Alignment of Magnetic Nanorods in Solidifying Films (original) (raw)
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Collective alignment of nanorods in thin Newtonian films
Soft Matter, 2013
In this paper, we provide a complete analytical description of the alignment kinetics of magnetic nanorods in magnetic field. Nickel nanorods were formed by template electrochemical deposition in alumina membranes from a dispersion in a water-glycerol mixture. To ensure uniformity of the dispersion, the surface of the nickel nanorods was covered with polyvinylpyrrolidone (PVP). A 40-70 nm coating prevented aggregation of the nanorods. These modifications allowed us to control alignment of the nanorods in a magnetic field and test the proposed theory. An orientational distribution function of nanorods was introduced. We demonstrated that the 0.04% volume fraction of nanorods in the glycerol-water mixture behaves as a system of non-interacting particles. However, the kinetics of alignment of a nanorod assembly does not follow the predictions of the single-nanorod theory. The distribution function theory explains the kinetics of alignment of a nanorod assembly and shows the significance of the initial distribution of nanorods in the film. It can be used to develop an experimental protocol for controlled ordering of magnetic nanorods in thin films.
A gradient field defeats the inherent repulsion between magnetic nanorods
Royal Society open science, 2014
When controlling the assembly of magnetic nanorods and chains of magnetic nanoparticles, it is extremely challenging to bring them together side by side while keeping a desired spacing between their axes. We show that this challenge can be successfully resolved by using a non-uniform magnetic field that defeats an inherent repulsion between nanorods. Nickel nanorods were suspended in a viscous film and a non-uniform field was used to control their placement. The in-plane movement of nanorods was tracked with a high-speed camera and a detailed image analysis was conducted to quantitatively characterize the behaviour of the nanorods. The analysis focused on the behaviour of a pair of neighbour nanorods, and a corresponding dynamic model was formulated and investigated. The complex two-dimensional dynamics of a nanorod pair was analysed analytically and numerically, and a phase portrait was constructed. Using this phase portrait, we classified the nanorod behaviour and revealed the exp...
Small, 2021
Magnetic colloids adsorbed at a fluid interface are unique model systems to understand self‐assembly in confined environments, both in equilibrium and out of equilibrium, with important potential applications. In this work the pearl‐chain‐like self‐assembled structures of superparamagnetic colloids confined to a fluid–fluid interface under static and time‐dependent actuations are investigated. On the one hand, it is found that the structures generated by static fields transform as the tilt angle of the field with the interface is increased, from 2D crystals to separated pearl‐chains in a process that occurs through a controllable and reversible zip‐like thermally activated mechanism. On the other hand, the actuation with precessing fields about the axis perpendicular to the interface induces dynamic self‐assembled structures with no counterpart in non‐confined systems, generated by the interplay of averaged magnetic interactions, interfacial forces, and hydrodynamics. Finally, how t...
One-dimensional alignment of nanoparticles via magnetic sorting
Applied Physics Letters, 2010
Near room temperature, thin films of MnAs spontaneously align into two phases, one ferromagnetic and the other paramagnetic. These two phases take the intriguing form of nanoscale "wires." In this experiment, we investigate the spontaneous formation of ordered linear arrays of magnetite nanoparticles on MnAs thin films using scanning probe microscopy.
Journal of Magnetism and Magnetic Materials, 2015
We present a dynamic study of soft magnetic, commercial Fe and Ni micrometer-sized particles dispersed in oleic acid and subjected to a variable (rotating) magnetic field in the horizontal plane. A very complex structure is formed after the particles decant towards the bottom liquid-solid interface and the magnetic field is applied for several minutes. The dynamics of structure formation was studied by means of the registration and analysis of microscopic video images, through a Matlab image analysis script. Several parameters, such as the number of clusters, the perimeter-based fractal dimension and circularity, were calculated as a function of time. The time evolution of the number of clusters was found to follow a power-law behavior, with an exponent consistent with that found in other studies for magnetic systems, whereas the typical formation time depends on the particle diameter and field configuration. Complementarily, the magnetic properties of the formed structure were studied, reproducing the experiment with liquid paraffin as the containing fluid, and then letting it solidify. The sample obtained was studied by vibrating sample magnetometry. The magnetization curves show that the material obtained is a planar magnetically anisotropic material, which could eventually be used as an anisotropic magnetic sensor or actuator.
