Polarity-Dependent Dielectric Torque in Nematic Liquid Crystals

The dielectric dispersion in the uniaxial nematic liquid crystals affects the switching dynamics of the director, as the dielectric torque is determined by not only the present values of the electric field and director but also by their past values. We demonstrate that this “dielectric memory” leads to an unusual contribution to the dielectric torque that is linear in the present field and thus polarity sensitive. This torque can be used to accelerate the “switch-off” phase of director dynamics.

Figure 1

Dielectric dispersion of two NLCs, with $\Delta \epsilon <0$ (a) and $\Delta \epsilon >0$ (b). The data for ${\epsilon }_{\Vert }$ are fitted by Eq. (1) with ${\epsilon }_{l\Vert }=6.60$, ${\epsilon }_{h\Vert }=3.91$, ${\epsilon }_{\perp }=7.31$, and $\tau =33\mu \mathrm{s}$ (a); ${\epsilon }_{l\Vert }=15.21$, ${\epsilon }_{h\Vert }=8.67$, ${\epsilon }_{\perp }=7.88$, and $\tau =20\mu \mathrm{s}$ (b).

Figure 2

Electro-optic response $I(t)$ of a $\Delta \epsilon <0$ NLC in a homeotropic cell driven by dc pulses with an instantaneous (i) and exponentially decaying back edges (e1–e3); “$r$” stands for “response.” The dashed lines show $I(t)$ simulated using Eqs. (4, 5). The inset shows $I(t)$ over a large time scale.

Figure 3

Electro-optic response $I(t)$ of a $\Delta \epsilon >0$ NLC in a planar cell driven by dc pulses with an instantaneous (i) and exponentially decaying back edges (e1, e2). (a) The cell is driven by a dc pulse of duration $310\mu \mathrm{s}$; (b) the cell is driven by a 5 V, 1 kHz ac pulse of duration 180 ms, followed by a dc pulse of duration $120\mu \mathrm{s}$; the inset shows a larger time scale. The dashed lines show $I(t)$ simulated using Eqs. (4, 5).

Figure 4

Electro-optic response $I(t)$ of 5CB in a planar cell driven by dc pulses with an instantaneous (i) and exponentially decaying back edges (e1, e2) of opposite polarity. (a) The cell is driven by a dc pulse of duration $121\mu \mathrm{s}$; (b) the cell is driven by a 5 V, 1 kHz ac pulse of duration 180 ms (not shown), followed by a dc pulse of duration $80\mu \mathrm{s}$.