Topological Metadefects: Tangles of Dislocations.

The concept of topological defects is universal. In condensed matter, it applies to disclinations, dislocations, or vortices that are fingerprints of symmetry breaking during phase transitions. Using as a generic example the tangles of dislocations, we introduce the concept of topological metadefects, i.e., defects made of defects. We show that in cholesterics, dextrogyre and levogyre primary tangles are generated through the D_{2}→C_{2} symmetry breaking from the coplanar dislocation pair called Lehmann cluster submitted to a high enough tensile strain. The primary tangles can be wound up individually into double helices. They can also annihilate in pairs or associate into tangles of higher orders following simple algebraic rules.

Tropisms of the Dowser Texture.

Due to its low symmetry C2v, the dowser texture is characterised by a 2D unitary vector field or alternatively by a unitary complex field. For the same symmetry reasons, the dowser texture is sensitive, in first order, to perturbations such as thickness gradients, electric fields or flows. We will focus on corresponding properties called respectively: cuneitropism, electrotropism and rheotropism. In particular we will show that topological defects, known as dowsons or monopoles, can be manipulated by means of these tropisms.

Electro-osmosis and flexo-electricity in the dowser texture.

Due to its low symmetry, the long-lived pseudo-planar texture, dubbed "the dowser texture", has a flexo-electric spontaneous polarisation [Formula: see text]. Being degenerated, the dowser texture is easily aligned by the electric torque [Formula: see text] acting on [Formula: see text]. The dowser texture can also be aligned by Poiseuille flows driven by electro-osmosis. The hydrodynamic torques due to the electro-osmotic flows are linear in [Formula: see text] like the electrical one. It is shown that in 5CB the electro-osmotic flows can alter measurements of the flexo-electric polarisation.

Flexo-electricity of the dowser texture.

The persistent quasi-planar nematic texture known also as the dowser texture is characterized by a 2D unitary vector field d. We show here that the dowser texture is sensitive, in first order, to electric fields. This property is due to the flexo-electric polarisation P collinear with d expected from R. B. Meyer's considerations on flexo-electricity in nematics. It is pointed out that due to the flexo-electric polarisation nematic monopoles can be manipulated by electric fields of appropriate geometry.

Rheotropism of the dowser texture.

In spite of its metastability, the pseudo-planar texture of a nematic layer confined between surfaces with homeotropic anchoring can be preserved indefinitely in certain conditions. The pseudo-planar texture, dubbed "the dowser texture", is degenerated and therefore sensitive to perturbations. It has been shown recently that the dowser texture is cuneitropic, that is to say, has a tendency to follow thickness gradients. Here, we point out that the dowser texture is also rheotropic or, in other words, has a weathercock behavior: it tends to follow the direction of Poiseuille flows. We observed this behavior in two experiments. In the first one it appears as a deformation of a wound up dowser texture submitted to a slowly alternating linear Poiseuille flow. In the second one we study rheotropic effects of elliptical flow patterns generated by modulation of the gap thickness in a system made of a rigid lens and a flexible glass slide. We show that such elliptical flows can wind the dowser texture and therefore can be used in studies of generation and annihilation of nematic monopoles.

Action of fields on captive disclination loops.

We report on two effects observed in experiments with captive disclination loops on polymeric fibers immersed in nematics and submitted to electric and/or magnetic fields. We show that the magnetic field oblique to a fiber with axial or helicoidal anchoring on its surface induces translation of disclination loops. Fields orthogonal to fibers with helicoidal anchoring make disclination loops rotate around the field direction. In the linear regime of this last chirogyral effect, the angle of rotation is proportional to the helix wave vector and its sense unveils the chirality of the helix. We propose a model explaining the origin and all features of these two effects.

Mind the Microgap in Iridescent Cellulose Nanocrystal Films.

