Three-dimensional morphology of field-induced chain-like aggregates of superparamagnetic microparticles

Reconfiguring ferromagnetic microrod chains by alternating two orthogonal magnetic fields

It is well-known that ferromagnetic microrods form linear chains under an external uniform magnetic field B and the chain length is strongly dependent on the applied field, the applied time duration, and the microrod density. When the chains become long enough and the B-field switches to its orthogonal direction, an irreversible morphological transition, i.e. a parallel linear chain array to a 2D network, is observed. The formation of the network depends on the ratio of the average chain length L and separation D, L/D, as well as the magnitude of the changed B-field. When the chain pattern has an L/D larger than a critical value, the network structure will be formed. Such a critical L/D ratio is a monotonic function of B, which determines the bending length of each magnetic chain during the change of B-fields. This morphological change triggered by external magnetic field can be used as scaffolds or building blocks for biological applications or design smart materials.

Reversible and irreversible aggregation of magnetic liposomes

Understanding stabilization and aggregation in magnetic nanoparticle systems is crucial to optimizing the functionality of these systems in real physiological applications. Here we address this problem for a specific, yet representative, system. We present an experimental and analytical study on the aggregation of superparamagnetic liposomes in suspension in the presence of a controllable external magnetic field. We study the aggregation kinetics and report an intermediate time power law evolution and a long time stationary value for the average aggregate diffusion coefficient, both depending on the magnetic field intensity. We then show that the long time aggregate structure is fractal with a fractal dimension that decreases upon increasing the magnetic field intensity. By scaling arguments we also establish an analytical relation between the aggregate fractal dimension and the power law exponent controlling the aggregation kinetics. This relation is indeed independent on the magnetic field intensity. Despite the superparamagnetic character of our particles, we further prove the existence of a population of surviving aggregates able to maintain their integrity after switching off the external magnetic field. Finally, we suggest a schematic interaction scenario to rationalize the observed coexistence between reversible and irreversible aggregation.

Nanocomposites of Highly Monodisperse Encapsulated Superparamagnetic Iron Oxide Nanocrystals Homogeneously Dispersed in a Poly(ethylene Oxide) Melt

Nanocomposite materials based on highly stable encapsulated superparamagnetic iron oxide nanocrystals (SPIONs) were synthesized and characterized by scattering methods and transmission electron microscopy (TEM). The combination of advanced synthesis and encapsulation techniques using different diblock copolymers and the thiol-ene click reaction for cross-linking the polymeric shell results in uniform hybrid SPIONs homogeneously dispersed in a poly(ethylene oxide) matrix. Small-angle X-ray scattering and TEM investigations demonstrate the presence of mostly single particles and a negligible amount of dyads. Consequently, an efficient control over the encapsulation and synthetic conditions is of paramount importance to minimize the fraction of agglomerates and to obtain uniform hybrid nanomaterials.

Dynamic scaling of ferromagnetic micro-rod clusters under a weak magnetic field

A controlled configurational change of micro-clusters in suspensions is essential for many smart material applications. In this paper, the dynamic process of ferromagnetic microrod clusters (FMRCs) under an external magnetic field was studied as a function of the cluster size N and the applied field B. The FMRCs rearranged from a side-by-side raft-like structure to an end-to-end chain-like structure, originating from coupled motions through the field-driven alignment of both ferromagnetic microrods and FMRCs. A theoretical model based on an extension of a zig-zag chain was developed, and both the cluster length and orientation could be characterized by a retardation time constant τ, with a relationship τ ∼ N²/B, which agrees well with the experimental results, τ ∼ N^2.2±0.2/B^0.8±0.1. Such a model can be used to predict other cluster dynamics or the magneto-elastic behavior of other soft matters consisting of FMRCs.

Reconstructing the fractal dimension of granular aggregates from light intensity spectra

There has been growing interest in using the fractal dimension to study the hierarchical structures of soft materials after realising that fractality is an important property of natural and engineered materials. This work presents a method to quantify the internal architecture and the space-filling capacity of granular fractal aggregates by reconstructing the three-dimensional capacity dimension from their two-dimensional optical projections. Use is made of the light intensity of the two-dimensional aggregate images to describe the aggregate surface asperities (quantified by the perimeter-based fractal dimension) and the internal architecture (quantified by the capacity dimension) within a mathematical framework. This method was tested on control aggregates of diffusion-limited (DLA), cluster-cluster (CCA) and self-correlated (SCA) types, stereolithographically-fabricated aggregates, and experimentally-acquired natural sedimentary aggregates. Statistics of the reconstructed capacity dimension featured correlation coefficients R ≥ 98%, residuals NRMSE ≤ 10% and percent errors PE ≤ 4% as compared to controls, and improved earlier approaches by up to 50%.

Formation of nanoparticles and nanostructures--an industrial perspective on CaCO3 , cement, and polymers

Nanotechnology enables the design of materials with outstanding performance. A key element of nanotechnology is the ability to manipulate and control matter on the nanoscale to achieve a certain desired set of specific properties. Here, we discuss recent insight into the formation mechanisms of inorganic nanoparticles during precipitation reactions. We focus on calcium carbonate, and describe the various transient stages potentially occurring on the way from the dissolved constituent ions to finally stable macrocrystals-including solute ion clusters, dense liquid phases, amorphous intermediates, and nanoparticles. The role of polymers in nucleating, templating, stabilizing, and/or preventing these structures is outlined. As a specific example for applied nanotechnology, the properties of cement are shown to be determined by the formation and interlocking of calcium-silicate-hydrate nanoplatelets. The aggregation of these platelets into mesoscale architectures can be controlled with polymers.

Aggregation and disaggregation dynamics of sedimented and charged superparamagnetic micro-particles in water suspension

This article presents results on the aggregation and disaggregation kinetics on a 1 μm diameter charged superparamagnetic particles dispersed in water under a constant uniaxial magnetic field in experiments with salt (KCl) added to the suspension in order to observe the behaviour of the system when the electrical properties of the particles have been screened. These particles have an electric charge and are confined between two separated 100 μm thick quartz windows, and sediment near the charged bottom wall. The electrostatic interactions that take place in this experimental setup may affect the micro-structure and colloidal stability of the suspension and thus, the dynamics of aggregation and disaggregation.