Flashing flexodomains and electroconvection rolls in a nematic liquid crystal

Abstract

Pattern forming instabilities induced by ultralow frequency sinusoidal voltages were studied in a rod-like nematic liquid crystal by microscopic observations and simultaneous electric current measurements. Two pattern morphologies, electroconvection (EC) and flexodomains (FD), were distinguished; both appearing as time separated flashes within each half period of driving. A correlation was found between the time instants of the EC flashes and that of the nonlinear current response. The voltage dependence of the pattern contrast C(U ) for EC has a different character than that for the FD. The flattening of C(U ) at reducing the frequency was described in terms of an imperfect bifurcation model. Analysing the threshold characteristics of FD the temperature dependence of the difference |e1 − e3| of the flexoelectric coefficients were also determined by considering elastic anisotropy.PACS numbers: 61.30. Gd, Recently interest has arisen to study the behaviour in another, subhertz frequency range, where the pattern growth/decay times are (much) shorter than the driving period, using compounds which may exhibit both EC and FD patterns. It has been proven experimentally that at such ultralow frequencies both for the dielectric [8] and the conductive [9] EC regimes, as well as for the FD [8,9] the patterns are flashing, i.e. they exist only in a small part of the driving period. It has been found that there is an f range (∼ 1 − 100 mHz) where both EC and FD patterns can exist in each driving half period in the form of successive (time shifted) flashes. Theoretical calculations based on the standard model of EC extended with flexoelectricity [6] (which is able to describe FDs too [10]) have justified that flashing patterns are indeed the solutions of the nemato-electrohydrodynamic equa-

Suppression of spatially periodic patterns by dc voltage

“…The two kinds of patterns occupy different locations; while FD is stationary, FDSW fluctuates with the driving voltage. Similarly, near the crossing of the EC and FDSW branches, FDSW may coexist with conductive EC rolls; in this case the patterns emerge at the same location, but in different time windows within the same driving period of the ac voltage (similarly to the ultralow f behaviour of calamitic materials [23]). The change of the dimensionless wave number q * = qd/π = 2d/λ (λ is the wavelength of the pattern) and the obliqueness angle α at threshold (along the SLC) is depicted in Figs.…”

“…2), there should exist a transition from the conductive EC roll pattern to FD as the ac frequency is reduced. This crossover occurs, however, at ultralow f at a few mHz [23], therefore it is not shown in Fig. 2.…”

“…The planarly aligned 1OO8 exhibits flexodomains as a first instability at pure dc driving, while it shows EC patterns when driven with ac voltage [23]. In Section III A we explore how superposition of dc and ac voltages alters the pattern morphologies, the onset characteristics (U c and q c ) and their frequency dependence, and up to what combination of the superposed voltages the uniform state can prevail.…”

“…The compound selected for the studies, 4-noctyloxyphenyl 4-n-methyloxybenzoate (1OO8), has nematic phase in the temperature range between 53 • C and 77 • C [23]. Its chemical structure is shown in Fig.…”

“…It has negative dielectric and positive conductivity anisotropies; hence standard EC develops at pure ac driving. At pure dc voltage, depending on the sample thickness d and on the electrical conductivity, it exhibits either flexodomains [23] or standard EC.…”