Spatio-temporal Analysis of Electroconvection in Nematics

Abstract

We investigate electroconvection in nematic liquid crystals by means of optical microscopy. Time resolved optical images are used to study the director dynamics. For the first time we present instant images in the dielectric regime. A numerical simulation of the optical transmission patterns is performed on the basis of Fermat's principle. In the instant images of dielectric rolls, the periodicity of the observed optical pattern is equal to the wavelength Ao of the convection rolls. The well known low contrast 'stationary' optical texture observed in conventional experiments results from time averaging of these instant images; its wavelength is Ao/2.

Laser diffraction by periodic dynamic patterns in anisotropic fluids

“…One of the problems encountered in this very efficient observation technique is the complexity of the optics in the periodically deformed director field. Simulations of the optical profiles have been presented by several authors [6,7,8,9,10,11,12], and the consequences of in-plane director twist have been considered [13,14]. Although light propagation in such a two-dimensionally inhomogeneous medium has been treated theoretically with different approximation methods, and the qualitative relation between director structure and observed intensity profile in the microscope is well established, the method fails to provide quantitative access to deflection amplitudes of the director field.…”

“…The wave vector spectrum can be determined qualitatively from a Fourier transform of digitized microscopic transmission images [12,15,16]. A more efficient, quantitative way is the quasi-optical Fourier transformation by means of laser diffraction.…”

Fundamental scaling laws of on-off intermittency in a stochastically driven dissipative pattern-forming system

“…We use the nematic mixture Mischung 5 (Halle) which consists of four disubstituted alkyl(oxy)phenylalkyloxybenzoates. This material has been used in previous EHC experiments [11,37,47,48]. The mixture is nematic at room temperature, its clearing point is 70.5 • C. The commercial sandwich cells (LINKAM) used in the experiments have cell gaps of 25.8 µm.…”

Preparation of subharmonic patterns in nematic electroconvection

“…In the low-frequency (conductive) regime, the charge density q͑t͒ alternates with the excitation, the director deflection preserves its sign. In the high-frequency (dielectric) regime, the situation is reversed, ͑t͒ alternates with the excitation (see, e.g., time-resolved observations in [6,7]), and q͑t͒ keeps its sign. In the conductive regime, similar to other convective instabilities, the selected wave number k th Ϸ / d is essentially determined by the container dimension.…”

“…Figure 2 shows typical diffraction images, taken slightly above the threshold voltage U l,th for pattern onset (see, Fig. 2(f) [7]. Fast intensity modulations due to the oscillation of ͑t͒ are averaged by the camera.…”