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Abstract
Dispersions of board-like goethite (α-FeOOH) particles with short-range repulsive interaction form a versatile colloidal model system, showing a nematic, smectic A and columnar phase. In high magnetic fields a biaxial nematic phase is induced with the shortest dimension of the particles aligned along the field. Moreover, if particles have a shape almost exactly in between rod-like and plate-like they can spontaneously, without external magnetic field, form biaxial nematic and biaxial smectic A phases, which is in accordance with theoretical predictions. The macroscopic domains were oriented by a magnetic field and their structure was revealed by small-angle X-ray scattering. Our results suggest that biaxial phases can be readily obtained by a proper choice of the particle shape.
Acknowledgements
This work is part of the research program SFB TR6 of the ‘Stichting voor Fundamenteel Onderzoek der Materie (FOM)’, which is financially supported by the ‘Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)’ and ‘Deutsche Forschungsgemeinschaft (DFG)’. The work of D.M.E. Thies–Weesie was performed as part of a NWO–CW TOP project. We thank P. Davidson for enlightening discussions and for his help, together with I. Dozov, with the birefringence measurements. We also thank the staff of the BM26 and ID02 beamlines of the ESRF for their excellent support and Th. Narayanan for sharing the magnet.