Small water droplets dispersed in a nematic liquid crystal exhibit a novel class of colloidal interactions, arising from the orientational elastic energy of the anisotropic host fluid. These interactions include a short-range repulsion and a long-range dipolar attraction, and they lead to the formation of anisotropic chainlike structures by the colloidal particles. The repulsive interaction can lead to novel mechanisms for colloid stabilization.

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Novel Colloidal Interactions in Anisotropic Fluids

In this article, we review the current state of knowledge concerning the physical and chemical properties of the eumelanin pigment. We examine properties related to its photoprotective functionality, and draw the crucial link between fundamental molecular structure and observable macroscopic behaviour. Where necessary, we also briefly review certain aspects of the pheomelanin literature to draw relevant comparison. A full understanding of melanin function, and indeed its role in retarding or promoting the disease state, can only be obtained through a full mapping of key structure-property relationships in the main pigment types. We are engaged in such an endeavor for the case of eumelanin.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1600-0749.2006.00345.x

The physical and chemical properties of eumelanin.

Physics of colloidal dispersions in nematic liquid crystals

Liquid crystals are remarkably useful for laboratory exploration of the dynamics of cosmologically relevant defects. They are convenient to work with, they allow the direct study of the "scaling solution" for a network of strings, and they provide a model for the evolution of monopoles and texture. Experiments described here support the simple "one-scale" model for cosmic string evolution, as well as some qualitative predictions of string statistical mechanics. The structure of monopoles and their apparent cylindrical but not spherical symmetry is discussed. A particular kind of defect known as texture is described and is shown to have a dynamical instability—it can decay into a monopole-antimonopole pair. This decay process has been observed occurring in the liquid crystal, and studied with numerical simulations.

https://science.sciencemag.org/content/sci/251/4999/1336.full.pdf

Cosmology in the Laboratory: Defect Dynamics in Liquid Crystals

Charge transfer transitions in solid tetracene and pentacene studied by electroabsorption

The fundamental concepts of the homotopy theory of defects in liquid crystals and the results of experimental studies in this field are presented. The concepts of degeneracy space, homotopy groups, and topological charge, which are used for classifying the topologically stable inhomogeneous distributions in different liquid-crystalline phases are examined (uni and biaxial nematics, cholesterics, smectics, and columnar phases). Experimental data are given for the different mesophases on the structure and properties of dislocations, disclinations, point defects in the volume (hedgehogs) and on the surface of the medium (boojums), monopoles, domain formations, and solitons. Special attention is paid to the results of studies of defects in closed volumes (spherical drops, cylindrical capillaries), and to the connection between the topological charges of these defects and the character of the orientation of the molecules of the liquid crystal at the surface. A set of fundamentally new effects that can occur in studying the topological properties of defects in liquid crystals is discussed.

https://iopscience.iop.org/article/10.1070/PU1988v031n03ABEH005710/pdf

Defects in liquid crystals: homotopy theory and experimental studies

Urbach rule and disordering processes in Cu6P(S1−xSex)5Br1−yIy superionic conductors

A comparative analysis of temperature behavior of optical absorption edge is performed for non-crystalline materials where the Urbach behavior is observed (SiO 2 ), with deviations from the Urbach behavior (20PbO · 80SiO 2 ) and with a mixed behavior (As 2 S 3 ). For glassy As 2 S 3 , the Urbach behavior of the absorption edge in a limited temperature range is explained from the point of view of two components of structural disordering – static and dynamic. A parallel red shift of the absorption edge in As 2 S 3 and, consequently, the temperature-independent Urbach energy at T

On the Urbach rule in non-crystalline solids

The main optical, dark and photoconductivity, photo-emf, and photoemission characteristics are studied for thin-layers of pentacene (Pc) crystals. It is shown that in the h = = 2.2 to 4.0 eV spectral region intrinsic photogeneration is dominant in Pc crystals, presumably proceeding via an autoionization mechanisms with a threshold value Ec = = (2.20 ± 0.05) eV. Whereas in the hvv = 1.4 to 2.2 eV spectral region an exciton photogeneration mechanism, namely, generation of excess charge carriers through triplet exciton-trapped hole interaction, seems most probably. The energy level spectra of neutral and ionized states of the Pc crystal are determined. The Pc crystal ionization energy Ic equals to (5.07 ± 0.05) eV; the forbidden energy gap ΔE0 = (2.20 ± 0.05) eV; the intrinsic dark conductivity activation energy E = ΔE0/2 = (1.15 ± 0.15) eV. The electron affinity Ac of the Pc crystal, determined by three independent methods equals to (2.87 ± 0.10) eV, (2.70 ± 0.15) eV, or (2.73 ± 0.2) eV, respectively. The characteristic energies of the quasi-continuous trap distribution for vacuum evaporated thin Pc crystals layers are kTc = 0.12 eV in an exponential approximation and ± = 0.28 eV in a Gaussian one as determined from SCLC measurements.



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Photoelectrical properties, energy level spectra, and photogeneration mechanisms of pentacene

The existence of the monopoles both with one x (+2) line and two x (+1) lines in cholesteric volumes with spherical shape is shown experimentally by microscopic observations with polarized light. The effect of disapperarnce of x lines near the cholesteric-smectic A transition is discovered.

M. V. Kurik

Papers

Defects in liquid crystals: homotopy theory and experimental studies

Urbach rule and disordering processes in Cu6P(S1−xSex)5Br1−yIy superionic conductors

On the Urbach rule in non-crystalline solids

Photoelectrical properties, energy level spectra, and photogeneration mechanisms of pentacene

Topological defects of cholesteric liquid crystals for volumes with spherical shape