A new photonic structure is produced from cellulose nanocrystal iridescent films reflecting both right and left circularly polarized light. Micrometer-scale planar gaps perpendicular to the films' cross-section between two different left-handed films' cholesteric domains are impregnated with a nematic liquid crystal. This photonic feature is reversibly tuned by the application of an electric field or a temperature variation.

Hedgehogs in the dowser state.

We show how to easily generate point defects called hedgehogs, in the so-called quasi-planar texture --the dowser state-- of a nematic layer confined between surfaces with homeotropic anchoring conditions. We point out that the dowser texture can be preserved infinitely in spite of its higher energy with respect to the homogeneous homeotropic texture. For topological reasons the dowser state in a squeezed droplet must contain at least one hedgehog. We submitted this hedgehog to a rotating magnetic field and controlled the continuous evolution, transitioning continuously between radial, hyperbolic and circular hedgehogs, which, just as in previous experiments by Lavrentovich et al., are topologically equivalent states. The dynamics of this transformation is shown to be directly sensitive to energy costs of different geometric configurations of the hedgehog defect and therefore can be used as a rough probe for elastic constants; knowing the principal elastic constants K1,2,3, one can retrieve information about the K24 constant. We propose also a method of generation of hedgehog pairs by application of a Poiseuille flow to a dowser state wound by a rotating magnetic field.

Persistent quasiplanar nematic texture: Its properties and topological defects.

In the so-called quasiplanar texture of a nematic layer confined between parallel plates with homeotropic anchoring conditions, the director field rotates by π between limit surfaces so that field lines have the shape of a dowsing Y-shaped wooden tool. The orientation of the director field at midheight of the layer is arbitrary for symmetry reasons and is thus very sensitive to perturbations. We point out that contrary to accepted ideas the quasiplanar texture can be preserved infinitely in spite of its metastability with respect to the homogeneous homeotropic texture. We propose to call such a long-lived version of the quasiplanar texture the dowser texture. We demonstrate both experimentally and theoretically that in samples of variable thickness, the director field is sensitive to the gradient of the sample thickness through a linear coupling term. As a result, it has a tendency to follow the direction of the thickness gradient. Because of its sensitivity to perturbations we propose to call the midplane director field the dowser field and its tendency to follow the thickness gradient cuneitropism. Under effect of the gradient field, the dowser field obeys the sine-Gordon equation and exhibits domain walls that correspond to the well-known solitonic solutions of the sine-Gordon model.

Sensing and tuning microfiber chirality with nematic chirogyral effect.

Microfibers with their elongated shape and translation symmetry can act as important components in various soft materials, notably for their mechanics on the microscopic level. Here we demonstrate the mechanical response of a micro-object to imposed chirality, in this case, the tilt of disclination rings in an achiral nematic medium caused by the chiral surface anchoring on an immersed microfiber. This coupling between chirality and mechanical response, used to demonstrate sensing of chirality of electrospun cellulose microfibers, is revealed in the optical micrographs due to anisotropy in the elastic response of the host medium. We provide an analytical explanation of the chirogyral effect supported with numerical simulations and perform an experiment to test the effect of the cell confinement and fiber size. We controllably twist the microfibers and demonstrate the response of the nematic medium. More generally the demonstrated study provides means for experimental discrimination of surface properties and allows mechanical control over the shape of disclination rings.

Physics of Free-Standing Lyotropic Films.

We explore the structures and properties of stable, free-standing films of lyotropic mesophases drawn on apertures of various shapes in an atmosphere of controlled humidity. New phenomena are uncovered and interpreted.

When boundaries dominate: dislocation dynamics in smectic films.

We discuss the influence of dissipation at a system boundary (film-meniscus interface) on the dynamics of dislocation loops inside a smectic film. This dissipation induces a strong coupling between dislocations-effectively independent of their separation-leading to their nontrivial dynamics. Because of these dynamics, the effective "dynamical" radius of nucleation can be 10 times larger than the usual static critical radius.