How to analyse the structure factor in ferrofluids with strong magnetic interactions: a combined analytic and simulation approach

Abstract

We present a theoretical model for calculating the structure factor for ferrofluids with strong inter-particle magnetic dipole–dipole interactions, where chain aggregates are known to exist. Our analytical model is based on the minimization of a free energy density functional that allows us to explicitly construct the radial distribution functions of the ferroparticles. Both mono- and bi-disperse model systems have been investigated in the absence of an external magnetic field. We perform an extensive comparison of the theoretical model predictions with the results of molecular dynamic computer simulations for a wide range of ferroparticle densities and coupling parameters, and find encouraging agreement between the simulation data and theory. The behaviour of the structure factor in the region of the first peak and in the region of large wave vectors is studied in detail, and related to the observed microstructure. Our results demonstrate that the combined method developed in the present study is suitable for revealing the connection between microstructure and scattering images, and thus can help to interpret experimental results such as small angle neutron scattering images.

Keywords:

• ferrofluids
• structure factor
• density functional theory
• molecular dynamics

Acknowledgements

We would like to dedicate this paper to Jean-Jacques Weis in appreciation of his important and inspiring contribution to science, and to dipolar systems in particular, and we are looking forward to hearing his new curious results from novel research fields in exotic lands. This research was carried out with the financial support of DFG-RFBR grant No. HO 1108/12-1, and a stipend to SK from the MPG. One of the authors (SK) was supported by CRDF grant Y3-P-05-11, and the Grant of President RF MK-412.2008.2. This research was partially supported by RFBR grant 08-02-00647 and CRDF grant PG07-005-02. One of the authors (ESP) is grateful to the Dynasty Foundation and was supported by the Government via a RF Stipend for PhD students. JC wants to thank the Spanish Ministerio de Educación y Ciencia for a postdoctoral grant (No. EXP2006-0931).