The optical anisotropy of sheared hematite suspensions

Abstract

Simultaneous linear dichroism and linear birefringence measurements were used to study sheared suspensions of colloidal hematite ( α-Fe 2O 3) particles. Particles of three different axis ratios were synthesized. The particles were characterized using electron microscopy, and the three samples had axis ratios of 2.6 ± 0.14, 3.61 ± 0.24, and 5.82 ± 0.58 with mean lengths of 0.315, 0.360, and 0.485 μm, repsectively. The particles were suspended in two Newtonian fluids, glycerin (viscosity = 17 P) and low molecular weight polybutene (viscosity = 220 P), and each suspension was subjected to transient shear flow in a Couette flow cell. The role of polydispersity in particle size on the relative orientations of the real and imaginary parts of the refractive index tensor was examined. It was demonstrated that when polydispersity in particle size exists, an asymmetric orienting field such as a simple shear flow generally results in birefringence and dichroism that are not coaxial. It was also found that at a wavelength of 632.8 nm, the particles can be adequately modeled as Rayleigh scatterers, and absorption can be neglected.