Measurements of the ratio of the Frank constants for splay and bend in nematics of disc-like molecules

Dynamics of the Frederiks transition in nematics consisting of disc-like molecules Thermal dependence of a bend viscosity coefficient

“…The temperature is expressed in reduced units TITNI or where TN and TNI, are the transition temperatures from the nematic phase to the isotropic liquid and to the columnar phase, respectively. Using the values for K3/Ax from our previous experiments the ratio yi*/Ax can be calculated as a function of temperature [5,24].…”

“…The exact thickness of the sample cells was determined interferometrically. A single domain magnetic distortion of the initially homeotropic sample was prepared as described in [5].…”

“…With this technique the measurements must be performed at or very close to the equilibrium state, so that the applied magnetic force is at all times exactly balanced by the restoring force against director deformations. As was reported in [5], due to the sluggish relaxation of the director distortion, it was found necessary to measure at different finite scanning speeds of the applied magnetic field and extrapolate to zero scanning speed in order to obtain the equilibrium value of the response to the applied magnetic field. In the present paper this relaxation of the director distortion is studied quantitatively.…”

“…From this continuum theory follows an expression for the free energy density (fd) due to deformations of a uniaxially aligned nematic material (see, for example, [4]): fd = 1/2[K1(V n)2 + K2(n V x n)2 + K3(n x V x n)2], (1) where n is the local axis of uniaxial symmetry, usually referred to as the director, and K1, K2 and K3 are the curvature elastic constants associated with splay, twist and bend distortions, respectively. In a previous study the magnetic Frederiks transition technique was used to obtain information about these elastic constants for some liquid crystals consisting of disc-like molecules [5]. With this technique the measurements must be performed at or very close to the equilibrium state, so that the applied magnetic force is at all times exactly balanced by the restoring force against director deformations.…”

“…2,3,7,8,12, 13-hexa(decanoyloxy)truxene (2a), 2, 3, 7, 8, 12, 13-hexa(tetradecanoyloxy)truxene (2b) and 2, 3, 7, 8, 12, 13-hexa(octadecanoyloxy)truxene (2c)) [22] (figure 1). Homeotropically aligned samples were prepared by placing the nematic material between two poly-imide coated (Dupont, P12566) [23] glass slides which were kept at a constant distance of some 50 pm by tungsten spacers [5]. The exact thickness of the sample cells was determined interferometrically.…”