Frequency-induced structure transition of nematic electroconvection in twist cells

We investigate electroconvection of nematic liquid crystals in planar 90 degrees twist cells in the dielectric regime. These cells provide competing boundary conditions for the nematic director at both electrodes. When the ratio of cell gap and roll width is not too small (in thin cells and at low frequencies), the convection rolls form along the director in the cell middle, which is diagonal to the anchoring directions. This is in accordance with known behavior in the conduction regime and with the assumption of a bulk instability (rolls traversing the cell). When the ratio of roll periodicity to cell gap is very small in the high-frequency range and the thick cells, a regular convection pattern sets in with wave vectors directed along the two alignment directions. We suggest the interpretation that two independent systems of convection rolls form localized near the electrode plates. It is very likely that a similar behavior occurs in nontwisted cells where it is difficult to identify experimentally.