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

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 favors 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.

Figure 1

Morphological phase diagram of a Phase 5 sample at $f=400$ Hz, and at $f=200$ Hz (inset from [19]). The dashed lines, as a guide for the eyes, indicate the trends of the stability limit curve. Stars indicate those $U_{\mathrm{ac}}$, $U_{\mathrm{dc}}$ combinations where the images covering an area of 52 $\mu \mathrm{m}$ $\times$ 52 $\mu \mathrm{m}$ were taken. The initial director lies along the horizontal direction.

Figure 2

Morphological phase diagram of a Phase 5 sample at $f=80$ Hz. The dashed lines, as a guide for the eyes, indicate the trends of the stability limit curve. The dotted line shows the boundary between the patternless basic state and the region of dielectric electroconvection. Stars indicate those $U_{\mathrm{ac}}$, $U_{\mathrm{dc}}$ combinations where the images covering an area of 52 $\mu \mathrm{m}$ $\times$ 52 $\mu \mathrm{m}$ were taken. The initial director lies along the horizontal direction.

Figure 3

Morphological phase diagram of a Phase 5 sample at $f=20$ Hz, and at $f=10$ Hz (inset from [19]). The dashed lines, as a guide for the eyes, indicate the trends of the stability limit curve. The dotted line shows the boundary between the patternless basic state and the region of conductive electroconvection. Stars indicate those $U_{\mathrm{ac}}$, $U_{\mathrm{dc}}$ combinations where the images covering an area of 52 $\mu \mathrm{m}$ $\times$ 52 $\mu \mathrm{m}$ were taken. The initial director lies along the horizontal direction.