Does the electric Lehmann effect exist in cholesteric liquid crystals?

Abstract

Abstract. Experiments have shown that cholesteric droplets or cholesteric fingers may be put into motion by the action of an electric field. The former rotate whereas the latter drift perpendicularly to their axes. In all cases, the texture moves without visible material transport. The electric Lehmann effect was initially used to interpret these observations but, recently, alternative explanations were found, based on electrohydrodynamics. Another experiment in this area was that of Padmini and Madhusudana (Liq. Cryst. 14, 497 (1993)). Performed in 1993 with a compensated cholesteric liquid crystal under fixed planar boundary conditions, it was also explained in terms of electric Lehmann effect. We conducted the same experiment and extended it to a π-twisted planar geometry. Although our experimental results agree with those of Padmini and Madhusudana, we demonstrate that they are incompatible with an electric Lehmann effect. By contrast, an explanation based on flexoelectricity allows us to interpret the whole data set obtained in both geometries. The consequence is that there is at the moment no clear experimental evidence of the electric Lehmann effect.PACS. 61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order -77.65.-j Piezoelectricity and electromechanical effects -42.70.Df Liquid crystals

Lehmann effect in a compensated cholesteric liquid crystal: Experimental evidence with fixed and gliding boundary conditions

“…For this reason, we redid their experiment with our mixture. Although our observations were essentially the same as theirs, we showed that they could not result from an electric Lehmann effect, but more simply from flexoelectricity [20].…”

“…Unfortunately, we cannot deduce from this preliminary experiment the value of the Lehmann coefficient because the temperature gradient in the liquid crystal layer is largely unknown. In addition, although we measured viscosity γ 1 in a previous article [20], its value may be different in the present sample because of the pollution by the glue. Thus, the next step will be to check the viscosity value and to make a special setup to impose a controlled temperature gradient.…”

“…We emphasize that we neglected the flexoelectric effects in the fitting procedure, which is justified as long as the flexoelectric coefficient difference e 3 − e 1 is smaller than 10 −11 Cm −1 [19]. This is the case in our mixture, in which we measured e 3 − e 1 =3× 10 −12 Cm −1 [20]. Second, we measured the Fréedériks transition in a π/2-twisted planar cell (thickness 6.8 µm, from Instec, Inc) at different temperatures close to the compensation point.…”

Direct measurement of the thermomechanical Lehmann coefficient in a compensated cholesteric liquid crystal

“…We then applied a large voltage (typically, 500 Vrms at frequency f = 10 kHz) between the two electrodes. Because the liquid crystal has a positive dielectric anisotropy (ε a ≈ 5 [16]), the director orients parallel to the electric field in the region lying between the two electrodes. As a consequence, the sample appears black in this region between crossed polarizer and analyzer when the polarizer is either parallel or perpendicular to the electric field.…”

QSPR Analysis of a Set of Liquid Crystals in the Benzylideneaniline Class According to their Transition Temperature

“…Some organic substances pass from crystaline, solid to isotropic, liquid states, not directly, but through a phase [1,2]. The mechanical and symmetrical properties of these phases are intermediate to the properties of liquids and crystals [3,4].…”