Uniform growth of clusters of magnetic nanoparticles in a rotating magnetic field

Abstract

It was recently shown that the exposure of magnetic microbeads to a homogeneous magnetic field, which rotates around the axis perpendicular to the field direction, generates highly ordered twodimensional particle arrays. In this work, the impact of downscaling such systems is analyzed. Dilutions of cobalt nanoparticles with an average diameter of 6 nm were brought into a rotating homogeneous magnetic field. A strong localization of the number of particles within a certain cluster size can be observed if the rotation frequency is adjusted to a specific particle concentration. In particular, we obtain an increase of 85 % of the maximum of the cluster size distribution, when changing the rotation frequency of the magnetic field from 300 to 750 rpm for a cobalt concentration of 35.95 mmol/l. We propose a heuristic model to explain the observed frequency dependence.

On the direct employment of dipolar particle interaction in microfluidic systems

“…13 Scanning electron micrograph of cobalt nanoparticle asemblies which result from cobalt concentrations of (a) c low = 3.66 mmol/l and (b, c) c high = 35.95 mmol/l. The experimental setup is schematically illustrated in (d) (Regtmeier et al 2012) Fig. 14 Particle ratio distributions which result from magnetic field rotation frequencies of 300, 500 and 750 rpm for the respective concentration (a) c low and (b) c high .…”

“…14 Particle ratio distributions which result from magnetic field rotation frequencies of 300, 500 and 750 rpm for the respective concentration (a) c low and (b) c high . (c) The frequency dependency of the particle ratio distribution is influenced by the nanoparticle concentration (Regtmeier et al 2012) magnetic particles. To understand the formation dynamics and stability criteria, fundamental investigations were presented.…”

“…Furthermore, the interplay between ferrofluids and magnetic (Erb et al 2009), as well as non-magnetic, material may be used to generate regularly ordered structures (Yellen et al 2005: Krebs et al 2009). In order to investigate the ordering of magnetic nanoparticles under the influence of a magnetic field that rotates perpendicular to the field direction, a spotting procedure similar to the method discussed in the previous section for microparticles was employed (Regtmeier et al 2012). Typical resulting assemblies of 6-nm Co nanoparticles are presented in Fig.…”

Microfluidic Magnetic Mixing at Low Reynolds Numbers and in Stagnant Fluids

“…The vortex induced by a rotating paramagnetic nanoparticle chain exerted long-range attracting interaction forces on adjacent chains that gradually minimized the distances between the chains. When the distance of two rotating chains (two induced vortices) became smaller than a critical value [121], the chains performed coaxial rotation and the two vortices merged resulting in strong vortexes. The particle chains rotated around themselves but simultaneously rotated about the center of the vortex due to the dynamic balance of the radial components of the interaction forces (Figure 7a).…”

Structure and dynamics of self-assembling colloidal monolayers in oscillating magnetic fields

“…In steady or slowly rotating magnetic fields such particles form chains, which align with the magnetic field lines [21][22][23][24][25]. If the field rotates too quickly, viscous drag breaks apart 3he chains [21], and the particles can instead form highly organized crystals [26][27][28]. However, previous experiments have not yet explored the role of hydrodynamic interactions in cluster formation.…”