of dipolar structures as a function of concentration and particle size.

We present the first real-space analysis on a single-particle level of the dipolar chains and branched clusters self-assembling in magnetic fluids in zero field. Spatial correlations and chain-length distributions directly obtained from tracked particle positions in vitrified films of synthetic magnetic (Fe3O4) dispersions provide a quantitative test for simulations and theory of dipolar fluids. A pertinent example is the cluster-size distribution that can be analyzed with a one-dimensional aggregation model to yield a dipolar attraction energy that agrees well with the dipole moment found from independent magnetization measurements.

Field-induced structures in a ferrofluid with well-defined magnetite nanoparticles with a permanent magnetic dipole moment are analyzed on a single-particle level by in situ cryogenic transmission electron microscopy (2D). The field-induced columnar phase locally exhibits hexagonal symmetry and confirms the structures observed in simulations for ferromagnetic dipolar fluids in 2D. The columns are distorted by lens-shaped voids, due to the weak interchain attraction relative to field-directed dipole-dipole attraction. Both dipolar coupling and the dipole concentration determine the dimensions and the spatial arrangement of the columns. Their regular spacing manifests long-range end-pole repulsions that eventually dominate the fluctuation-induced attractions between dipole chains that initiate the columnar transition. DOI: 10.1103/PhysRevLett.97.185702 PACS numbers: 64.70.Nd, 75.50.Mm, 82.70.Dd Nanoparticles dispersed in a solvent and with a sufficiently large permanent magnetic dipole moment selfassemble into a variety of magnetic equilibrium structures such as (flux-closure) rings and wormlike, branched dipole chains [1,2]. The morphology of these clusters, formed in absence of an external field, has been examined in detail, together with a determination of pair correlation functions and chain-length distributions [3]. In contrast, much less is known about the structural transitions induced by an external (homogeneous) magnetic field for fluids of permanent dipoles. Interestingly, magnetic colloids in an external field are nevertheless frequently encountered in practical applications [4] and biomedicine [5,6].The structure formation and phase behavior of colloidal systems in reduced dimensions is not necessarily equivalent to that of three-dimensional (3D) systems [7][8][9]. In particular, for permanent dipolar spheres confined to two dimensions (2D) a field-induced transition to a columnar phase with local hexagonal symmetry was predicted [10], although a conclusive experimental real-space analysis is still lacking. Elongated iron-particle clusters have been imaged [1] but the irregular particle shape and the bidisperse size distribution obstruct the wanted single-particle analysis. Parallel structures have also been observed for maghemite ferrofluids dried in the presence of a homogeneous field [11,12]. However, we have shown elsewhere that drying procedures may drastically change structure morphology [2]. Moreover, dipole interactions in conventional ferrofluids are in general too weak for a realistic comparison to the purely dipolar spheres from simulations.In this Letter, we report unequivocal real-space evidence for the predicted columnar phase transition [10] from in situ cryo-TEM images of monodisperse magnetic colloids with dominating dipolar interactions. The particle positions are confined by a 2D film whereas the dipole orientations can thermally fluctuate in 3D. Our imaging results, in addition, allow to quantify positional and angular interparticle correlations showing, among other things, a pr...

Microwave flash sintering of inkjet printed colloidal silver dispersions on thin polymer substrates was studied as a function of the antenna area and initial resistance. The presence of conductive antennae promotes nanoparticle sintering in pre‐dried ink lines (see figure). For dried nanoparticle inks connected to antennae, sintering times of 1 s are sufficient to obtain pronounced nanoparticle sintering and conductivities between 10 and 34% compared to bulk silver.

We show that the equilibrium size of single-layer shells composed of polyoxometalate macroions is inversely proportional to the dielectric constant of the medium in which they are dispersed. This behavior is consistent with a stabilization mechanism based on Coulomb repulsion combined with charge regulation. We estimate the cohesive energy per bond between macroions on the shells to be approximately ÿ6kT. This number is extracted from analysis based on a charge regulation model in combination with a model for defects on a sphere. The value of the cohesive bond energy is in agreement with the model-independent critical aggregate concentration. This observation points to a new class of thermodynamically stable shell-like objects. We point out the possible relevance our findings have for certain surfactant systems.

The surface of magnetite (Fe 3 O 4 , d ¼ 6:3 AE 0:7 nm) nanoparticles dispersed in cyclohexane was studied in the presence of oleic acid and oleylamine using in situ FTIR spectroscopy. Equimolar mixtures of these surfactants are widely used in the chemical synthesis of nanoparticles with a low polydispersity. Here, the IR spectra indicate that oleic acid molecules adsorb to the magnetite surface as a carboxylate. Measurements as a function of surfactant concentration yield an adsorption isotherm, with about two surfactant molecules adsorbed per nm 2 of magnetite at 1 mM surfactant concentration and about 3.5 molecules per nm 2 at 310 mM, of the order expected for full monolayer coverage. No spectral indication is found of oleylamine molecules at the surface of magnetite. In solution, however, almost every oleylamine molecule combines with an oleic acid molecule to form an acid-base complex, with an association constant of 3:5 Â 10 4 dm 3 mol À1 .

We show by cryogenic transmission electron microscopy that PbSe and CdSe nanocrystals of various shapes in a liquid colloidal dispersion self-assemble into equilibrium structures that have a pronounced dipolar character, to an extent that depends on particle concentration and size. Analyzing the cluster-size distributions with a one-dimensional (1D) aggregation model yields a dipolar pair attraction of 8−10 k B T at room temperature. This accounts for the long-range alignment of the crystal planes of individual nanocrystals in self-assembled superstructures and for anisotropic nanostructures grown via oriented attachment.

Field-induced structure formation in a ferrofluid with well-defined magnetite nanoparticles with a permanent magnetic dipole moment was studied with small-angle neutron scattering ͑SANS͒ as a function of the magnetic interactions. The interactions were tuned by adjusting the size of the well-defined, single-magnetic-domain magnetite ͑Fe 3 O 4 ͒ particles and by applying an external magnetic field. For decreasing particle dipole moments, the data show a progressive distortion of the hexagonal symmetry, resulting from the formation of magnetic sheets. The SANS data show qualitative agreement with recent cryogenic transmission electron microscopy results obtained in 2D ͓Klokkenburg et al., Phys. Rev. Lett. 97, 185702 ͑2006͔͒ on the same ferrofluids.