Physical properties of aqueous suspensions of goethite ( $\alpha$ -FeOOH) nanorods

Abstract

At volume fractions larger than 8.5%, aqueous suspensions of lath-like goethite (alpha-FeOOH) nanorods form a lyotropic nematic phase. In this article, we first discuss the nematic ordering within statistical-physics models of the isotropic/nematic phase transition. We then describe the influence of a magnetic field on the nematic phase. Because the nanorods bear permanent magnetic moments, the nematic suspensions have dipolar order and very low Frederiks thresholds. Moreover, the nematic phase aligns parallel to a small magnetic field but realigns perpendicular to a high field because of a competition between the permanent moments of the nanorods and their negative anisotropy of magnetic susceptibility. This magneto-optical study of the nematic phase is completely consistent with that of the isotropic phase of the same suspensions published in Part I (this issue, p. 291). Besides, we demonstrate the field-induced biaxiality of a nematic single domain aligned perpendicular to the field. We also describe here preliminary experiments where an a.c. electric field is applied to the nematic phase. Both field amplitude and frequency were found to control the alignment direction and homeotropic-to-planar alignment transitions were observed. From this data, simple models were used to estimate some physical constants of the nematic phase.

NIR transmittance tuneability under a magnetic field of colloidal suspensions of goethite (α-FeOOH) nanorods

“…Note that goethite crystal structure can be also described by the Pnma space group, as reported in other publications. 21 In that case, axes are switched and axis c of Pbnm corresponds to b axis of Pnma, along which the magnetic moment is oriented.…”

Liquid crystalline phase in ethanol suspension of anisometric 4-cyano-4-n-hexylbiphenyl microcrystals: Witnessed by dielectric spectroscopy

“…Theoretical consideration on their phase behavior was pioneered by Onsager, 4,5 which was followed by statistical models 6 and numerical simulations. 7,8 Meanwhile, the liquid crystalline phase and the phase transition with the change of particle concentration have been witnessed in a mass of colloidal suspensions composed of anisometric inorganic, [9][10][11][12] organic, 13,14 and biological 3,15,16 particles. In most of these suspensions, however, the employed particles were obtained from chemical synthesis, which is time consuming, has low output, and is generally result in broad particle size distribution.…”

Influence of polydispersity on the phase behavior of colloidal goethite

“…Goethite can easily form liquid crystals that are susceptible to external fields. Lemaire et al [7][8][9][10][11] further revealed that the boardlike goethite particles have a permanent magnetic moment along their long axis, presumably due to uncompensated spins within their antiferromagnetic crystal structure, combined with an induced magnetic moment oriented predominantly along the shortest particle dimension. The interplay between them leads to peculiar field-strength-dependent reorientation phenomena.…”