Morphogenesis of defects and tactoids during isotropic-nematic phase transition in self-assembled lyotropic chromonic liquid crystals

We explore the structure of nuclei and topological defects in the first-order phase transition between the nematic (N) and isotropic (I) phases in lyotropic chromonic liquid crystals (LCLCs). The LCLCs are formed by self-assembled molecular aggregates of various lengths and show a broad biphasic region. The defects emerge as a result of two mechanisms. 1) Surface anisotropy mechanism that endows each N nucleus (tactoid) with topological defects thanks to preferential (tangential) orientation of the director at the closed I-N interface, and 2) Kibble mechanism with defects forming when differently oriented N tactoids merge with each other. Different scenarios of phase transition involve positive (N-in-I) and negative (I-in-N) tactoids with non-trivial topology of the director field and also multiply connected tactoids-in-tactoids configurations. The closed I-N interface limiting a tactoid shows a certain number of cusps; the lips of the interface on the opposite sides of the cusp make an angle different from pi. The N side of each cusp contains a point defect-boojum. The number of cusps shows how many times the director becomes perpendicular to the I-N interface when one circumnavigates the closed boundary of the tactoid. We derive conservation laws that connect the number of cusps c to the topological strength m of defects in the N part of the simply-connected and multiply-connected tactoids. We demonstrate how the elastic anisotropy of the N phase results in non-circular shape of the disclination cores. A generalized Wulff construction is used to derive the shape of I and N tactoids as the function of I-N interfacial tension anisotropy in the frozen director field of various topological charges m. The complex shapes and structures of tactoids and topological defects demonstrate an important role of surface anisotropy in morphogenesis of phase transitions in liquid crystals.Comment: 31 pages, 13 figure

Polarity-dependent dielectric torque in nematic liquid crystals

The dielectric dispersion in the uniaxial nematic liquid crystals affects the switching dynamics of the director, as the dielectric torque is determined by not only the present values of the electric field and director but also by their past values. We demonstrate that this dielectric memory leads to an unusual contribution to the dielectric torque that is linear in the present field and thus polarity-sensitive. This torque can be used to accelerate the switch-off phase of director dynamics.Comment: 12 pages, 4 figure

55.2: Fast Switching Dual-Frequency Liquid Crystal Optical Retarder, Driven by an Amplitude and Frequency Modulated Voltage

Abstract We Introduction Nematic cells are widely used as optical retarders in various applications where 0 ε is the electric constant, 1 γ is the rotational viscosity of the nematic liquid crystal, ∆ε=ε II -ε ⊥ is the dielectric anisotropy, ε II and ε ⊥ are the principal dielectric permittivities referred to the nematic director, threshold value of the applied voltage, K is the characteristic elastic constant. According to Eq.(1), one can decrease τ on by increasing the applied voltage. However, the relaxation time τ off depends only on the material parameters and the thickness of the cell and cannot be shortened by a higher electric field, see Eq.(2) τ off ~d 2 . The drawback is that smaller d causes smaller optical retardation (the optical path difference for ordinary and extraordinary waves) ∆L, as ∆L~d. The goal of our work was to resolve contradictory requirements of fast (sub-millisecond) switching and the broad range of switched optical retardations (∆L ≥1µm). Fast Switching Dual-Frequency Liquid Crystal Optical Retarder We use the so-called dual-frequency nematic materials in cells with a high pretilt angle (α≈45 degrees) driven by a sequence of electric pulses of different frequency and amplitude. We assembled nematic cells with an anti-parallel fashion from plates with a high pretilt angle, which was achieved by oblique deposition of SiO layers We used the optical setup with the cell placed between two crossed polarizer prisms to measure the time evolution of the optical response of the cell

Liquid Crystals with Patterned Molecular Orientation as an Electrolytic Active Medium

Transport of fluids and particles at the microscale is an important theme both in fundamental and applied science. One of the most successful approaches is to use an electric field, which requires the system to carry or induce electric charges. We describe a versatile approach to generate electrokinetic flows by using a liquid crystal (LC) with surface-patterned molecular orientation as an electrolyte. The surface patterning is produced by photo-alignment. In the presence of an electric field, the spatially varying orientation induces space charges that trigger flows of the LC. The active patterned LC electrolyte converts the electric energy into the LC flows and transport of embedded particles of any type (fluid, solid, gaseous) along a predesigned trajectory, posing no limitation on the electric nature (charge, polarizability) of these particles and interfaces. The patterned LC electrolyte exhibits a quadratic field dependence of the flow velocities; it induces persistent vortices of controllable rotation speed and direction that are quintessential for micro- and nanoscale mixing applications.Comment: 35 pages, 10 figure