Interactions and confinement of particles in liquid crystals: Novel particles and defects

The motivating topic for this project was to work towards building a new type of electronic paper. Current electronic paper technologies are not able to perform at frame rates high enough to display video. The project was inspired by a prototype device based on the ‘Janus particle’ (particles with two halves of different properties, for example, colour), and to try to reduce the power consumption. The starting point for the project was an investigation of 500nm Janus particles and spheres. The question of whether the addition of particles affected physical properties of the bulk liquid crystal was investigated. Above concentrations up to 1% wt/wt, aggregations formed quickly. At this weight percentage, no significant changes in order parameter, refractive indices or elastic constants of the LC could be seen. It is known qualitatively that topological defects in liquid crystals can attract or repel particles. Importantly the strength of interaction of particles with defects has been quantified in this work. A passive method of microrheology was implemented to quantify the confinement strength. Depending on the system, confinement strengths ranged between 10 and 10,000pN/µm. Particles treated for strong surface anchoring were found to be more strongly confined than particles with weak surface anchoring. Further, particles in liquid crystals with higher elastic constants were found to have higher confinement strengths than in particles in liquid crystals with lower elastic constants. Particle size was not found to affect confinement strength significantly in the size range studied. Finally, the topic of Janus particles was readdressed. In the size regime ~5- 10µm Janus particles show evidence of hybrid alignment and rotation in an electric field. In conclusion, the idea of using Janus particles in a device appears promising: we hope this work is continued in future.