The cage elasticity and under-field structure of concentrated magnetic colloids probed by small angle X-ray scattering (original) (raw)
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In the present study we probe the bulk modulus and the structure of concentrated magnetic fluids by Small Angle Xray Scattering. The electrostatically stabilized nanoparticles experience a repulsive interparticle potential modulated by dipolar magnetic interactions. On the interparticle distance lengthscale, we show that nanoparticles are trapped under-field in oblate cages formed by their first neighbours. We propose a theoretical model of magnetostriction for the field-induced deformation of the cage. This model captures the anisotropic features of the experimentally observed scattering pattern on the local scale in these strongly interacting colloidal dispersions.
Structure and dynamics of charged magnetic colloids
Journal of Physics: Condensed Matter, 2006
By coating cobalt ferrite nanoparticles with a silica shell, the polydispersity of the resulting core-shell particles can be reduced. Hereby, opposite to the case for conventional ferrofluids, self-organization to liquid-like and even crystalline structures in aqueous media is enabled. The resulting structures mainly originate from the predominant electrostatic repulsion of colloidal macroions bearing charged groups at the surface of the silica shell. Due to the small magnetic moment of the cobalt ferrite cores, however, these structures can be influenced by external magnetic fields or field gradients. While field gradients act as a magnetic trap for these particles, homogeneous fields induce an aligning of the magnetic momenta. Hereby a decrease of symmetry from spherical to cylindrical symmetry of the structures appearing can be observed. Due to collective phenomena, even interactions significantly smaller than the thermal energy can induce clearly observable structural distortions. Even in the absence of an external field, suspensions of such magnetic particles show an unexpected slow diffusion caused by hydrodynamic interactions.
The European Physical Journal E, 2001
Small-angle X-ray scattering (SAXS) was performed on a series of Electric Double-Layered Magnetic Fluids (EDL-MF) composed of ferrite type-CoFe2O4, MnFe2O4, ZnFe2O4, NiFe2O4 and CuFe2O4nanoparticles of different crystalline sizes (DXR ranging from 40 to 139Å, as determined by X-ray diffraction). The information concerning the scattering objects was obtained through the analysis of the distance distribution function p(r) and of the size distribution function D(R), both retrieved from SAXS data. The results show that EDL-MF, in the absence of an applied magnetic field, are composed of small magnetic particle aggregates in solution. These agglomerates are elongated in one direction (chain-like) with the longest dimension varying from 240 to 330Å. The cross-section size is of the order of D XR. The data also demonstrate that the maximum dimension of these aggregates is independent of the ferrite type. On the other hand, the number of aggregated magnetic particles is nanoparticle-size-dependent. Accordingly, larger ferrite-type nanoparticles as those with D XR = 139Å form aggregates composed of 2-3 magnetic particles, whereas smaller ones with DXR ∼ = 40Å form agglomerates of about 6 magnetic particles in solution. As the nanoparticle size is reduced, it might increase the particle surface defects. Such occurrence would affect the particle surface charge density, which could reduce the electrostatic screening, favoring the agglomeration phenomenon.
Quantitative Real-Space Analysis of Self-Assembled Structures of Magnetic Dipolar Colloids
Physical Review Letters, 2006
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 (Fe 3 O 4 ) 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.
Applying SAXS to study the structuring of Fe3O4 magnetic nanoparticles in colloidal suspensions
DYNA, 2019
In this work, Small Angle X-ray Scattering (SAXS) patterns, obtained from two different aqueous colloidal suspensions of magnetite nanoparticles electrostatically stabilized with citric acid, were fitted using three different mathematical models in order to describe the particle size distribution and aggregation state. The colloidal suspensions differ in the mean particle size (4.5±1.0 nm and 5.5±1.1 nm) and the aqueous stabilization, allowing control of the strength of the interaction strength between particles. The models used for SAXS analysis, reveal that the particles are almost spherical with a broad size distribution, and that particles in each suspension are aggregated and are subject to an attractive interaction potential, typical for magnetic nanoparticles. For the better-stabilized sample, ramified chain-like aggregates were found, and for the less-stabilized sample, a more compact structure was determined. The size distribution obtained by applying SAXS mathematical mode...
Tuning the Colloidal Crystal Structure of Magnetic Particles by External Field
Angewandte Chemie, 2014
Manipulation of the self-assembly of magnetic colloidal particles by an externally applied magnetic field paves a way toward developing novel stimuli responsive photonic structures. Using microradian X-ray scattering technique we have investigated the different crystal structures exhibited by self-assembly of core-shell magnetite/silica nanoparticles. An external magnetic field was employed to tune the colloidal crystallization. We find that the equilibrium structure in absence of the field is random hexagonal close-packed (RHCP) one. External field drives the self-assembly toward a body-centered tetragonal (BCT) structure. Our findings are in good agreement with simulation results on the assembly of these particles.
Communications Chemistry
Competition between attractive and repulsive interactions drives the formation of complex phases in colloidal suspensions. A major experimental challenge lies in decoupling independent roles of attractive and repulsive forces in governing the equilibrium morphology and long-range spatial distribution of assemblies. Here, we uncover the ‘dual nature’ of magnetic nanoparticle dispersions, particulate and continuous, enabling control of the short-range attraction and long-range repulsion (SALR) between suspended microparticles. We show that non-magnetic microparticles suspended in an aqueous magnetic nanoparticle dispersion simultaneously experience a short-range depletion attraction due to the particulate nature of the fluid in competition with an in situ tunable long-range magnetic dipolar repulsion attributed to the continuous nature of the fluid. The study presents an experimental platform for achieving in situ control over SALR between colloids leading to the formation of reconfig...
Journal of Physics: Condensed Matter, 2008
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.
Aggregation of superparamagnetic colloids in magnetic fields: the quest for the equilibrium state
Soft Matter, 2011
Experimental and simulation studies of superparamagnetic colloids in strong external fields have systematically shown an irreversible aggregation process in which chains of particles steadily grow and the average size increases with time as a power-law. Here we show, by employing Langevin dynamics simulations the existence of a different aggregation behavior: aggregates form during a transient period and the system attains an equilibrium distribution of aggregate sizes. A thermodynamic self-assembly theory supports the simulation results and it also predicts that the average aggregate size in the equilibrium state depends only on a dimensionless parameter combining the volume fraction of colloids φ0 and the magnetic coupling parameter Γ. The conditions under which this new behavior can be observed are discussed. PACS numbers: 83.10.Mj, 61.43.Hv, 82.70.Dd, 83.80.Gv Colloidal aggregation is a subject of active research for both practical (e.g. stability of many industrial products) and fundamental reasons (as a test field for statistical-mechanical theories, for example). Our interest here is in the new physics arising in the aggregation behavior of superparamagnetic colloids. These systems are a successful example of implementation of a new behavior typical of the nanoscale (superparamagnetism) in new materials with many exciting practical applications, ranging from environmental waste capture [1] to biomedicine . Superparamagnetic materials show a large magnetic dipole in presence of external field, saturation magnetization similar to that of ferromagnetic materials but no coercitivity nor remanence at the working temperature. Superparamagnetic colloids are typically made by embedding superparamagnetic nanocrystals in a non-magnetic matrix (such as polystyrene, nanoporous silica or others) .