Dynamics of concentrated ferrofluid with labelled particles

In the study of ferrofluid dynamics the separation of self and pair correlation is the essential problem at both intermediate and high concentrations of the magnetic phase. The neutron spin-echo (NSE) total scattering function St(q, t)=βSp(q, t)+(1-β)Ss(q, t) contains both contributions, where the parameter β=σp/(σp+σs) depends on the corresponding cross sections σs,p. To distinguish these different kinds of dynamics we eliminated one of them by using a mixture of magnetic particles with different scattering amplitudes and concentrations. The NSE experiments (LLB, CE-Saclay) at momentum transfer q1∼0.4 nm ∼1/Dp (Dp is the particle diameter) and q2∼0.7 nm1/Dp have been carried out for these opposite situations. The functions Sp(q, t), Ss(q, t), from these data behave quite differently. The autocorrelations reveal a stretched relaxation at low q: Ss(q, t)=exp[-q2Γ(t)/2], where Γ(t)=2Dtα is the squared particle displacement, the exponent α=0.5±0.1 at q1, and α=0.8±0.1 at q2. The magnitude α<1 indicates the strong influence of dipole forces on the motion of a particle. It resembles the segmental relaxation in a polymer chain. On the other hand, the pair correlations show the oscillations (period∼40 ns, amplitude∼1 nm) mixed with diffusion. (Copyright (c) 1999 Elsevier Science B.V., Amsterdam. All rights reserved.)