Light scattering study of particle dynamics in dipolar magnetic fluids

Halsey and Toor theoretical model (HT) predicts that a thermally induced lo ng-range chain-chain interaction might be responsible for chain-chain later al aggregation in dipolar fluids. However, this model fails with experiment s. To characterize thermally induced chain vibrations, we measured the dyna mics of particles in a chain configuration in a very dilute ferrofluid emul sion using Dynamic Light Scattering (DLS). The transverse particle diffusio n coefficient at short times is studied as a function of the scattering wav e vector, q, and the coupling constant, lambda. By varying q, transverse mo tions of the whole chain can be well separated from transverse motions of t he chain components (particles). The results show that the characteristic f requency of particle position fluctuations scales as aq(3)/root lambda wher e a is the particle radius. This is interpreted using a polymer analogy inc luding Hydrodynamic Interactions (HI) known as Rouse-Zimm model. Since HI a re not included in HT model, HI could be responsible for the discrepancy wi th experiments. When the volume fraction increases, chains gather and DIS i s inappropriate to study the system. A more recently developed technique, k nown as Diffusing Wave Spectroscopy (DWS) is used to obtain the mean square displacement of particle within columns. Since the particle dynamics is cl osely related to the viscoelastic properties of the system, studying partic le dynamics may help us to understand better the rheological properties of MR fluids.