A diffraction technique to investigate the orientational alignment of anisotropic particles: studies of clay under flow

Measurement of the orientation distribution of anisotropic particles is of considerable interest. The free energy of such materials depends strongly on the relative orientation of the particles and will effect the order-to-disorder transitions associated with liquid-crystal phases and alignment under the influence of external fields. This paper illustrates the use of a diffraction technique that measures the full orientation distribution of anisotropic crystalline particles in a fluid. Neutron diffraction data for dispersions of plate-like particles in pipe flow are presented and the changes in orientational order with flow and shape are described. This technique gives data about orientation which is decoupled from effects of both inter-particle order and the shape of the particles. Wider applications of this method, which provides order parameters for liquid crystals to arbitrary high order, are discussed.