Grandjean-Cano Wedge Research Articles

Confinement and magnetic-field effect on chiral ferroelectric nematic liquid crystals in Grandjean-Cano wedge cells.

We explore the structure and magnetic-field response of edge dislocations in Grandjean-Cano wedge cells filled with chiral mixtures of the ferroelectric nematic mesogen DIO. Upon cooling, the ordering changes from paraelectric in the cholesteric phase N^{*} to antiferroelectric in the smectic SmZ_{A}^{*} and to ferroelectric in the cholesteric N_{F}^{*}. Dislocations of the Burgers vector b equal to the helicoidal pitch P are stable in all three phases, while dislocations with b=P/2 exist only in the N^{*} and SmZ_{A}^{*}. The b=P/2 dislocations split into pairs of τ^{-1/2}λ^{+1/2} disclinations, while the thick dislocations b=P are pairs of nonsingular λ^{-1/2}λ^{+1/2} disclinations. The polar order makes the τ^{-1/2} disclinations unstable in the N_{F}^{*} phase, as they should be connected to singular walls in the polarization field. We propose a model of transformation of the composite τ^{-1/2} line-wall defect into a nonsingular λ^{-1/2} disclination, which is paired up with a λ^{+1/2} line to form a b=P dislocation. The SmZ_{A}^{*} behavior in the in-plane magnetic field is different from that of the N_{F}^{*} and N^{*}: the dislocations show no zigzag instability, and the pitch remains unchanged in the magnetic fields up to 1 T. The behavior is associated with the finite compressibility of smectic layers.

Chiral ferronematic liquid crystals: a physico-chemical analysis of phase transitions and induced helical twisting

ABSTRACT A possibility was assessed to modify physico-chemical properties of ferronematic mixtures by introducing additional mesogenic and non-mesogenic components. The mixture, consisting of RM734 and DIO at the 70:30 weight ratio of the components, was doped with nematic 5CB and its chiral isomer CB15. This allowed a substantial lowering of the ferronematic temperature range, as well as induction of helical twisting in both nematic (N) and ferronematic (NF) phases. The effects of 5CB and CB15 on the NF to N phase transition were nearly identical, but the isotropic transition temperature decreased more strongly upon addition of CB15. Optical transmission vs. temperature measurements showed lower transmittance in the NF phase, probably due to scattering on the domain boundaries. In the vicinity of the NF → N transition, a noticeable range of anomalously low transmission was noted both on heating and cooling, suggesting a complex character of molecular reorientation features. In the chiral NF (N*F) system, helical twisting was recorded using Grandjean-Cano wedge, with the helical pitch decreasing upon cooling in both N* and N*F phases. In the cooling mode, transient undulations emerged in the N*F phase. The complex picture of the ferronematic transition is supported by POM images taken at different temperatures.

Advances in model-independent restoration of the liquid crystal surface anchoring potential

We extend further the theoretical and experimental studies of the actual surface anchoring potential restoration by polarization microscope technique in a wedge-shaped cell with weak surface anchoring forces filled by a chiral nematic liquid crystal (in Grandjean-Cano Wedge). To realize the theoretically predicted options of observation of large director deviation angles from the easy axes the experimental studies of director distribution in Grandjean-Cano zones were performed for differing easy axes orientations at the wedge surfaces. A weak surface anchoring at one wedge surface was obtained by a photo-alignment technique. A strong surface anchoring at the second wedge surface was also obtained by a photo-alignment technique with an enlarged irradiation dose. There were observed walls with weak jumps of the director orientation between first neighboring Grandjean-Cano zones. Some experimentally obtained polarization microscope images demonstrate partially unwound director structures in zero Grandjean-Cano zone. To ensure optimal parameters of the further experiments on the actual surface anchoring potential restoration theoretical calculations of the director distribution in individual Grandjean-Cano zones were performed for various model surface anchoring potentials. It was found that especially promising for description of the experimentally found director distribution is the D-potential. As a result of the present studies the angular range of a model-independent actual azimuthal surface anchoring potential restoration was enlarge (up to π/3) and the ways of an actual surface anchoring potential restoration in a whole angular range of its determination are discussed.

