LIGHT SCATTERING STUDY OF PARTICLE DYNAMICS IN DIPOLAR MAGNETIC FLUIDS

Abstract

Halsey and Toor theoretical model (HT) predicts that a thermally induced long-range chain-chain interaction might be responsible for chain-chain lateral aggregation in dipolar fluids. However, this model fails with experiments. To characterize thermally induced chain vibrations, we measured the dynamics of particles in a chain configuration in a very dilute ferrofluid emulsion using Dynamic Light Scattering (DLS). The transverse particle diffusion coefficient at short times is studied as a function of the scattering wave vector, q, and the coupling constant, λ. By varying q, transverse motions of the whole chain can be well separated from transverse motions of the chain components (particles). The results show that the characteristic frequency of particle position fluctuations scales as [Formula: see text], where a is the particle radius. This is interpreted using a polymer analogy including Hydrodynamic Interactions (HI) known as Rouse-Zimm model. Since HI are not included in HT model, HI could be responsible for the discrepancy with experiments. When the volume fraction increases, chains gather and DLS is inappropriate to study the system. A more recently developed technique, known as Diffusing Wave Spectroscopy (DWS) is used to obtain the mean square displacement of particle within columns. Since the particle dynamics is closely related to the viscoelastic properties of the system, studying particle dynamics may help us to understand better the rheological properties of MR fluids.