Browsing by Author "Ras, Robin, Dr., Aalto University, Department of Applied Physics, Finland"

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Magnetic Nanoparticles : From Synthesis Concepts to Biomimetics, Dynamic Self-Assemblies and Collective Phenomena
(2013) Timonen, Jaakko
School of Science | Doctoral dissertation (article-based)
Magnetic nanoparticles exhibit size-dependent magnetic properties that make them promising building blocks for advanced materials and devices. This thesis covers several aspects of magnetic nanoparticles, ranging from their synthesis to applications. In Publications I and II, nucleation and growth of monodisperse cobalt nanoparticles in a widely used hot-injection method were investigated. In contrast to the prior understanding, the nucleation was shown to be delayed and kinetically controlled, which was applied to designing more robust heating-up methods without sacrificing the particle uniformity. A facile test tube synthesis method was introduced for rapid screening of different reaction conditions that would otherwise be expensive and laborsome. In Publications III and IV, magnetic nanoparticles were utilized in the contexts of biocomposites and biomimetics. Magnetically active organic-inorganic biofibers were produced by synthesizing cobalt ferrite nanoparticles onto biofibers of native cellulose nanofibrils. A template-free method for creating biomimetic magnetic cilia was demonstrated through magnetically guided self-assembly of micron-sized cobalt particles with elastomeric polymers. The cilia were applied to mixing of liquids by actuation via an external magnetic field. In Publications V and VI, iron oxide nanoparticles were used as force mediators to drive water droplets on superhydrophobic surfaces. Individual droplets placed in a confining magnetic field were shown to oscillate with decreasing amplitude, from which the dissipative forces were determined as a function of normal force. Concentrated magnetic droplets were unstable in a perpendicular magnetic field, leading to droplet splitting and self-assembly into complex patterns. A controlled transition from equilibrium self-assembly to dissipative self-organization was observed under dynamic magnetic field. In Publication VII, microwave dynamics of dipolarly coupled single-domain magnetic nanoparticles interacting in the near-field regime were studied analytically and numerically. The uniform Kittel mode was shown to be replaced by quasi-uniform collective modes. The resonant frequency and width were determined to be dependent on the way the nanoparticles were assembled with respect to each other. The results of this thesis contribute to the understanding of the physics and chemistry of magnetic nanoparticles. The presented concepts pave way towards modern applications, such as robust magnetic microfluidic mixers, surface analysis methods, programmable microdroplet chemistry and tunable microwave materials.