Optical waveguiding in bent-core liquid-crystal filaments

Fluid Ferroelectric Filaments

Freestanding slender fluid filaments of room-temperature ferroelectric nematic liquid crystals are described. They are stabilized either by internal electric fields of bound charges formed due to polarization splay or by external voltage applied between suspending wires. The phenomenon is similar to those observed in dielectric fluids, such as deionized water, except that in ferroelectric nematic materials the voltages required are three orders of magnitudes smaller and the aspect ratio is much higher. The observed ferroelectric fluid threads are not only unique and novel but also offer measurements of basic physical quantities, such as the ferroelectric polarization and viscosity. Ferroelectric nematic fluid threads may have practical applications in nano-fluidic micron-size logic devices, switches, and relays.

Confinement-sensitive optical response of cholesteric liquid crystals in electrospun fibers

Soft self-assembling photonic materials such as cholesteric liquid crystals are attractive due to their multiple unique and useful properties, in particular, an optical band gap that can be continuously and dynamically tuned in response to weak external influences, easy device integration, compatibility with flexible architectures, and, as shown here, potential for submicrometer optical applications. We study such a system formed by a short-pitch cholesteric confined in the core of polymer fibers produced by coaxial electrospinning, showing that the selective reflection arising from the helical photonic structure of the liquid crystal is present even when its confining cavity is well below a micrometer in thickness, allowing as little as just half a turn of the helix to develop. At this scale, small height variations result in a dramatic change in the reflected color, in striking difference to the bulk behavior. These conclusions are made possible by combining focused ion beam (FIB) dissection and imaging of the internal fiber morphology with optical microscopy. The FIB dissection further reveals that the cross section of the cavity within the fiber can have a shape that is quite different from that of the outside fiber. This is critical for the photonic behavior of the composite fiber because different optical textures are generated not only by change in thickness but also by the shape of the cavity. Our results provide insights into the behavior of cholesterics in submicrometer cavities and demonstrate their potential at such dimensions.

Tension of freely suspended fluid filaments

Stable fluid filaments with diameters of several micrometers and slenderness ratios well above 1000 are unique objects formed by some liquid crystalline phases of bent-core mesogens. We present a technique to determine filament tensions from their deflection under defined loads. A strong temperature dependence is observed, with a minimum near the clearing temperature. Both the nonlinear relation between filament tension and diameter and the substantial increase of the tension with lower temperatures indicate contributions of volume terms, in addition to surface capillary forces. We discuss a model that relates these bulk terms to elastic forces, originating from the undulated smectic layer structure. This model can explain the origin of the filament stability.

Pattern-stabilized decorated polar liquid-crystal fibers

Geometric frustration gives rise to new fundamental phenomena and is known to yield the formation of exotic states of matter, such as incommensurate crystals, modulated liquid-crystalline phases, and phases stabilized by defects. In this Letter, we present a detailed study of polar structure of freely suspended fluid filaments in a polarization modulated liquid-crystal phase. We show that a periodic pattern of polarization-splay stripes separated by defect boundaries and decorating smectic layers can stabilize the structure of fluid fibers against the Rayleigh-Plateau instability. The instability is suppressed by the resistance of the defect structure to a radial compression of the cylindrical fibers. Our results provide direct experimental observation of a link between the stability of the liquid fibers, internal polar order, and geometrical constraints. They open a new perspective on a wide range of fluid polar fiber materials.

Boundary-layer method for the analytical calculation of stable textures of bent-core liquid crystal fibers

We study the equilibrium textures of molecular orientation inside cylindrical fibers made of coaxial layers of bent-core smectics. We propose a free-energy model taking into account surface-like and bulk contributions--including layer-compression and electrostatic terms among others--with constant values of the material parameters. We follow the usual variational procedure of minimization of the free energy with respect to the tilt-angle profile θ(r) and obtain an Euler-Lagrange equation and its boundary condition. We solve the variational equations for the equilibrium configurations using a boundary-layer approximation and find multiple solutions. Since the equilibrium tilt profiles are found to be radially inhomogeneous, we select those with minimum distortions in order to find the lowest free-energy state. We minimize further the free energy of the system with respect to the fiber radius and find wider intervals of stability than those previously reported, depending on the balance of the material's spontaneous polarization, elastic and electric divergence-of-polarization constants, and surface-tension coefficients. The bulk and surface-layer structures thus found could be used to calculate the allowed modes of propagation of electromagnetic waves inside the fiber.