title: In situ imaging of field-induced hexagonal columns in magnetite ferrofluids.
abstract: Field-induced structures in a ferrofluid with well-defined magnetite nanoparticles with a permanent magnetic dipole moment are analyzed on a single-particle level by in situ cryogenic transmission electron microscopy (2D). The field-induced columnar phase locally exhibits hexagonal symmetry and confirms the structures observed in simulations for ferromagnetic dipolar fluids in 2D. The columns are distorted by lens-shaped voids, due to the weak interchain attraction relative to field-directed dipole-dipole attraction. Both dipolar coupling and the dipole concentration determine the dimensions and the spatial arrangement of the columns. Their regular spacing manifests long-range end-pole repulsions that eventually dominate the fluctuation-induced attractions between dipole chains that initiate the columnar transition.
authors.affiliation: Van 't Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
authors.forename: Mark
authors.initials: M
authors.lastname: Klokkenburg
authors.forename: Ben H
authors.initials: BH
authors.lastname: Erné
authors.forename: Johannes D
authors.initials: JD
authors.lastname: Meeldijk
authors.forename: Albrecht
authors.initials: A
authors.lastname: Wiedenmann
authors.forename: Andrei V
authors.initials: AV
authors.lastname: Petukhov
authors.forename: Roel P A
authors.initials: RP
authors.lastname: Dullens
authors.forename: Albert P
authors.initials: AP
authors.lastname: Philipse