In this paper, we investigate the absolute stability of several Translationally Invariant Configurations (TICs) observed in cholesteric liquid crystal samples. The bounding plates of the samples may impose homeotropic anchoring (case 1), slightly tilted anchoring when the plates are also rubbed in either the same (case 2) or opposite (case 3) directions, or hybrid anchoring with planar on one plate and homeotropic on the other. In each case, the stability is examined as a function of the confinement ratio—defined as the ratio of the sample thickness to the cholesteric pitch—and of the applied field (electric and/or magnetic).

Quasiparticles in liquid crystals, such as torons and skyrmions, represent a new class of topologically protected solitonic excitations, offering a promising route toward soft microrobotics. Here we demonstrate that torons can be propelled by modulated electric fields and magnetically steered with full directional control, thus achieving programmable trajectories without net liquid flow. Within microfluidic architectures, we guide ensembles of torons through confined channels and realize targeted pick-up, transport, and release of colloidal cargo. By combining experiments and numerical simulations, we uncover how magnetic alignment reshapes toron structure, speed, and stability, while confinement within microchannels gives rise to novel transport regimes. Unlike conventional colloidal inclusions, torons are intrinsically uniform, soft, and reconfigurable, establishing them as both an ideal model system for studying emergent phenomena in active topological matter and a versatile platform for next-generation soft robots, adaptive delivery systems, and smart active matter.

We analyze experimentally and theoretically the winding and unwinding of the c[over ⃗] director in a chiral smectic-C^{★} film crossed by an ethanol flow. This leads to a target pattern under crossed polarizers when the +1 defect imposed by the boundary conditions is pinned on the edge of the film. We show that the target is deformed at the center of the film when it is subjected to a flow of ethanol because of the presence of two recirculation vortices of chemohydrodynamical origin. This deformation and the two vortices disappear during the unwinding of the target when the ethanol flow is stopped. This unambiguously shows that the target deformation is due to the vortices and not to the elastic anisotropy. These two points are confirmed theoretically. An estimate of the two so-called chemomechanical and chemohydrodynamical Leslie coefficients is also derived from this study.

In this paper, we numerically and experimentally show that the director field orientation degeneracy within the Translationally Invariant Configuration (TIC) of a cholesteric liquid crystal under an electric field can be lifted by imposing a magnetic field B→ parallel to the electrodes. The configuration can be either parallel or perpendicular to the magnetic field depending on the values of the sample thickness, pitch, and applied voltage, with two equiprobable orientations in each case. The transition between the parallel and perpendicular orientations has hysteresis, suggesting that it is first order. When B→ is slightly tilted with respect to the electrode plane, the indeterminacy on the TIC orientation is removed when the TIC is directed along B→.

Ce chapitre traite des effets thermomécaniques et thermohydrodynamiques dans les cholestériques et les nématiques déformés soumis à un gradient thermique. Plusieurs expériences statiques ou dynamiques sur la déformation, la rotation ou la translation de textures torsadées (TICs et doigts) sont analysées et démontrent que ces effets ne peuvent expliquer la rotation Lehmann des gouttes cholestériques.

The structure of the nematic (cholesteric) drops that form at the clearing temperature of a mixture of the bent-core molecule CB7CB and the rodlike molecule 8CB doped with a surfactant is optically determined. Using experimental observations and numerical simulations, it is demonstrated that the director field inside these drops is not escaped concentric, as previously proposed, but twisted bipolar. The Lehmann rotation of these drops in the presence of a temperature gradient is described. Their rotation velocity is shown to be proportional to the temperature gradient and to the surface twist angle of the director field and inversely proportional to the drop radius, thus revealing a fundamental scaling law for the Lehmann effect of nematic and cholesteric twisted-bipolar droplets.

