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

Abstract

The magnetic alignment of organically modified montmorillonite platelets in magnetic fieldsis studied using small-angle x-ray scattering. When suspended in non-mesogenic solventssuch as dodecanol the platelets are found to align parallel to the magnetic field due tothe negative anisotropy of the magnetic susceptibility of the montmorillonite.Alignment of the clay in the nematic phase of 5CB was possible for sufficiently dilutesamples and at sufficiently high field strengths. However, in this case the plateletsaligned with their normals parallel to the field. It is argued that this is due tohomeotropic anchoring of the liquid crystal at the clay particle surfaces leading to theparticle orientation being slaved to the director alignment. Alignment of the 5CBdirector parallel to the field was only apparent at high fields and low particleconcentrations, where the magnetic coherence length of the liquid crystal wouldbe smaller than the typical size of a domain between stacks of clay platelets. Asimple theoretical model is presented to aid the interpretation of the observations.The observed degree of particle alignment is often less than predicted, probablyas the result of interactions between stacks of particles opposing reorientation.