Studies of the liquid crystal surface anchoring potential using Grandjean–Cano wedge

ABSTRACTWe extend further the theoretical and experimental studies of the actual surface anchoring potential restoration by polarisation microscope technique and using micro-images in a wedge-shaped cell with weak surface anchoring forces filled by a chiral nematic liquid crystal (in Grandjean–Cano (GC) Wedge). To realize the theoretically predicted options of observation of large director deviation angles from the easy axes, the experimental studies of director distribution in GC zones were performed for differing easy axes orientations at the wedge surfaces in white light. A weak surface anchoring at one wedge surface was obtained by a photo-alignment technique. A strong surface anchoring at the second wedge surface was obtained by a rubbing. There were observed for the first time jump-less walls (without jumps of the director orientation) between neighbouring GC zones and jumps in colour in the Newton’s rings (lines) at the positions of the walls between GC zones. Qualitative explanations of the both phenomena are presented and an expression for the jump in colour value in the Newton’s rings (lines) at the positions of the walls between GC zones is presented. Some experimentally found by the polarisation microscope and by the microphoto technique images demonstrate jump-less walls between the first GC zones. To ensure optimal parameters of the further experiments on the actual surface anchoring potential restoration, theoretical calculations of the director distribution in individual GC zones were performed for various model surface anchoring potentials. It was found that especially promising for description of the experimentally found director distribution looks the modified D-potential. As a result of the present studies, the ways to enlarge (compared to the previous studies) angular range of the actual surface anchoring potential restoration were proposed.

Surface Anchoring and Director Distribution in a Grandjean–Cano Wedge

Theoretical calculations of the director distribution in a Grandjean–Cano wedge in relation to the anchoring strength and mutual orientation of the easy axes at the wedge surfaces are performed for various model surface anchoring potentials. Comparison of the theory and experiment allows one to propose the socalled model D-potential, quadratic in angle of director deviation from the easy axes, as the best one in fitting the experiment in the angular range of performed measurements. To satisfy general requirements on the wedge surface, a modified D-potential is proposed. The optimal conditions of the experiment aimed at restoring the potential in the whole range of its definition (in particular, nonparallel orientation of the easy axes at the wedge surfaces) are formulated.

On the way of reconstruction of the liquid crystal surface anchoring potential

We extend further the theoretical and experimental studies of the actual surface anchoring potential reconstruction by polarization microscope technique in a wedge-shaped cell with weak surface anchoring forces filled by a chiral nematic liquid crystal (in Grandjean-Cano Wedge) and present the material for the sake of the reader convenience with a short coherent survey of our previous results. To realize the theoretically predicted options of observation of large director deviation angles from the easy axes φ at a wedge surfaces (up to π/2) the experimental studies of director distribution in Grandjean-Cano zones were performed. A weak surface anchoring at one wedge surface was obtained by photo-alignment technique. A strong surface anchoring at the second wedge surface was obtained by rubbing. The measurements were performed using polarizing microscope. The experimentally found director distributions in Grandjean-Cano zones were compared with theoretical ones calculated for various model surface anchoring potentials. As a result of the present studies the ways to enlarge (compared to the previous studies) angular range of the actual surface anchoring potential restoration were proposed and it was found that the experimentally found surface potential is nicely fitted by a new model surface anchoring potential, the so called model D-potential WD(φ)~φ2.

Lasing in grandjean–cano wedge as an option to study surface anchoring

Lasing in grandjean–cano wedge as an option to study surface anchoring

The measurement of weak chirality using nematic liquid crystals

ABSTRACTTwo recently reported methods for measuring weak chirality using nematic liquid crystals are described. These are between one and two orders of magnitude more sensitive than the standard Grandjean-Cano wedge. Pitch lengths of 50 mm have been measured with prospects for extending this beyond 100 mm; this is equivalent to measuring the presence of 1 ppm of a strong chiral dopant. Results for a number of standard chiral dopants and nematic liquid crystal hosts are reported.

Director distribution in Grandjean-Cano wedge and restoration of surface anchoring potential

ABSTRACTWe extend further the performed early both theoretically and experimentally studies of the director distribution of a chiral nematic liquid crystal placed in a wedge-shaped cell with weak surface anchoring aimed to restore the actual surface anchoring potential. As a result of the present and previous experimental studies [1–3] it was found that the experimentally found surface potential (obtained from the measured potential derivative) is very nice fitted by a new model surface anchoring potential, the so called model D-potential (Ws(ϕ) = (W/2)ϕ2), in a wide angular range −π/4< ϕ<π/4.

