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.