Liquid Crystal Institute

About: Liquid Crystal Institute is a based out in . It is known for research contribution in the topics: Liquid crystal & Phase (matter). The organization has 1037 authors who have published 1979 publications receiving 53749 citations.

Fast liquid-crystal elastomer swims into the dark

Abstract: Liquid-crystal elastomers (LCEs) are rubbers whose constituent molecules are orientationally ordered. Their salient feature is strong coupling between the orientational order and mechanical strain. For example, changing the orientational order gives rise to internal stresses, which lead to strains and change the shape of a sample. Orientational order can be affected by changes in externally applied stimuli such as light. We demonstrate here that by dissolving-rather than covalently bonding-azo dyes into an LCE sample, its mechanical deformation in response to non-uniform illumination by visible light becomes very large (more than 60 degrees bending) and is more than two orders of magnitude faster than previously reported. Rapid light-induced deformations allow LCEs to interact with their environment in new and unexpected ways. When light from above is shone on a dye-doped LCE sample floating on water, the LCE 'swims' away from the light, with an action resembling that of flatfish such as skates or rays. We analyse the propulsion mechanism in terms of momentum transfer.

Thermal Conductivity and Particle Agglomeration in Alumina Nanofluids: Experiment and Theory

Abstract: In recent years many experimentalists have reported an anomalously enhanced thermal conductivity in liquid suspensions of nanoparticles. Despite the importance of this effect for heat transfer applications, no agreement has emerged about the mechanism of this phenomenon, or even about the experimentally observed magnitude of the enhancement. To address these issues, this paper presents a combined experimental and theoretical study of heat conduction and particle agglomeration in nanofluids. On the experimental side, nanofluids of alumina particles in water and ethylene glycol are characterized using thermal conductivity measurements, viscosity measurements, dynamic light scattering, and other techniques. The results show that the particles are agglomerated, with an agglomeration state that evolves in time. The data also show that the thermal conductivity enhancement is within the range predicted by effective medium theory. On the theoretical side, a model is developed for heat conduction through a fluid containing nanoparticles and agglomerates of various geometries. The calculations show that elongated and dendritic structures are more efficient in enhancing the thermal conductivity than compact spherical structures of the same volume fraction, and that surface (Kapitza) resistance is the major factor resulting in the lower than effective medium conductivities measured in our experiments. Together, these results imply that the geometry, agglomeration state, and surface resistance of nanoparticles are the main variables controlling thermal conductivity enhancement in nanofluids.

Making waves in a photoactive polymer film

Abstract: Oscillating materials that adapt their shapes in response to external stimuli are of interest for emerging applications in medicine and robotics. For example, liquid-crystal networks can be programmed to undergo stimulus-induced deformations in various geometries, including in response to light. Azobenzene molecules are often incorporated into liquid-crystal polymer films to make them photoresponsive; however, in most cases only the bending responses of these films have been studied, and relaxation after photo-isomerization is rather slow. Modifying the core or adding substituents to the azobenzene moiety can lead to marked changes in photophysical and photochemical properties, providing an opportunity to circumvent the use of a complex set-up that involves multiple light sources, lenses or mirrors. Here, by incorporating azobenzene derivatives with fast cis-to-trans thermal relaxation into liquid-crystal networks, we generate photoactive polymer films that exhibit continuous, directional, macroscopic mechanical waves under constant light illumination, with a feedback loop that is driven by self-shadowing. We explain the mechanism of wave generation using a theoretical model and numerical simulations, which show good qualitative agreement with our experiments. We also demonstrate the potential application of our photoactive films in light-driven locomotion and self-cleaning surfaces, and anticipate further applications in fields such as photomechanical energy harvesting and miniaturized transport.

Observation of a Biaxial Nematic Phase in Potassium Laurate-1-Decanol-Water Mixtures

Abstract: The phase diagram of the ternary system potassium laurate-1-decanol-D$_{2}\mathrm{O}$ was studied over concentration ranges where nematic phases are likely to occur. Two uniaxial nematic phases which are separated by a biaxial nematic phase are found. In limited concentration range the following phase sequence may be observed reversibly on heating and on cooling: isotropic-uniaxial nematic (positive optical anisotropy)-biaxial nematic-uniaxial nematic (negative optical anisotropy)-biaxial nematic-uniaxial nematic (positive optical anisotropy)-isotropic.

Tunable Mirrorless Lasing in Cholesteric Liquid Crystalline Elastomers

Abstract: Cholesteric liquid crystals are chiral nematics, where the handedness of the constituent molecules causes the orientation of the local nematic director to vary in space. In the helical cholesteric structure, the director is perpendicular to the helix axis, and its orientation varies linearly with position along the helix axis. The spatial period of the structure is the pitch, which is determined by the concentration and the helical twisting power of the chiral constituents. As a consequence of the periodicity of the helical cholesteric structure and the birefringence of the liquid crystal, for a range of wavelengths, light propagation along the helix axis is forbidden for one of the normal modes. Since propagation is forbidden, incident light with a wavelength in this band and with the same helicity as the cholesteric is strongly reflected. The edges of this reflection band are at wavelengths that are equal to the refractive indices times the pitch. [1] Because of the existence of the selective reflection band, cholesteric liquid crystals are 1D photonic bandgap materials. The bandgap structure of cholesteric liquid crystals allows for the possibility of lasing without external mirrors that usually form a laser cavity. When a fluorescent dye is dissolved in the cholesteric host so that the peak of the fluorescent emission of the dye is in the selective reflection band of the cholesteric, propagation of one normal mode of the emitted light is forbidden. As a consequence, at low pump intensities, the fluorescence spectrum of the dye is modified, [2] showing suppression of emission in the reflection band, and enhanced emission near the band edge. As the pump intensity is increased, the linewidth of the enhanced fluorescence at the band edge narrows, and, above a pump threshold, lasing occurs. [2] Thin samples, typically 15‐30 lm in thickness, of low molecular weight cholesteric liquid crystals incorporating a variety of dyes [3] have been shown to lase. The primary role of the cholesteric liquid crystal in these systems is to act as a distributed cavity. Lasing occurs at the band edges, [2‐4] as pre