We experimentally investigate, in detail, electromechanical effects in liquid-crystal elastomers (LCEs) previously swollen with low-molecular-weight liquid crystals (LMWLCs). Both polydomain (POLY) and monodomain (MONO) LCEs were studied. We used a well known LMWLC, 4-n-pentyl-4-cyanobiphenyl (5CB) as a solvent. After swelling POLY and MONO LCEs (LSCE) with 5CB, shape changes were measured by recording the displacement of the edge of the swollen LCE at different voltages, V, and temperature. With 100 microm distance between electrodes, measurable shape changes (approximately 1-20 microm) are observed with small voltages (V approximately 0.5-10 V). In particular, we note that, compared to unswollen L(S)CEs, a dramatic approximately 200 times decrease of the threshold field was found for electromechanical effects in swollen L(S)CEs. While swollen MONO LCEs showed electromechanical effects in the planar geometry, homeotropic MONO swollen with homeotropically oriented 5CB did not. This is easy to understand because, in the homeotropic case, the liquid-crystal preferred axis is already aligned with the field so the field has no reorienting effect. The inverse of the response time when the field was switched on in both POLY and MONO was proportional to E2, which is the same field dependence as the response time of LMWLCs. When the field was switched off, the relaxation time showed a field dependence different from that of LMWLCs that we attribute to relaxation of the LCE network.

We report on chevrons ͑herringbonelike patterns͒ observed in homeotropically aligned liquid crystals with high electric conductivity. We focus our attention on two types of chevrons observed in the conduction regime. The threshold voltage and the characteristic double periodicity of chevrons ͑i.e., the short wavelength 1 of the striated rolls and the long wavelength 2 of the chevron bands͒ have been measured as functions of the applied electric frequency f. With the aid of a crossed polarizer set, we have, in addition, determined the director field which shows a periodic in-plane rotation for our chevrons ͑with a wavelength 2 ). We arrived at the types of chevrons after qualitatively different bifurcation sequences with increasing voltage. The frequency dependence of 2 also shows a qualitatively different behavior with respect to the two types of chevrons. The experimental results are discussed in terms of recent theoretical investigations.

The experimental study of electroconvection in a homeotropically aligned nematic ͑MBBA͒ is reported. The system undergoes a supercritical bifurcation ''rest state-spatiotemporal chaos.'' The chaos is caused by longwavelength modulation of the orientation of convective rolls. For the first time the observations both below and beyond the Lifshitz point are accompanied by quantitative analysis of temporal autocorrelation functions of turbulent modes. The dependence of the correlation time on the control parameter is obtained. A secondary bifurcation from normal to abnormal rolls is discussed. ͓S1063-651X͑97͒51412-7͔PACS number͑s͒: 47.27.Cn, 47.52.ϩj, 47.65.ϩa, 61.30.Gd Being a rich pattern-forming system, ac-driven electrohydrodynamic convection ͑EHC͒ in a nematic layer attracts a good deal of attention of both theoreticians and experimentalists ͓1,2͔. The system consists of a thin layer of a nematic liquid crystal sandwiched between two parallel electrodes. The convection occurs when the applied to the layer ac voltage V exceeds a certain threshold V c ͓3͔. There are two conventional realizations of EHC, namely planar, where the director n lies in plane of the electrodes (x-y plane͒, and homeotropic, where the director is perpendicular to the electrodes. It is clear that in the planar alignment the nonconvective state is anisotropic in the x-y plane, contrary to the homeotropic geometry, where spatially uniform distribution of the director parallel to the z axis yields isotropy in the x-y plane.In EHC most attention has been paid to the planar orientation; the homeotropic alignment has not been studied in detail yet. Meanwhile, in the homeotropic case EHC instability is usually preceded by Fréedericksz transition: beyond a certain threshold voltage V F the director tilts with respect to the z axis ͓3͔. Thus, at VϾV F the director has a nonzero projection on the x-y plane (n ʈ ), which therefore brings about spontaneous breaking of isotropy in this plane. Since orientation of n ʈ is not imposed by any external factor, the system is degenerate with respect to arbitrary rotations in the x-y plane. In other words, at the threshold of homeotropic EHC the system has additional ͑compared to the planar alignment͒ continuous one-parameter group of symmetry, which provides grounds for qualitatively different dynamics.Theoretical analysis of pattern formation at small values of the normalized control parameter ͓ϵ(V 2 ϪV c 2 )/V c 2 Ӷ1] shows that planar and homeotropic EHC do differ dramatically ͓4,5͔. In particular, in the planar case at small one can have steady stable normal rolls ͑the pattern's wave vector k is parallel to undistorted n͒ at the frequency of the applied ac-voltage f lying beyond the so-called Lifshitz point ͑LP͒, and oblique rolls ͑a certain finite angle between k and n͒ below LP ͓1,2͔.Contrary to this, in the homeotropic case while beyond LP the governing equations also have steady solutions of the normal-roll type; all of them are unstable under very general conditions ͓5͔. The instability occurs due ...

We report on noise-induced patterns and threshold characteristics for the ac-driven electrohydrodynamic system in nematic liquid crystals. The wave number-dependence of the threshold for the stochastic system was experimentally investigated with controlling intensity rate of noise. The results are discussed in comparison with the previous experimental and theoretical ones. Moreover, unexpected patterns as well as pre-transitional ones are presented.

We report external noise-induced threshold shift and pattern formation in ac-driven electroconvection (EC) in nematic liquid crystals. We investigate the noise intensity dependence of the threshold and determine the relationship f(c)(*)=hf(cd)(a) (α ∼ 1.4, h ∼ 0.1) between the characteristic cutoff frequency (f(c)(*)) of the noise and the characteristic ac frequency (f(cd)) of EC. For f(c)>f(c)(*), the noise contributes to stabilizing EC (i.e., upward threshold shift), whereas for f(c)