Marat S. Soskin

Carbon Nanotubes in Liquid Crystals: Fundamental Properties and Applications

The structure and properties of liquid crystalline suspensions filled by carbon nanotubes (CNTs) are critically reviewed. Special attention is paid to interactions between CNTs and molecules of the liquid crystals (LC), which lead to formation of ordered supramolecular structures . These structures, in turn, determine unique physical properties of LC + CNT suspensions, including electrical conductivity, dielectric permittivity , phase transitions , optical transmission , memory effects that can be used in electrooptic and optoelectronic devices, etc. Great variety of LC phases are considered as a host media, such as nematics, cholesterics, smectics of different types (including ferroelectrics ), lyotropic , chromonic , ionic and hydrogen-bonded liquid crystals. Alongside multi- and single-walled carbon nanotubes, the suspensions can also contain the platelets of organoclays used for facilitation of CNT dispersing. Recent practical applications of LC + CNT suspensions and nanomaterials based thereon are also outlined.

LC nanocomposites: induced optical singularities, managed nano/micro structure, and electrical conductivity

Microstructure, phase transitions, electrical conductivity, and optical and electrooptical properties of multiwalled carbon nanotubes (NTs), dispersed in the cholesteric liquid crystal (cholesteryl oleyl carbonate, COC), nematic 5CB and their mixtures, were studied in the temperature range between 255 K and 363 K. The relative concentration X=СОС/(СОС+5CB) was varied within 0.0-1.0. The concentration C of NTs was varied within 0.01-5% wt. The value of X affected agglomeration and stability of NTs inside СОС+5CB. High-quality dispersion, exfoliation, and stabilization of the NTs were observed in COC solvent (“good” solvent). From the other side, the aggregation of NTs was very pronounced in nematic 5CB solvent (“bad” solvent). The dispersing quality of solvent influenced the percolation concentration Cp, corresponding to transition between the low conductive and high conductive states: e.g., percolation was observed at Cp≈1% and Cp≈0.1% for pure COC and 5CB, respectively. The effects of thermal pre-history on the heating-cooling hysteretic behavior of electrical conductivity were studied. The mechanism of dispersion of NTs in COC+5CB mixtures is discussed. Utilization of the mixtures of “good” and “bad” solvents allowed fine regulation of the dispersion, stability and electrical conductivity of LC+NTs composites. The mixtures of COC and 5CB were found to be promising for application as functional media with controllable useful chiral and electrophysical properties.

Chain topological reactions in developing random light fields

It was investigated development of singular generic elliptic speckle patterns generated and driven by laser beam in LiNbO3:Fe crystal by quick-action stokes polarimetry. It is realized through totality of local topological transitions in random space-time point governed by smoothly varying control parameter (amplitude of funnel bottom in the place of forthcoming vortices pair nucleation). Vortices pair nucleates when zero amplitude value is reached. Total development of singular light fields proceeds through the direct and chain topological reactions. Direct reactions possess short spacetime loop trajectories. Chain reaction trajectory consists from sequence of singularities pair nucleation and annihilation with singularities from other pairs. They can finish generically on field borders only, dont touch and intersect. Existing wave front singularities are arranged in the topological network which plays role of its skeleton and keeps integrity during field development. Its genesis was realized in the speckle field appeared after PDLC cell with variable applied constant electric field. Nucleation of first optical vortices pairs was observed firstly.

Singular trajectories: space-time domain topology of developing speckle fields

It is shown the space-time dynamics of optical singularities is fully described by singularities trajectories in space-time domain, or evolution of transverse coordinates(x, y) in some fixed plane z0. The dynamics of generic developing speckle fields was realized experimentally by laser induced scattering in LiNbO3:Fe photorefractive crystal. The space-time trajectories of singularities can be divided topologically on two classes with essentially different scenario and duration. Some of them (direct topological reactions) consist from nucleation of singularities pair at some (x, y, z0, t) point, their movement and annihilation. They possess form of closed loops with relatively short time of existence. Another much more probable class of trajectories are chain topological reactions. Each of them consists from sequence of links, i.e. of singularities nucleation in various points (xi yi, ti) and following annihilation of both singularities in other space-time points with alien singularities of opposite topological indices. Their topology and properties are established. Chain topological reactions can stop on the borders of a developing speckle field or go to infinity. Examples of measured both types of topological reactions for optical vortices (polarization C points) in scalar (elliptically polarized) natural developing speckle fields are presented.

