Colloidal particles at the interface between an isotropic liquid and a chiral liquid crystal

Abstract

We create an interface between a cholesteric liquid crystal (CLC) and an isotropic liquid (silicone oil) at which homeotropic anchoring leads to a well aligned cholesteric layer and the formation of the fingerprint texture. Fluorescent colloidal particles with planar surface anchoring are dispersed in the CLC and subsequently imaged using confocal microscopy. A majority of these particles decorate the interface between the CLC and the silicone oil. We present a detailed study of the position of the particles along the direction perpendicular to the interface: the final distribution of particles perpendicular to the interface has a clear dependence on the ratio between the particle size and the pitch of the CLC. This suggests, supported by simulations, that there is a particle size and pitch length dependent drive to expel particles, due to the elastic energy cost of remaining in the CLC. We use polarizing optical microscopy to observe changes to the fingerprint texture as the particles perturb the interface. This is combined with a qualitative study of the in-plane ordering of the particles. Chains of particles form perpendicular to the helical axis (parallel to the cholesteric layers), whereas disordered aggregates are seen where the direction of the helical axis is not uniform.