REPOSITORY > RESULTS

Abstract

The thesis presents experimental studies of colloidal interactions and novel colloidal structures
in confined nematic liquid crystal using optical tweezers. Liquid crystal colloids are a
novel class of composite materials with well-controlled periodic structures in the nanometer
and micrometer ranges. Moreover, they also posses novel mechanical, optical and rheological
properties, which makes them potentially interesting for applications in photonics, electrooptics,
sensors and metamaterials. Likewise, they are promising to play an important role
in the future development of self-assembling soft matter systems.
This work is primarily focused on two-dimensional assembly of colloidal inclusions in thin
nematic layers. The inclusions induce distortions and topological defects, characterized by
dipolar and quadrupolar symmetry of the molecular orientation field in the continuous liquid
crystalline matrix. These distortions can mediate elastic interactions among dispersed particles
and direct their spatial ordering into variety of self-assembled colloidal architectures. It
has been shown that regular two-dimensional nematic colloidal crystals of elastic dipoles and
quadrupoles can be assembled using optical tweezers and stabilized by nematic elasticity. The
assembly process was to some extent automated by the application of external electric field,
which enabled the massive and directed transport of colloidal particles on macroscopic scale.
Furthermore, a novel binding mechanism has been demonstrated, where colloidal particles
are entangled by delocalized disclination lines. Nematic braids, formed by such disclinations,
stabilize new topological objects and entrap particles in a complex manner.
The main results of the thesis are organized in three independent sections. Firstly, we
present the experimental analysis of elastic distortions around rod-like particles in homogeneous
nematic layers and demonstrate that interparticle forces do not crucially depend on
the shape of inclusions. Secondly, we show that the chiral ordering of the underlying complex
fluid strongly influences defect formation and colloidal interactions. Escaped hyperbolic
defect rings with multiple configurations are energetically stabilized by non-uniform director
geometry of a twisted nematic cell. Here, chiral dimers and novel two-dimensional colloidal
crystals of pure and mixed chirality, intercalated with a lattice of escaped defect loops, are
reported. Thirdly, we have observed that chiral nematic field reverses the stability and
metastability of entangled and non-entangled colloidal structures. This finding allowed us to
create reconfigurable entangled two-dimensional colloidal clusters and periodic crystals with
an enormous number of conformations. Such conformations possess topological structure of
various knots and links, formed by topological frustrations of the liquid crystal orientation
field. They have been precisely analyzed applying the mathematical theory of knots and we
believe that this work opens a new link between colloidal science and knot theory. Moreover,
we anticipate that such knotted patterns may also find use as a model system for studying
knotted conformations in confined and chiral environment, as well as light-addressed surface
memory or as topological target for the scaffolding of dispersed biopolymers.