In 1968, de Gennes and Meyer independently predicted that a cholesteric phase can form a stable oblique helicoidal (or heliconical) structure provided that K_{3}<K_{2} where K_{3} (K_{2}) is the bend (twist) constant. This structure usually develops under electric field when the material is of positive dielectric anisotropy and was observed for the first time in 2014 by Xiang et al. in a cholesteric phase made of a liquid crystal dimer material (CB7CB) in which K_{3} is anomalously small. Following a recent theoretical prediction by Poy and Žumer, I show that confining a similar cholesteric phase between two glass plates treated for unidirectional anchoring can lead to a similar heliconical instability. In that case, the confinement induces a surface field that acts as an effective electric field E with E≡1/d where d is the sample thickness. The experiment was conducted in a mixture of CB7CB +50 wt% 8CB doped with a small amount of the chiral molecule R811. In addition, I show that this mixture presents an unexpected compensation point near the transition to the N_{TB} phase.

We show experimentally and theoretically that the heliconical fluctuations that develop in a cholesteric phase (Ch) close to a transition to a chiral twist-bend nematic phase (N_{TB}) may lead to the appearance of a compensation point. At this point, the equilibrium twist of the cholesteric phase vanishes and changes sign. Mixtures of the flexible dimer CB7CB and the rodlike molecules 8CB or 5CB, doped with a small amount of the chiral molecules R811, S2011, CC, or CB15, are used in experiments to determine the conditions for the appearance of a compensation point.

Capacitance measurements have been extensively used to measure the anchoring extrapolation length L at a nematic–substrate interface. These measurements are extremely delicate because the value found for L often critically depends on the sample thickness and the voltage range chosen to perform the measurements. Several reasons have been proposed to explain this observation, such as the presence of inhomogeneities in the director distribution on the bounding plates or the variation with the electric field of the dielectric constants. In this paper, I propose a new method to measure L that takes into account this second effect. This method is more general than the one proposed in Murauski et al. Phys. Rev. E 71, 061707 (2005) because it does not assume that the anchoring angle is small and that the anchoring energy is of the Rapini–Papoular form. This method is applied to a cell of 8CB that is treated for planar unidirectional anchoring by photoalignment with the azobenzene dye Brilliant Yellow. The role of flexoelectric effects and the shape of the anchoring potential are discussed.

We propose to measure the ratio κ31 of the bend elastic constant K3 over the splay elastic constant K1 of a nematic phase by using an optical method based on the measurement of the birefringence of homeotropic/planar hybrid samples. This method is compared to the classical measurements using the Freedericksz transition under AC electric field and its advantages and disadvantages are discussed. Both methods are applied to the measurement of K1 and K3 close to the nematic-to-SmA phase transition of the liquid crystal 8CB and the nematic-to-NTB phase transition of the mixture 8CB + 50 wt% CB7CB. The role of flexoelectricity in these measurements is analysed, as well as the role of the pretilt angle and the anchoring energy of the molecules on the plates limiting the samples.

This paper deals with the Lehmann rotation of banded cholesteric droplets subjected to a temperature gradient when they coexist with their own isotropic liquid. I show that their angular rotation velocity increases --in absolute value-- when they are subjected to an additional AC electric field in the conducting regime. This velocity increase is correlated with a prolate distortion of the droplets and the probable presence of electrohydrodynamical toroidal circulation flows inside and outside the droplets. I propose that the coupling between these flows and the director field is responsible for the increase of the angular velocity of the texture. The origin of these flows is discussed qualitatively in the framework of the leaky dielectric model by taking into account the generation of charges both in the bulk via a Carr-Helfrich mechanism (Tarasov, Krekhow and Kramer model) and at the surface of the droplet (Taylor-Melcher model).

The Lehmann effect is the continuous rotation of cholesteric droplets subjected to a temperature gradient. Discovered by Otto Lehmann in 1900, this effect was re-observed recently by several authors not only in cholesterics but also in nematics when the director field is twisted inside the droplets. In most experiments, the droplets coexist with their isotropic liquid, but the Lehmann effect can also be observed when the droplets are dispersed in an isotropic liquid in which the LC is partly miscible. After a brief history on the Lehmann effect and its first explanation by Leslie in 1968, we will review the main experimental results obtained on this subject from 2008. In particular, the role of the temperature gradient, of the size of the droplets, of the textures and their orientation with respect to the temperature gradient, of the confinement effects, of the impurities and of the concentration of chiral molecules will be described. A special emphasis will also be placed on the research of hydrodynamic effects to answer the fundamental question of whether it is just the texture or the droplet itself that rotates. We will then review the different models proposed in the literature to explain the Lehmann effect. Among them are two thermomechanical models directly based on the Leslie explanation (named TM1 and TM2 models), a thermomechanical model of rotating texture 'surfing' on a heat wave (TM3 model), a model of melting-growth (MG model) that only applies when the droplets coexist with their own isotropic liquid, and a pure hydrodynamic model (H model) based on the existence of Marangoni flows – currently, only evidenced in emulsified cholesterics. The strengths and weaknesses of each model will be discussed in relation with the experimental results.

