8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Fri 911:10-14:00P3.35Biaxiality and nematic-nematic phase separation incolloidal goethite dispersionsEsther van den Pol, 1 Andreea Lupascu, 1 Mihai Diaconeasa, 1 DominiqueThies-Weesie, 1 Dmytro Byelov, 1 Andrei Petukhov, 1 and Gert Jan Vroege 11 Utrecht University, Padualaan 8, 3584 CH, Utrecht, NetherlandsColloidal dispersions of goethite (α-FeOOH) contain charged boardlike crystallites (of volumeLxWxT) in water of pH=3. They form nematic [1], smectic [2] and columnar liquid crystals,which is the richest liquid crystal phase behavior found in mineral liquid crystals. We usedpolarisation microscopy together with (microradian) small angle X-ray scattering to study thesedifferent structures. Goethite particles also display peculiar magnetic properties, combininga considerable permanent magnetic moment along their length (L) with an induced magneticmoment predominantly along the shortest particle dimension (T) [1]. Therefore, particles alignparallel to a small external magnetic field but perpendicular to a large magnetic field. We recentlyemployed the magnetic properties at very low fields to prove the existence of biaxial nematic andsmectic phases if particles have specific dimensions in between rodlike and platelike (L/W≈W/T)[3, 4]. Here, we show that applying intermediate magnetic fields, which induces some of thegoethite nanorods to rotate, leads to nematic-nematic phase separation [5]. This phenomenon canbe explained as an Onsager transition with as its driving force the large excluded volume betweenparticles possessing mutually perpendicular orientations. Surprisingly, this leads to two separatenematic phases with two mutually perpendicular directors, which form droplets and give twomacroscopically separated phases. Different associated interfacial phenomena occur during thisphase transition monitored by polarization microscopy.[1] B. J. Lemaire et al, Phys. Rev. Lett. 88, 125507 (2002).[2] G. J. Vroege et al, Adv. Mater. 18, 2565 (2006).[3] E. van den Pol et al, Phys. Rev. Lett. 103, 258301 (2009).[4] E. van den Pol et al, Liquid Crystals 37, 641 (2010).[5] E. van den Pol et al, J. Phys. Chem. Lett. 1, 2174 (2010).35