INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 18 (2006) S2685–S2696 doi:10.1088/0953-8984/18/38/S11 Understanding the structure and the dynamics of magnetic fluids: coupling of experiment and simulation G M´ eriguet 1 , E Dubois 1 , M Jardat 1 , A Bourdon 1 , G Demouchy 1 , V Dupuis 1 , B Farago 2 , R Perzynski 1 and P Turq 1 1 Universit´ e P et M Curie—Paris 6, UMR-UPMC-CNRS-ESPCI 7612, Laboratoire LI2C, Case 51, 4 place Jussieu, Paris F-75005, France 2 Institut Laue Langevin, BP 156, 38042 Grenoble Cedex 9, France E-mail: meriguet@ccr.jussieu.fr Received 3 May 2006, in final form 13 July 2006 Published 8 September 2006 Online at stacks.iop.org/JPhysCM/18/S2685 Abstract Experiments and Brownian dynamics simulations have been coupled in order to better understand structural and dynamical properties of ferrofluids, especially the role of the magnetic dipolar interaction. The ferrofluid used is a ‘home- made’ well defined suspension, the experimental characteristics of which are introduced in the modelled system. In this system, the determination of the structure using simulations and small angle neutron scattering (SANS) experiments shows no sign of chaining in the suspensions, both without and with a magnetic field. Nevertheless, on the scale of the interparticle distance, the structure is strongly anisotropic. This is at contrast with the weak anisotropy of the translational diffusion coefficient under magnetic field on the same scale. Moreover, on the macroscopic scale, both structure and translational dynamics are strongly anisotropic. Also the rotational diffusion is strongly modified if determined without field or after a weak orientation of the particles with a weak field. These results all emphasize the role of the collective phenomena associated with the dipolar interaction. (Some figures in this article are in colour only in the electronic version) 1. Introduction Magnetic fluids, or ferrofluids, are colloidal dispersions of magnetic nanoparticles in a liquid [1], the properties of which, for example viscosity, microstructure or apparent density, can be tuned in a constant magnetic field or in a field gradient. Therefore, numerous applications based on the magnetic dipolar interactions have been or can be developed [2, 3]. The understanding of the specific properties of ferrofluids compared to those of classical colloids is therefore an important question, which has attracted considerable interest in the literature. However, up to now, there is no clear and definitive answer on several points, in 0953-8984/06/382685+12$30.00 © 2006 IOP Publishing Ltd Printed in the UK S2685