Onsager Revisited: Magnetic Field Induced Nematic−Nematic Phase Separation in Dispersions of Goethite Nanorods

Abstract

We found, using polarization microscopy and small-angle X-ray scattering, that for goethite, a low polydispersity suffices to form two separate nematic phases, while previous theory showed that this is only possible for mixtures of particles with extremely different lengths or diameters. Applying a critical magnetic field, which induces some of the goethite nanorods to rotate, leads to sufficient excluded volume between the particles to cause macroscopic phase separation between two orthogonal nematic phases. The larger the polydispersity of the system, the broader the range of field strengths where nematic-nematic phase separation occurs. This is a new phase separation mechanism which is expected to lead to interesting interfacial phenomena.

Non-monotonic response of a sheared magnetic liquid crystal to a continuously increasing external field

“…Moreover, we want to emphasize that, in contrast to Ref. 92 , being out-of-equilibrium is essential for the observed demixing in our system as one of the favored orientations is the shear-induced orientation.…”

“…Although the aforementioned Onsager analysis relies on equilibrium arguments, it does suggest that demixing in the present nonequilibrium system is possible. Experimentally, a similar demixing has been realized in a system of nano-rods 92 , where a certain degree of polydispersity is required to induce particle misalignments. In contrast to that, in our study, the demixing occurs between monodisperse particles.…”

Particle shape effects in colloidal crystals and colloidal liquid crystals: Small-angle X-ray scattering studies with microradian resolution

“…In LC phases this couples to the orientational and partial positional order, which for instance leads to the reorientation of the nematic phase by 90 o and promotes the formation of a columnar from a nematic LC phase [48]. A new type of nematicnematic phase separation into domains with particle director oriented either perpendicular or parallel to the applied field was also found, due to the fact that larger particles in a polydisperse sample start to reorient at a weaker magnetic field causing packing problems so that they spontaneously separate into a different phase [60]. Polydispersity also plays a role in the smectic phase [61] transforming to a columnar phase at high polydispersity and an intermediate smectic-Clike zig-zag structure at low polydispersity.…”

Liquid Crystalline Anisotropic Nanoparticles: From Metallic and Semiconducting Nanoparticles to Carbon Nanomaterials

“…Therefore, the particles have the opposing tendencies to align with their long axis parallel to the field at a low magnetic field strength and to orient perpendicular to a higher magnetic field. This magnetic property of goethite nanorods has been [45][46][47][48][49][50][51][52]. Goethite nanorods form nematic phases in their aqueous suspensions.…”

“…Magnetic field induced nematic-nematic phase separation in goethite nanorods has been demonstrated recently. Small angle X-ray scattering measurements proved that the two resulting nematic phases have orthogonal orientations, a uniaxial nematic phase with particles parallel to the field and a biaxial nematic phase with particles perpendicular to the field [52]. Maeda et al prepared monodisperse colloidal β-FeOOH particles and found that smectic ordering occurs over a wide region from evaporating suspensions of the particle on substrates.…”