Manipulation and transport of microparticles and even fluorescent molecules by thermally induced gradient of the order parameter is demonstrated in the nematic liquid crystal. IR light absorption of a focused beam of the laser tweezers is used to heat locally a thin layer of the nematic liquid crystal by several degrees, thus creating a spatial gradient of temperature of the nematic liquid crystal over tens of micrometers. It is observed that a colloidal particle with dipolar symmetry of the director configuration is attracted into the hot spot of the tweezers. The strength of trapping potential increases linearly with particle radius, which indicates that the trapping is due to elastic energy of the distorted nematic liquid crystal around the particle. By using fluorescent molecules instead of colloidal particles, we observed that this thermal trapping of colloidal particles is efficient down to the nanoscale, as fluorescent molecules are also attracted to the hotter regions of the liquid crystal. This effect is absent in the isotropic phase.
The interactions between different types of colloidal particles are measured and analyzed. We use these interactions to
build different self-assembled microstructures, such as dimers, chains, wires, crystals and superstructures. In the
experiments we have used different size, different symmetry of colloids (elastic dipoles and quadrupoles) and different
way of colloidal binding (via localized defects and via entangled defects). We use optical tweezers for directed selfassembly
of colloidal particles. Special attention is devoted to the hierarchical superstructures of large and small
particles. We show that smaller, submicron colloidal particles are trapped into the topological defect rings or loops,
twisting around larger colloidal particles, which are sources of strong nematic deformations. Various possible
applications are discussed, especially in photonics and metamaterials.
We describe and analyze experiments, where optical manipulation of small colloidal particles in the nematic liquid
crystal (NLC) was used to create artificial colloidal structures, such as 1D chains and 2D colloidal crystals, and
superstructures of different types of colloids. In all cases, the colloidal particles are strongly bound to each other, with a
typical pair interaction energy of several 1000 kBT per 1μm size particle. There are two distinct mechanisms of colloidal
binding in a spatially homogeneous NLC: (i) binding via spatially localized topological (point) defects, and (ii) binding
via entangled topological defects, where the defect line winds around and wraps several colloidal particles.
Colloidal structures assembled in confined nematic liquid crystals are examined. Theoretical predictions based on
Landau-type approaches are complemented with the latest studies of laser assisted colloidal assembling. Effective
colloidal interactions are particularly sensitive to the confinement and external fields. Their complexity leads to
numerous stable or metastable colloidal superstructures not present in isotropic solvents. Particularly important are
colloidal structures coupled by entangled disclinations. Such a string-like coupling is very robust and opens new routes
to assemble new photonic materials.
The ability to generate regular spatial arrangements of particles on different length scales is one of the central issues of
the "bottom-up" approach in nanotechnology. Current techniques rely on single atom or molecule manipulation by the
STM, colloidal particle manipulation by laser or optoelectronic tweezers, microfluidics, optofluidics, micromanipulation
and classical lithography. Of particular interest is self-assembly, where the pre-determined spatial arrangements of
particles, such as 3D photonic crystals, could be realized spontaneously. Dispersions of particles in liquid crystals show
several novel classes of anisotropic forces between inclusions, which result in an amazing diversity of self-assembled
patterns, such as linear chains and 2D photonic crystals of microspheres. The forces between the particles in nematic
colloids are extremely strong and long-range, resulting in several thousand times stronger binding compared to the
binding in water based colloids. The mechanisms of self-assembly in nematic colloids are discussed, showing this is a
novel paradigm in colloidal science, which can lead to new approaches in colloidal self-assembly for photonic devices.
We describe and analyze laser trapping of small colloidal particles in a nematic liquid crystal, where the index of refraction of colloidal particles is smaller compared to the indices of the liquid crystal. Two mechanisms are identified that are responsible for this anomalous trapping: (i) below the optical Freedericksz transition, the trapping is due to the anisotropic dielectric interaction of the polarized light with the inhomogeneous director field around the colloidal particle, (ii) above the optical Freedericksz transition, the optical trapping is accompanied by the elasticity-mediated interaction between the optically distorted region of a liquid crystal and the particle. In majority of the experiments, the trapping above the Freedericksz transition is highly anisotropic. Qualitative agreement is found with a numerical analysis, considering nematic director elastic distortion, dielectric director-light field coupling and optical repulsion due to low refraction index colloid in a high index surroundings.
We report on an observation of attractive gradient force on dielectric particles suspended in a medium with a higher refractive index. This unexpected phenomenon was observed with micron sized silica spheres (n=1.37) suspended in optically anisotropic nematic liquid crystal (no=1.5 and ne=1.7). The newly discovered interaction has a range an order of magnituded bigger than the laser beam waist diameter. Above transition temperature, where nematic order of a liquid crystal is lost, the gradient force becomes repulsive and it's range is reduced to expected values. We attribute an anomalous gradient force in nematic phase to two phenomena: particle dressing with a liquid crystal molecules resulting in a colloid with a higher effective index of refraction than surroundings and laser field induced distortino of nematically ordered liquid crystal molecules.