Manipulation of modified clay particles in a nematic solvent by a magnetic field

The magnetic alignment of organically modified montmorillonite platelets in magnetic fields is studied using small-angle X-ray scattering. When suspended in non-mesogenic solvents such as dodecanol the platelets are found to align parallel to the magnetic field due to the negative anisotropy of the magnetic susceptibility of the montmorillonite. Alignment of the clay in the nematic phase of 5CB was possible for sufficiently dilute samples and at sufficiently high field strengths. However in this case the platelets aligned with their normals parallel to the field. It is argued that this is due to homeotropic anchoring of the liquid crystal at the clay particle surfaces leading to the particle orientation being slaved to the director alignment. Alignment of the 5CB director parallel to the field was only apparent at high fields and low particle concentrations, where the magnetic coherence length of the liquid crystal would be smaller than the typical size of a domain between stacks of clay platelets. A simple theoretical model is presented to aid the interpretation of the observations. The observed degree of particle alignment is often less than predicted, probably as the result of interactions between stacks of particles opposing reorientation.