Dynamics of concentrated ferrofluid with labelled particles

In the study of ferrofluid dynamics the separation of self and pair correla tion is the essential problem at both intermediate and high concentrations of the magnetic phase. The neutron spin-echo (NSE) total scattering functio n S-t(q, t) = beta S-p(q, t) + (1 - beta)S-s(q, t) contains both contributi ons, where the parameter beta = sigma(p)/(sigma(p) + sigma(s)) depends on t he corresponding cross sections sigma(s,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 q(1) approximate to 0.4 n m similar to 1/D-p (D-p is the particle diameter) and q(2) approximate to 0 .7 nm > 1/D-p have been carried out for these opposite situations. The func tions S-p(q, t), S-s(q, t), from these data behave quite differently. The a utocorrelations reveal a stretched relaxation at low q: S-s(q, t) = exp[ - q(2)r(t)/2], where Gamma(t) = 2Dt(alpha) is the squared particle displaceme nt, the exponent alpha = 0.5 +/- 0.1 at q(1), and alpha = 0.8 +/- 0.1 at q( 2). The magnitude alpha < 1 indicates the strong influence of dipole forces on the motion of a particle. It resembles the segmental relaxation in a po lymer chain. On the other hand, the pair correlations show the oscillations (period similar to 40 ns, amplitude - 1 nm) mixed with diffusion. (C) 1999 Elsevier Science B.V. All rights reserved.