State transition at electrohydrodynamic convection of twisted nematic liquid crystals

Nontrivial ultraslow dynamics under electric-field in nematics of bent-shaped molecules

“…To give a convincible relationship between the material parameters and the reorientational process, let us summarize the characteristics of the stripes and COO 2 material: (1) De of COO 2 is positive; (2) stripes run perpendicular to the rubbing direction; (3) the wavelength of the stripes relies on the thickness, d, of samples and exhibits the similar length order of d; (4) the Freedericksz transition occurs before the stripe pattern appears in the planar cell; and (5) the director cannot follow a high-frequency electric field, e.g., 1000 Hz square waves. According to the well-studied standard and nonstandard EHC phenomena in the planar cell geometry used here, there are only four possibilities of material parameter combinations where the stripe (roll) instability can occur: 33,34 In case (B), the stripes are known to be parallel to the rubbing direction. As a result, only case (C) can well explain the experimental observation with no contradiction.…”

“…, 1000 Hz square waves. According to the well-studied standard and nonstandard EHC phenomena in the planar cell geometry used here, there are only four possibilities of material parameter combinations where the stripe (roll) instability can occur: 33,34 (A) Δ ε < 0 and Δ σ > 0 (Δ σ is the anisotropy of conductivity); (B) Δ ε > 0 and Δ σ < 0; (C) Δ ε > 0 and Δ σ > 0; and (D) Δ ε < 0 and Δ σ < 0. The cases (A) and (D) are immediately excluded because Δ ε > 0 in the present case.…”

Reflective Bistable Chiral Splay Nematic Liquid Crystal for Low-Power Heat Sensor

“…K 11 , K 22 , and K 33 are the splay, twist and bend elastic constants of the liquid crystal, respectively, and q 0 is the chirality related to the pitch P 0 by q 0 = 2 π / P 0 . Under one constant approximation ( K 11 = K 22 = K 33 = K ), the free energy could be simplified as The surface anchoring energy F s is taken into account by the Rapini–Papoular surface potential [ 15 ] Here, A p and A a are the polar and azimuthal anchoring coefficients, respectively. Using Equation (5) and Equation (6), and given the director profiles, the variations of the Gibbs free energy per unit area with respect to the twist angle, namely, the bistable curve, can be obtained by a straightforward calculation.…”