Microbial biofilms are present on biotic and abiotic surfaces and have a significant impact on many fields in industry,
health care and technology. Thus, a better understanding of processes that lead to development of biofilms and their
chemical and mechanical properties is needed. In the following paper we report the results of active laser tweezers
microrheology study of optically inhomogeneous extracellular matrix secreted by Visbrio sp. bacteria. One particle and
two particle active microrheology were used in experiments. Both methods exhibited high enough sensitivity to detect
viscosity changes at early stages of bacterial growth. We also showed that both methods can be used in mature samples
where optical inhomogeneity becomes significant.
We have developed a magneto-optic tweezers that offer new experimental possibilities when laser tweezers were
traditionaly used. The magneto-optic tweezers combine a multi-trap optical tweezers based on acousto optic deflectors
and homogeneous magnetic field which direction and magnitude can be time modulated in arbitrary fashion.
Superparamagnetic beads that are readily available from several commercial sources are used as trap handles. They can
be manipulated using optical tweezers in a well known way. By applying magnetic field additional repulsive or attractive
interaction between the particles can be induced, giving rise to new micromanipulation possibilities. Several examples of
how magneto-optic traps can be used in colloidal physics reasearch and potential applications in biophysics and
microfluidic systems are presented.
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 (n<sub>o</sub>=1.5 and n<sub>e</sub>=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.