8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Wed 711:10-14:00P5.11Diffusive motion of nanoparticles under externalmagnetic fieldManuela Belzik 1 and Moshe Gottlieb 11 Ben Gurion University of teh Negev, Beer Sheva, 84105, Beer Sheva, IsraelBelzik1 M. , Dobbrow2 C. , Schmidt2 A. , Gottlieb1 M. 1 Chemical Engineering Dept. , Ben-Gurion University, Beer-Sheva 84105, Israel 2 Department Chemie, Universität zu Köln, Köln50939, Germany We investigate the diffusive motion of magnetic nanoparticles suspended in aliquid under the influence of external magnetic field. As result of the magnetic field processes suchas chain formation, self-assembly or micro-phase separation occur. The magnetic cobalt nanoparticlesexamined in this work are stabilized by a combination of steric and electrostatic effects. Theabsence of charges on the surface leads to chain-formation due to magnetic dipole-dipole interactions.By adding tri-n-octylphosphine oxide (TOPO) to the synthesis mixture, a surface charge iscreated, and occurring electrostatic repulsion overcomes magnetic dipole attraction and Van derWaals forces. Thus, the particle-particle interactions are dependent on the TOPO concentration.The influence of TOPO on the inter-particle interactions is affirmed by Cryo-TEM (TransmissionElectron Microscopy). We employed Dynamic Light Scattering (DLS) and Depolarized DynamicLight Scattering (DDLS) to study the dynamic behavior of magnetic nanoparticle samples thatdiffer in the amount of TOPO. The translational diffusion (Dt) coefficients were obtained by DLSmeasurements. The change of Dt with concentration can shed light on particle-particle interactions.DDLS measurements allow determination the rotational diffusion coefficient of chain structuresfrom which the dimension, thus the length of the chains can be deduced. Comparison of thedynamics of these systems without an external field, and while subjected to an external magneticfield of 3. 7 mT has been carried out. We find that under the influence of the magnetic field thediffusion coefficient of all system examined decreases. The decrease of the diffusion coefficient ismost likely due to chain formation or cluster formation under the magnetic field. We show that forparticles with considerable surface charge (high TOPO concentration) the slowing down of the diffusionunder an external magnetic field is a time-dependent process, which surprisingly diminisheswith increasing concentration.11