9 research outputs found
Travelling waves in electroconvection of the nematic Phase 5: A test of the weak electrolyte model
We investigated travelling waves appearing as the primary pattern-forming
instability in the nematic Phase 5 (Merck) in the planar geometry in order to
test the recently developed weak electrolyte model of ac-driven
electroconvection [M. Treiber and L. Kramer, Mol. Cryst. Liq. Cryst 261, 311
(1995)]. Travelling waves are observed over the full conductive range of
applied frequencies for three cells of different layer thickness d. We also
measured the elastic constants, the electric conductivity, and the dielectric
constant. The other parameters of Phase 5 are known, apart from the (relatively
unimportant) viscosity and the two parameters of the weak
electrolyte model that are proportional to the geometric mean of the
mobilities, and the recombination rate, respectively. Assuming a sufficiently
small recombination rate, the predicted dependence of the frequency of the
travelling waves at onset (Hopf frequency) on d and on the applied frequency
agreed quantitatively with the experiments, essentially without fit parameters.
The absolute value of the Hopf frequency implies that the geometric mean of the
mobilities amounts to .Comment: ReVTeX, 24 pages, 4 figures, to appear in Journal de Physique I
Abnormal Rolls and Regular Arrays of Disclinations in Homeotropic Electroconvection
We present the first quantitative verification of an amplitude description
for systems with (nearly) spontaneously broken isotropy, in particular for the
recently discovered abnormal-roll states. We also obtain a conclusive picture
of the 3d director configuration in a spatial period doubling phenomenon
involving disclination loops (CRAZY rolls). The first observation of two
Lifshitz frequencies in electroconvection is reported.Comment: 4 pages; 4 figure
Patterns driven by combined AC and DC electric fields in nematic liquid crystals
The effect of superimposed ac and dc electric fields on the formation of
electroconvection and flexoelectric patterns in nematic liquid crystals was
studied. For selected ac frequencies an extended standard model of the
electro-hydrodynamic instabilities was used to characterize the onset of
pattern formation in the two-dimensional parameter space of the magnitudes of
the ac and dc electric field components. Numerical as well as approximate
analytical calculations demonstrate that depending on the type of patterns and
on the ac frequency, the combined action of ac and dc fields may either enhance
or suppress the formation of patterns. The theoretical predictions are
qualitatively confirmed by experiments in most cases. Some discrepancies,
however, seem to indicate the need to extend the theoretical description
Flexoelectricity in liquid crystals: theory, experiments and applications
This book intends to give a state-of-the-art overview of flexoelectricity, a linear physical coupling between mechanical (orientational) deformations and electric polarization, which is specific to systems with orientational order, such as liquid crystals. Chapters written by experts in the field shed light on theoretical as well as experimental aspects of research carried out since the discovery of flexoelectricity. Besides a common macroscopic (continuum) description the microscopic theory of flexoelectricity is also addressed. Electro-optic effects due to or modified by flexoelectricity as well as various (direct and indirect) measurement methods are discussed. Special emphasis is given to the role of flexoelectricity in pattern-forming instabilities. While the main focus of the book lies in flexoelectricity in nematic liquid crystals, peculiarities of other mesophases (bent-core systems, cholesterics, and smectics) are also reviewed. Flexoelectricity has relevance to biological (living) systems and can also offer possibilities for technical applications. The basics of these two interdisciplinary fields are also summarized