Spontaneous liquid crystal and ferromagnetic ordering of colloidal magnetic nanoplates
Nature Communications, 2016
Ferrofluids are familiar as colloidal suspensions of ferromagnetic nanoparticles in aqueous or organic solvents. The dispersed particles are randomly oriented but their moments become aligned if a magnetic field is applied, producing a variety of exotic and useful magnetomechanical effects. A longstanding interest and challenge has been to make such suspensions macroscopically ferromagnetic, that is having uniform magnetic alignment in the absence of a field. Here we report a fluid suspension of magnetic nanoplates that spontaneously aligns into an equilibrium nematic liquid crystal phase that is also macroscopically ferromagnetic. Its zero-field magnetization produces distinctive magnetic self-interaction effects, including liquid crystal textures of fluid block domains arranged in closed flux loops, and makes this phase highly sensitive, with it dramatically changing shape even in the Earth’s magnetic field.
Reorientation kinetics of superparamagnetic nanostructured rods
The attractive interactions between oppositely charged species (colloids, macromolecules etc) dispersed in water are strong, and the direct mixing of solutions containing such species generally yields to a precipitation, or to a phase separation. We have recently developed means to control the electrostaticallydriven attractions between nanoparticles and polymers in water, and at the same time to preserve the stability of the dispersions. We give here an account of the formation of supracolloidal aggregates obtained by co-assembly of 7 nm particles with copolymers. Nanostructured rods of length comprised between 5 and 50 μm and diameter 500 nm were investigated. By application of a magnetic field, the rods were found to reorient along with the magnetic field lines. The kinetics of reorientation was investigated using step changes of the magnetic field of amplitude /2. From the various results obtained, among which an exponential decay of the tangent of the angle made between the rod and the field, we concluded that the rods are superparamagnetic. @ :
Assembly of microscopic highly magnetic droplets: Magnetic alignment versus viscous drag
Physical Review E, 1999
We report here the collective behavior of droplets in the scale 10 Ϫ6 -10 Ϫ5 m made of a concentrated magnetic fluid elongated and oriented by a magnetic field while rotating synchronously with respect to the carrier liquid. Distribution of droplet sizes is studied as a function of a magnetoviscous number N mv that quantifies the competition between magnetic field and local vorticity. The liquid state and very low interfacial tension enable both breakup and coalescence processes, which are undergone by the droplet population to reach dynamic equilibrium. Theoretical analysis of a single drop motion is extended to the case of the drop assembly. Experiments combining rotation and field modulation show a regime of nonsteady rotation in good agreement with theory.
Self-Assembly of Magnetic Nanoparticles in Evaporating Solution
Journal of the American Chemical Society, 2011
When deposited from an evaporating solution onto a substrate, even nondescript nanoparticles can organize into intricate spatial patterns. Here we show that a simple but long-ranged anisotropy in nanoparticles' interactions can greatly enrich this scenario. In experiments with colloidal Co nanocrystals, which bear a substantial magnetic dipole, we observe assemblies quite distinct from those formed by nonmagnetic particles. Reflecting the strongly nonequilibrium nature of this process, nanocrystal aggregates also differ substantially from expected low-energy arrangements. Using coarse-grained computer simulations of dipolar nanoparticles, we have identified several dynamical mechanisms from which such unusual morphologies can arise. For particles with modest dipole moments, transient connections between growing domains frustrate phase separation into sparse and dense regions on the substrate. Characteristic length scales of the resulting cellular networks depend non-monotonically on the depth of quenches we use to mimic the effects of solvent evaporation. For particles with strong dipole moments, chain-like aggregates formed at early times serve as the agents of assembly at larger scales. Their effective interactions drive the formation of layered loop structures similar to those observed in experiments.
Soft Matter, 2014
Manufacturing new soft materials with specific optical, mechanical and magnetic properties is a significant challenge. Assembling and manipulating colloidal particles at fluid interfaces is a promising way to make such materials. We use lattice-Boltzmann simulations to investigate the response of magnetic ellipsoidal particles adsorbed at liquid-liquid interfaces to external magnetic fields. We provide further evidence for the firstorder orientation phase transition predicted by Bresme and Faraudo [Journal of Physics: Condensed Matter 19 , 375110]. We show that capillary interface deformations around the ellipsoidal particle significantly affect the tilt-angle of the particle for a given dipole-field strength, altering the properties of the orientation transition. We propose scaling laws governing this transition, and suggest how to use these deformations to facilitate particle assembly at fluid-fluid interfaces.