title: Magnetic-field-induced nematic-nematic phase separation and droplet formation in colloidal goethite.
abstract: We demonstrate the suitability of polarization microscopy to study the recently discovered (parallel) nematic-(perpendicular) nematic phase separation. This novel type of phase transition is induced by applying an external magnetic field to a nematic liquid crystal of boardlike colloidal goethite and is due to an interplay between the intrinsic magnetic properties of goethite and the collective effect of liquid crystal formation. It is shown that the intense ochre colour of goethite does not preclude the use of polarization microscopy and interference colours, and that dichroism can give valuable qualitative information on the nature of the phases, their anchoring and their sedimentation and order parameter profiles. We also apply these techniques to study 'nematic-nematic tactoids': nematic droplets sedimenting within a nematic medium with mutually perpendicular orientations.
authors.affiliation: Van' t Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute for Nanomaterials Science, Utrecht University, Utrecht, The Netherlands.
authors.forename: E
authors.initials: E
authors.lastname: van den Pol
authors.forename: A A
authors.initials: AA
authors.lastname: Verhoeff
authors.forename: A
authors.initials: A
authors.lastname: Lupascu
authors.forename: M A
authors.initials: MA
authors.lastname: Diaconeasa
authors.forename: P
authors.initials: P
authors.lastname: Davidson
authors.forename: I
authors.initials: I
authors.lastname: Dozov
authors.forename: B W M
authors.initials: BW
authors.lastname: Kuipers
authors.forename: D M E
authors.initials: DM
authors.lastname: Thies-Weesie
authors.forename: G J
authors.initials: GJ
authors.lastname: Vroege
mesh_terms: D001718:Birefringence; D003460:Crystallization; D058085:Iron Compounds / Q000138:chemical synthesis* / Q000737:chemistry*; D050866:Liquid Crystals / Q000737:chemistry*; D008280:Magnetics / Q000295:instrumentation*; D008859:Microscopy, Polarization / Q000379:methods*; D008863:Microspheres; D008903:Minerals / Q000138:chemical synthesis* / Q000737:chemistry*; D044367:Phase Transition; D013499:Surface Properties
chemical_list: D058085:Iron Compounds; D008903:Minerals; C094886:goethite