Edge dislocations are linear defects that locally break the positional order of the layers in smectic A liquid crystals. As in usual solids, these defects play a central role for explaining the plastic properties of the smectic A phase. This work focuses on the dynamical properties of dislocations in bulk samples prepared between two glass plates and in free-standing films. The emphasis will be put on the measurement of the mobility of edge dislocations in liquid crystals either pure or doped with nanoparticles. The experimental results will be compared to the existing models.

We show experimentally and theoretically that the Lehmann effect recently observed by Yoshioka and Araoka (Nat. Commun., 2018, 9, 432) in emulsified cholesteric liquid crystal droplets under temperature gradients is due to Marangoni flows rather than to the thermomechanical or chemomechanical couplings often invoked to explain the phenomenon. Using colloidal tracers we visualize convection rolls surrounding stationary cholesteric droplets in vertical temperature gradients, while a shift in the position of internal point defects reveals the corresponding inner convection in nematic droplets thermomigrating in a horizontal temperature gradient. We attribute these phenomena to the temperature dependence of the surface tension at the interface between these partially-miscible liquids, and justify their absence in the usual case of purely lyophobic emulsions. We perform a theoretical analysis to help validate this hypothesis, demonstrating the strong dependence of the precession velocity on the configuration of the cholesteric director field.

We propose a complete characterization of the chemical Leslie effect in a Langmuir monolayer of a chiral liquid crystal. To reach this goal, we developed new experimental techniques using an electric field and a humidifier to prepare large monodomains in which the molecules can freely rotate. We also designed six independent experiments to precisely measure the four material constants involved in the dynamics of the monolayer, namely the Leslie coefficient, the rotational viscosity, the curvature elasticity constant and the surface polarization. The relevance of the inverse Leslie effect is also discussed.

The Lehmann effect is the rotation of cholesteric droplets when they are submitted to a temperature gradient. So far, this effect was only observed in the coexistence region between the cholesteric phase and its isotropic liquid. This zone of coexistence is due to the presence in the LC of impurities. In this paper, we show that the rotation velocity of the droplets does not depend on the choice of the impurity and on its concentration, providing that the variations of the equilibrium twist and the rotational viscosity are taken into account. These results were obtained by doping the cholesteric LC (a diluted mixture of 7CB and R811) with nonmesogenic and mesogenic impurities. The nonmesogenic impurities used are the biphenyl, the hexachloroethane, and a fluorinated polyether polymer. The mesogenic impurity is the LC I52. From these experiments we conclude that the Lehmann effect is certainly not due to a chemical torque of the type described by Leslie, Akopyan, and Zel'dovich. Finally, we propose alternative avenues that might be explored to understand the Lehmann effect.

We revisit a theoretical paper of Akopyan and Zel'dovich about the thermomechanical coupling terms in nematic liquid crystals. We show that the expressions of these terms given by these authors must be corrected to satisfy the Onsager reciprocity relations, a point already stressed by Pleiner and Brand in 1987. We then extend this calculation to the cholesteric phase and show that there are no additional terms in the uniaxial approximation of this phase. Finally, we give the correspondence between the Akopyan and Zel'dovich terms and those calculated by Pleiner and Brand in 1996 by making a different choice for the forces and the fluxes in the theory.

New creep experiments under sinusoidal compression/dilation deformation of a homeotropic sample of smectic-A liquid crystal (8CB) show that its response is nonlinear at very small amplitude of deformation. This behavior is explained by taking into account the crossing between the edge dislocations that climb parallel to the layers and the screw dislocations joining the two surfaces limiting the sample. The activation energy of the crossing process and the density of the screw dislocations as a function of the sample thickness are estimated experimentally.