Aggregation of magnetic holes in a rotating magnetic field

Abstract

We have experimentally investigated field induced aggregation of nonmagnetic particles confined in a magnetic fluid layer when rotating magnetic fields were applied. After application of a magnetic field rotating in the plane of the fluid layer, the single particles start to form two-dimensional (2D) clusters, like doublets, triangels, and more complex structures. These clusters aggregated again and again to form bigger clusters. During this nonequilibrium process, a broad range of cluster sizes was formed, and the scaling exponents, z and z ′ , of the number of clusters N (t) ∼ t z ′ and average cluster size S(t) ∼ t z were calculated. The process could be characterized as diffusion limited clustercluster aggregation. We have found that all sizes of clusters that occured during an experiment, fall on a single curve as the dynamic scaling theory predicts. Hovewer, the characteristic scaling exponents z ′ , z and crossover exponents ∆ were not universal. A particle tracking method was used to find the dependence of the diffusion coefficients D s on cluster size s. The cluster motions show features of Brownian motion. The average diffusion coefficients < D s > depend on the cluster sizes s as a power law < D s >∝ s γ where values of γ as different as γ = −0.62 ± 0.19 and γ = −2.08 ± 0.51 were found in two of the experiments.

Magnetic Propulsion of Self-Assembled Colloidal Carpets: Efficient Cargo Transport via a Conveyor-Belt Effect

“…The colloidal clusters are stable only in presence of the driving field, while immediately disintegrate due to thermal fluctuations once the applied field is switched off. As reported in previous works on assembly of "magnetic holes" [29] or Janus colloids [30], we observe that when the cluster is formed, it continues to rotate but at a smaller angular frequency than that imposed by the driving field. The cluster rotation arises from an unbalanced viscous shear force experienced by the torque particles located at the edge of the cluster [30,31].…”

Two-dimensional melting of colloids with long-range attractive interactions

“…9,[18][19][20] Recently, it has been shown that suspended paramagnetic particles under a rotating magnetic field (PURM) can be used to generate a long-range tunable interaction potential between colloids that decays as r À3 , characteristic of a Lennard-Jones potential. [21][22][23][24] The precise calculation of the interaction potential allows us to readily simulate the PURM system using molecular simulations. 25 Thus, the phase behavior of the PURM system can be experimentally probed and simulated with Monte Carlo simulations.…”

Aggregation dynamics and magnetic properties of magnetic micrometer-sized particles dispersed in a fluid under the action of rotating magnetic fields

“…Nevertheless, we have found that the kinetics of formation of such structures has not yet been extensively investigated. The formation of the filamentary structures in the uniaxial configuration has been previously studied by means of microscopic image analysis, in magnetic particles in a fluid [18,19], and non-magnetic particles in a magnetic fluid [20]. Previously, we have published a preliminary work as a conference proceeding, regarding this topic [21].…”