Igor Muševič

Reconfigurable knots and links in chiral nematic colloids.

Colloidal particles inside a liquid crystalline solvent can be manipulated to create knots of arbitrary shape and complexity. Tying knots and linking microscopic loops of polymers, macromolecules, or defect lines in complex materials is a challenging task for material scientists. We demonstrate the knotting of microscopic topological defect lines in chiral nematic liquid-crystal colloids into knots and links of arbitrary complexity by using laser tweezers as a micromanipulation tool. All knots and links with up to six crossings, including the Hopf link, the Star of David, and the Borromean rings, are demonstrated, stabilizing colloidal particles into an unusual soft matter. The knots in chiral nematic colloids are classified by the quantized self-linking number, a direct measure of the geometric, or Berry’s, phase. Forming arbitrary microscopic knots and links in chiral nematic colloids is a demonstration of how relevant the topology can be for the material engineering of soft matter.

3D microlasers from self-assembled cholesteric liquid-crystal microdroplets

We demonstrate a tunable and omnidirectional microlaser in the form of a microdroplet of a dye-doped, cholesteric liquid crystal in a carrier fluid. The cholesteric forms a Bragg-onion optical microcavity and the omnidirectional 3D lasing is due to the stimulated emission of light from the dye molecules in the liquid crystal. The lasing wavelength depends solely on the natural helical period of the cholesteric and can be tuned by varying the temperature. Millions of microlasers can be formed simply by mixing a liquid crystal, a laser dye and a carrier fluid, thus providing microlasers for soft-matter photonic devices.

Assembly and control of 3D nematic dipolar colloidal crystals.

Topology has long been considered as an abstract mathematical discipline with little connection to material science. Here we demonstrate that control over spatial and temporal positioning of topological defects allows for the design and assembly of three-dimensional nematic colloidal crystals, giving some unexpected material properties, such as giant electrostriction and collective electro-rotation. Using laser tweezers, we have assembled three-dimensional colloidal crystals made up of 4 μm microspheres in a bulk nematic liquid crystal, implementing a step-by-step protocol, dictated by the orientation of point defects. The three-dimensional colloidal crystals have tetragonal symmetry with antiparallel topological dipoles and exhibit giant electrostriction, shrinking by 25-30% at 0.37 V μm(-1). An external electric field induces a reversible and controllable electro-rotation of the crystal as a whole, with the angle of rotation being ~30° at 0.14 V μm(-1), when using liquid crystal with negative dielectric anisotropy. This demonstrates a new class of electrically highly responsive soft materials.

Human stefin B readily forms amyloid fibrils in vitro

Human stefin B (cystatin B) is an intracellular cysteine proteinase inhibitor broadly distributed in different tissues. Here, we show that recombinant human stefin B readily forms amyloid fibrils in vitro. It dimerises and further oligomerises, starting from the native-like acid intermediate, I(N), populated at pH 5. On standing at room temperature it produces regular (over 4 microm long) fibrils over a period of several months. These have been visualised by transmission electron microscopy and atomic force microscopy. Their cross-sectional diameter is about 14 nm and blocks of 27 nm repeat longitudinally. The fibrils are smooth, of unbranched surface, consistent with findings of other amyloid fibrils. Thioflavin T fluorescence spectra as a function of time were recorded and Congo red dye binding to the fibrils was demonstrated. Adding 10% (v/v) trifluoroethanol resulted in an increased rate of fibrillation with a typical lag phase. The finding that human stefin B, in contrast to the homologue stefin A, forms amyloid fibrils rather easily should promote further studies of the proteins behaviour in vivo, and/or as a model system for fibrillogenesis.

Surfactant sensing based on whispering-gallery-mode lasing in liquid-crystal microdroplets

Lasing of whispering-gallery modes in nematic liquid-crystal microdroplets, floating in water, is demonstrated. It is shown that millimolar concentrations of sodium dodecyl sulfate in water effect the orientation of liquid-crystal molecules in the microdroplet, which changes the lasing spectrum. The presence of targeted molecules in water can be monitored by simply measuring and recognizing the spectrum of light, lasing from a small liquid-crystal droplet in water.

Surfaces and interfaces of liquid crystals

Prologue.- 1 Introduction.- 2 Surface-Induced Order Detected by Deuteron Nuclear Magnetic Resonance.- 3 Interfacial and Surface Forces in Nematics and Smectics.- 4 Linear Optics of Liquid Crystal Interfaces.- 5 Solid-Liquid Crystal Interfaces Probed by Optical Second-Harmonic Generation.- 6 Liquid Crystal Alignment on Surfaces with Orientational Molecular Order: A Microscopic Model Derived from Soft X-ray Absorption Spectroscopy.- 7 Scanning Probe Microscopy Studies of Liquid Crystal Interfaces.- 8 Introduction to Micro- and Macroscopic Descriptions of Nematic Liquid Crystalline Films: Structural and Fluctuation Forces.- 9 Applications.- Epilogue.

Interactions of micro-rods in a thin layer of a nematic liquid crystal

We present an experimental analysis of the topological properties of a nematic liquid crystal in the vicinity of small cylindrical objects (micro-rods), dispersed in the nematic liquid crystal 5CB (4-n-pentyl-4-cyanobiphenyl). Depending on the type of liquid crystal anchoring on the surfaces of micro-rods, we have observed two types of the symmetry in the surrounding liquid crystal molecules: dipolar and quadrupolar. Using the manipulation of micro-rods with laser tweezers, the strength and separation dependencies of various pair interaction potentials have been determined. Our results are in qualitative agreement with previous theoretical predictions for 2D interactions of micro-rods in the nematic liquid crystals.

Design of 2D Binary Colloidal Crystals in a Nematic Liquid Crystal

In this paper, we examine directed self-assembly in a 2D binary system of dipolar and quadrupolar colloidal particles with normal surface boundary conditions, dispersed in the nematic liquid crystal. Using the laser tweezers, we assembled a large variety of stable 2D colloidal crystal structures. In all analyzed structures, the particles, their surface treatment and the cell conditions were the same, which gives us the ability to systematically follow the evolution of colloidal assembly when many particles are present. We present an analogy between molecular self-assembly and organization of colloidal microspheres in liquid crystalline medium to extend the strategy for designing colloidal crystalline structures of different level of complexity.

Self-assembly of nematic colloids

Dispersions of colloidal particles in nematic liquid crystals show new classes of interparticle forces, which are anisotropic, long range, and several thousand times stronger than van der Waals forces in water-based colloids. These forces are responsible for a variety of new self-assembled colloidal microstructures, which cannot be observed in isotropic solvents, such as chains of microspheres and cellular soft solid materials. Basic principles of particle self-assembly in 2D nematic colloids are discussed, showing this is a novel paradigm in colloid science, which could lead to new approaches of colloidal self-assembly for photonic devices.

Theoretical and experimental study of the nanoparticle-driven blue phase stabilisation

Abstract.We have studied theoretically and experimentally the effects of various types of nanoparticles (NPs) on the temperature stability range