Complex light with optical singularities induced by nanocomposites

The nanocomposites on the base of long (5-10μm, o-MWCNTs) and short (~ 2μm, m-MWCNTs) multi-walled carbon nanotubes (MWCNTs) hosted by nematic 5CB were investigated in details by means of polarizing microscopy, studies of electrical conductivity and electro-optical behaviour. The spontaneous self-organization of MWCNTs was observed and investigated both theoretically and experimentally. The efficiency of MWCNT aggregation in these composites is controlled by strong, long ranged and highly anisotropic van der Waals interactions and Brownian motion of individual nanotubes and their aggregates. The simple Smoluchowski approach was used for estimation of the half-time of aggregation. It was shown that aggregation process includes two different stages: fast, resulting in formation of loose aggregates (L-aggregates) and slow, resulting in formation of compacted aggregates (C-aggregates). Both L- and C- aggregates possess extremely ramified fractal borders. Formation of the percolation structures was observed for o-MWCNTs at C=Cp≈0.025-0.05 % wt and for m-MWCNTs at C=Cp≈0.1- 0.25 % wt. A physical model describing formation of C-aggregates with captured 5CB molecules inside was proposed. It shows good agreement with experimentally measured characteristics. It was shown that MWCNTs strongly affect the structural organization of LC molecules captured inside the MWCNT skeleton and of interfacial LC layers in the vicinity of aggregate borders. Moreover, the structure of the interfacial layer, as well as its birefringence, drastically changed when the applied electric voltage exceeded the Freedericksz threshold. Finally, formation of the inversion walls between branches of the neighbouring MWCNT aggregates was observed and discussed for the first time.

Self-organized composites of multiwalled carbon nanotubes and nematic liquid crystal 5CB: optical singularities and percolation behavior in electrical conductivity

This work discusses optical singularities and electrical conductivity behavior in a thin electrooptical cell filled with composites including multi-walled carbon nanotubes (MWCNTs) and nematic liquid crystal (LC). The MWCNTs with high aspect ratio L/d≈300 ÷ 1000 and nematic LC 5CB (4-pentyl-40-cyanobiphenyl) were used. The composites were prepared by introduction of MWCNTs (0.0001÷0.1% wt) into LC solvent with subsequent sonication. The increase of MWCNT concentration (between 0.005÷0.05 % wt) resulted in self-organization of MWCNTs and formation of micronsized aggregates with fractal boundaries. The visually observed formation of spanning MWCNT networks near the percolation threshold at ~0.025 % wt was accompanied with transition from non-conductive to conductive state and generation of optical singularities. The observed effects were explained by the strong interactions between MWCNTs and LC medium and planar orientation of 5CB molecules near the lateral surface of MWCNTs. It was speculated that optical singularities arose as a results of interaction of an incident laser beam with LC perturbed interfacial shells covering the MWCNT clusters. Behavior of the interfacial shell thickness in external electric field and in the vicinity of the nematic to isotropic transition was discussed.

Front Matter: Volume 8274

This PDF file contains the front matter associated with SPIE Proceedings Volume 8274, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Fine topological structure of coherent complex light created by carbon nanocomposites in LC

Fine complex light structure, optical singularities and electroconductivty of nematic 5CB doped by multi-walled carbon nanotubes (MWCNTs) were investigated. MWCNTs gather spontaneously to system of micro scale clusters with random fractal borders at small enough concentration. They are surrounded by the striped micro scale cladding which creates optical singularities in propagating laser beam. Applied transverse electric field above the Freedericksz initiates homeotropic arrangement of 5CB and the striped inversion walls between nanotubes clusters what diminishes free energy of a composite. Theory of their appearance and properties was built. Simultaneously the striped cladding disappears what can be treated as new mechanism of structure orientation nonlinearity in nonlinear photonics. Polarization singularities (circular C points) were measured firstly. Percolation of clusters enhances strongly electrical conductivity of the system and creates inversion walls even without applied field. Carbon nanotubes composites in LC form bridge between nano dopants and micro/macro system and are promising for applications. Elaborated protocol of singular optics inspection and characterization of LC nanocomposites is promising tool for applications in modern nanosience and technique.

Experimental investigation of critical points in optical coherence function

By using an unconventional holography, referred to as coherence holography, developed recently, we will explore the whole phase field in an optical coherence function and present the direct experimental investigation to the coherence critical points, including coherence phase saddles, coherence phase extrema and coherence phase singularities. We have observed the local phase structures around the coherence critical points, and studied the relationship between the saddles and the extrema in the optical coherence function. Some topological rules associated with the coherence critical points, such as topological sign rule governing the coherence vortices and topological index conservation during the reaction of the coherence vortices, are also investigated by experiments.