Geometrically-controlled twist transitions in nematic cells.

We study geometrically controlled twist transitions of a nematic confined between a sinusoidal grating and a flat substrate. In these cells, the transition to the twisted state is driven by surface effects. We have identified the mechanisms responsible for the transition analytically and used exact numerical calculations to study the range of surface parameters where the twist instability occurs. Close to these values, the cell operates under minimal external fields or temperature variations.