State transition at electrohydrodynamic convection of twisted nematic liquid crystals

We study electrohydrodynamic convection (EHC) patterns in twisted nematic liquid crystal (TNCL) cells with an initial homeotropic alignment. In addition, we demonstrate switching characteristics due to the transition between the 90° and $-{90}^{\circ }$ twisted states. The pattern formations are similar to those obtained in a parallel-aligned nontwisted liquid crystal cell. Calculations of the free energy in an initially, homeotropically aligned TNLC in an electric field demonstrate that the energy barrier between the two states decreases with the increase of the electric field. The mutual transition is caused by the fluctuating flow at the dynamic scattering mode 2, where the energy barrier between the two states is reduced by applying a strong electric field. These properties may be employed for the development of a redirectable dynamic lenticular lens, as an extended version of the dynamic lenticular lens using EHC. The transition between the two states caused by EHC is reversible, and the convection roll pattern of the Williams domain rotates by 90° at each transition.

Figure 1

Microscopic images of various EHC patterns according to frequency and applied voltage at homeotropic 90° TNLC cell. (a) Typical Williams domain, (b) a chevron pattern, (c) scattering mode, (d) a chevron pattern rotated by 90°, (e) William domain region rotated by 90°, (f) the chevron pattern rotated by 90°, (g) scattering mode, (h) a chevron pattern rotated by 90° again, and (i) Williams domain region rotated by 90° again.

Figure 2

Schematic, EHC mode diagram, and the microscopic images of EHC patterns of 90° twisted state. (a) Schematic of 90° twisted state with EHC, (b) bifurcation diagram of 90° twisted state with EHC, (c) soft mode turbulence (SMT) observed at 500 Hz and 15.5 V, (d) Williams domain (WD) observed at 1000 Hz and 16.4 V, (e) the fluctuating Williams domain (FWD) observed at 1000 Hz and 17 V, (g) chevron pattern observed at 1000 Hz and 18 V, (f) grid pattern (GP) observed at 1000 Hz and 24 V, and (h) dynamic scattering mode2 (DSM2) observed at 1000 Hz and 40 V.

Figure 3

Schematic and EHC mode diagram of EHC patterns of $-{90}^{\circ }$ twisted state. (a) Schematic of $-{90}^{\circ }$ twisted state with EHC and (b) bifurcation diagram of $-{90}^{\circ }$ twisted state with EHC.

Figure 4

The change of bistable curve according to the strength of the fields at TNLC cell with orthogonally rubbed two surface boundaries. (a) Free energy density at 90° twisted state and (b) free energy density of $-{90}^{\circ }$ twisted state.

Figure 5

Images showing that the two states of which convection roll patterns are mutually orthogonal are switched when voltage with a proper frequency is applied to the top and bottom electrodes that generate vertical electric field.