Director distribution and surface anchoring potential in Grandjean-Cano wedge

Theoretical and experimental studies of the director distribution of a chiral nematic liquid crystal placed in a wedge-shaped cell with weak surface anchoring were performed. We measured local value of pitch as function of position inside each Grandjean-Cano zone by registering selective reflection spectra from narrow stripes cut from each Cano zone. We also measured the rotation of the polarisation plane as function of thickness of the wedge sample. These data allowed determination of the angle between the local director and the easy direction on the surface. The obtained director distributions in individual Cano zones were compared with the results of theory and thus used for restoration of the actual shape of the surface anchoring potential.

Spatial distribution control of polymer nanoparticles by liquid crystal disclinations

A perylene-labeled hyper-branched polymer with a mesogenic shell was observed to migrate toward a field with a large distortion of directors. This polymer was captured by disclination lines of 1/2 strength in its nematic and chiral nematic phases using a confocal fluorescence microscope. In particular, the well-aligned particle array of the hyper-branched polymers was produced by the chiral nematic phase confined in a wedge cell. The hyper-branched polymer with its mesogenic shell was completely dissolved into an isotropic phase. Immediately after a phase transition to the chiral nematic phase, numerous droplets appeared over a wide area owing to phase separation, and the free droplets moved toward the disclination lines via Brownian motion. Finally, the droplets were rapidly attracted to the disclination lines and trapped within them. A fluorescent-labeled hyper-branched polymer with mesogenic shell was captured by the disclination lines of a liquid crystal. In a Grandjean–Cano wedge cell, the particles of the hyper-branched polymer migrated toward the periodically aligned defect line and thus formed a two-dimensional lattice array.

Atomic force microscope based method of measuring short cholesteric pitch in liquid crystals

We present a new, high-resolution method for measuring the helical pitch in cholesteric liquid crystals (CLCs) using an atomic force microscope. This method is based on the measurement of force between a microsphere and a flat surface in the presence of CLCs. The measured force is periodic due to the structural periodicity of the CLC and the pitch can be determined with the accuracy of several nanometres. Besides high accuracy there is no limitation for measuring very short pitches below 200 nm, which is not possible with the standard optical Grandjean–Cano wedge method.

A Simple Method of Determining the Pitch of a Chiral Nematic Liquid Crystal

The pitch value P of a chiral nematic liquid crystal is a critical parameter for azimuthal anchoring strength measurement. The Grandjean-Cano wedge method is a conventional technique for pitch determination, but the perfect wedge cell with known angle and uniform disclination lines are required. In this work, we have developed an improved Grandjean-Cano method, which is simple and gives accurate pitch value. The sensitivity of this new method is ±λ/2Δn.

Optical method for measuring the azimuthal anchoring strength of liquid crystals using pitch values determined in imperfect samples

An improved Grandjean–Cano wedge method for measuring the pitch of a chiral nematic liquid crystal is demonstrated. This method is easy to implement and can yield results of high accuracy when it is used for measuring the liquid crystal surface anchoring strength.

Electric-field-induced disclination migration in a Grandjean-Cano wedge

The behavior of a disclination line in a Grandjean-Cano wedge is investigated under an electric field applied normal to the cell substrates. The focus of attention is the first disclination, separating the untwisted and the π-twisted area. This disclination line shows a field dependent migration process that was observed in the cholesteric mixture ZLI-1132/S811. The dynamics of this migration process is studied experimentally and is compared with two theoretical models. One model utilizes a two-dimensional lattice Boltzmann approach to predict the initial migration movement, whereas a one-dimensional continuum model allows a full analysis of the experimental results.

Defect dynamics in a smectic Grandjean-Cano wedge.

An array of edge dislocation forms spontaneously in a Grandjean-Cano wedge filled by a smectic liquid crystal. In the vicinity of the smectic A to smectic C transition, these defects are visible under the microscope [R. B. Meyer, B. Stebler, and S. T. Lagerwall, Phys. Rev. Lett. 41, 1393 (1978)]. This paper deals with their dynamics under controlled deformation (dilation and compression). First, we characterize several regimes of dislocation mobility occurring with increasing strain epsilon or strain rate epsilon;. We relate these regimes to the interactions between screw and edge dislocations. We also show that screw dislocations give rise to loops of edge dislocations under sufficient strain, which strengthens the model of loop nucleation by helical instability of screw dislocations. Lastly, we discuss several models for the microscopic origin of the interactions between defects.