Inhibited pattern formation by asymmetrical high-voltage excitation in nematic fluids

Abstract

In contrast to the predictions of the standard theory of electroconvection (EC), our experiments showed that the action of superposed ac and dc voltages rather inhibits pattern formation than favours the emergence of instabilities; the patternless region may extend to much higher voltages than the individual ac or dc thresholds. The pattern formation induced by such asymmetrical voltage was explored in a nematic liquid crystal in a wide frequency range. The findings could be qualitatively explained for the conductive EC, but represent a challenging problem for the dielectric EC.

Suppression of spatially periodic patterns by dc voltage

“…10(a)]. In contrast to that, in the experiments a substantial increase of the ac threshold upon dc bias was detected (for both conductive and dielectric EC), similarly to the recent findings on the nematic Phase 5 [22]. The recent theoretical analysis of flexodomains [6] has pointed out that in case of pure ac driving the equations have two solutions with different time symmetries: the conductive mode implies stationary modulation of the out-of-plane director component, while for the dielectric mode it oscillates with the ac frequency.…”

“…Figs. 1 and 2 of [22]), we have to emphasize an important difference: the upper branch of the SLC here corresponds to the threshold of flexodomains (instead of the dc mode of EC).…”

“…The above theoretical results have been compared with experiments, which explored the influence of combined ac+dc driving on standard EC in a nematic mixture (Merck Phase 5) [21,22]. A comparison of experimental data with theoretical predictions yielded a good match for case A, however, only at the lowest frequencies [21].…”

“…In extreme cases (in the dielectric regime) it resulted in the 'opening' of the SLC: the superposed ac and dc voltage reached the upper limit of the voltage source without inducing pattern, even though the voltage(s) exceeded several times the pure ac or dc thresholds (see Fig. 1 of [22]). Later, in Section IV, we provide an explanation of this pattern inhibition.…”

Influence of DC voltage on the dielectric properties of nematics

“…Experimental studies of the electrooptical properties of nematics, as well as most display applications of them utilize ac driving voltage; investigations under dc voltage are rather scarce. Recently, electroconvection and flexoelectric patterns occurring under superposed action of ac and dc voltages have been studied and, unexpectedly, an inhibition of the pattern formation at high voltages (an increase of the thresholds) was found [1][2]. The fact that conductivity is an important factor determining the threshold voltage, motivated the present testing of the effect of dc bias on the electrical conductivity.…”