Metric approach for sound propagation in nematic liquid crystals

Motility of acoustically powered micro-swimmers in a liquid crystalline environment

Suspensions of microswimmers in liquid crystals demonstrate remarkably complex dynamics and serve as a model system for studying active nematics. So far, experimental realization of microswimmers suspended in liquid crystalline media has relied on biological microorganisms that impose strict limitations on the compatible media and makes it difficult to regulate activity. Here, we demonstrate that acoustically powered bubble microswimmers can efficiently self-propel in a lyotropic liquid crystal. The velocity of the swimmers is controlled by the amplitude of the acoustic field. Histograms of swimming directions with respect to the local nematic field reveal a bimodal distribution: the swimmers tend to either fully align with or swim perpendicular to the director field of the liquid crystal, occasionally switching between these two states. The bubble-induced streaming from a swimmer locally melts the liquid crystal and produces topological defects at the tail of the swimmer. We show that the defect proliferation rate increases with the angle between the swimmer's velocity and the local orientation of the director field.

Concurrent guiding of light and heat by transformation optics and transformation thermodynamics via soft matter

Controlling light and heat via metamaterials has presented interesting technological applications using transformation optics (TO) and transformation thermodynamics (TT). However, such devices are commonly mono-physics and mono-purpose, because the used metamaterial is designed to deal with one type of physical mechanisms. Here we demonstrate, for the first time, how to connect TO and TT via the liquid crystal 4-Cyano-4'-pentylbiphenyl (5CB) and, to exemplify such link, we present a multiphysics, multi-purpose device that simultaneously controls light and heat using such material. The anisotropic multiphysics properties of 5CB bond TO and TT, expanding the usage of these theories. The device, composed by 5CB confined between two right circular concentric cylinders, concentrates light (as a converging lens) and simultaneously repels heat from the inner cylinder when the molecules are along the direction [Formula: see text] and it disperses light (as a diverging lens) and concurrently concentrates heat to the inner cylinder, without disturbing the external temperature field, when the molecules are along the direction [Formula: see text], contributing for saving materials and designing miniaturized multiphysics systems.

Thermal and shape topological robustness of heat switchers using nematic liquid crystals

One interesting way to control heat is to use devices designed by transformation thermics, where artificial media are used. However, once manufactured (either repelling or concentrating heat, for example), besides being mono-purpose, such devices are designed according to a specific geometric boundary conditions. Another problem is the temperature dependence of the materials employed, since their properties are sometimes considered temperature-invariant. In this paper, we show that a previously proposed bi-objective heat switcher (Phys. Rev. E 89, 020501(R) (2014)) is in fact robust against temperature and geometric deformations, due to the topological properties of the molecular nematic orientation. Using a geometrical approach for heat propagation, by performing finite element simulations, we show that a device made by concentric cylinders with thermotropic nematic liquid crystal between them, sustains its functionality even with their molecular thermal conductivities depending on the temperature, achieving a 60% increase and a 44% decrease in the heat flux for each mode. Utilizing topological arguments we show that deformations on the surface of the outer cylinder do not break the operating mode (repeller or concentrator). We present a comparison between our geometrical approach and the transformation thermodynamics to give an additional explanation for the obtained results. We hope the presented device is useful for heat control under mechanical and thermal influence of the external environment.

Retrieving the saddle-splay elastic constant K24 of nematic liquid crystals from an algebraic approach

The physics of light interference experiments is well established for nematic liquid crystals. Using well-known techniques, it is possible to obtain important quantities, such as the differential scattering cross section and the saddl-splay elastic constant K24. However, the usual methods to retrieve the latter involve adjusting of computational parameters through visual comparisons between the experimental light interference pattern or a (2) H-NMR spectral pattern produced by an escaped-radial disclination, and their computational simulation counterparts. To avoid such comparisons, we develop an algebraic method for obtaining of saddle-splay elastic constant K24. Considering an escaped-radial disclination inside a capillary tube with radius R0 of tens of micrometers, we use a metric approach to study the propagation of the light (in the scalar wave approximation), near the surface of the tube and to determine the light interference pattern due to the defect. The latter is responsible for the existence of a well-defined interference peak associated to a unique angle [Formula: see text] . Since this angle depends on factors such as refractive indexes, curvature elastic constants, anchoring regime, surface anchoring strength and radius R0, the measurement of [Formula: see text] from the interference experiments involving two different radii allows us to algebraically retrieve K24. Our method allowed us to give the first reported estimation of K24 for the lyotropic chromonic liquid crystal Sunset Yellow FCF: K 24 = 2.1 pN.

Principles of thermal design with nematic liquid crystals

Highly engineered materials are arousing great interest because of their ability to manipulate heat, as described by the coordinate transformation approach. Based on recently developed analog gravity models, we present how a simple device based on nematic liquid crystals can achieve in principle either thermal concentration or expulsion. These outcomes are shown to stem from the topological properties of a disclination-like structure, induced in the nematic phase by anchoring conditions.