title: What tunes the structural anisotropy of magnetic fluids under a magnetic field?
abstract: In the present study, the structure of monophasic ionic magnetic fluids under a static magnetic field is explored. In these aqueous electrostatically stabilized ferrofluids, we vary both the isotropic interparticle interactions and the anisotropic dipolar magnetic interaction by tuning the ionic strength and the size of the nanoparticles. Small angle neutron scattering measurements carried out on nanoparticles dispersed in light water exhibit miscellaneous 2D nuclear patterns under a magnetic field with various q-dependent anisotropies. In this nondeuterated solvent where the magnetic scattering is negligible, this anisotropy originates from an anisotropy of the structure of the dispersions. Both the low q region and the peak of the structure factor can be anisotropic. On the scale of the interparticle distance, the structure is better defined in the direction perpendicular to the field. In the thermodynamic limit (q-->0), the model previously described in ref 10 matches the data without any fitting parameters: the interparticle interaction is more repulsive in the direction parallel to the magnetic field. At low q, the amplitude of the anisotropy of the pattern is governed by the ratio of two interaction parameters: the reduced parameter of the anisotropic magnetic dipolar interaction, gamma/Phi, over the isotropic interaction parameter, , in zero field, which is proportional to the second virial coefficient.
authors.affiliation: Laboratoire Liquides Ioniques et Interfaces Chargées, UPMC, UMR CNRS-UPMC-ESPCI 7612, case 51, 4 place Jussieu, 75252 Paris Cedex 05, France.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Mériguet
authors.forename: Fabrice
authors.initials: F
authors.lastname: Cousin
authors.forename: Emmanuelle
authors.initials: E
authors.lastname: Dubois
authors.forename: François
authors.initials: F
authors.lastname: Boué
authors.forename: Andrejs
authors.initials: A
authors.lastname: Cebers
authors.forename: Bela
authors.initials: B
authors.lastname: Farago
authors.forename: Régine
authors.initials: R
authors.lastname: Perzynski