Measurement of the twist elastic constant of nematic liquid crystals using pi-cell devices

We present a fast and accurate method for measurement of the twist elastic constant of nematic liquid crystals using pi-cell devices. We observe that there are two bifurcations during the transition from the bend state to the planar 180 degrees twisted state, each of which leads to a voltage threshold. The Freedericksz threshold voltage due to a tilt bifurcation occurs at a lower voltage and is softened by the required surface pretilt in the device. The higher of the two thresholds is due to a bifurcation between left- and right-handed 180 degrees twisted states and we term this the T to V state threshold. This voltage threshold is independent of device thickness, and is not softened by the pretilt. We show that the T to V state threshold is highly sensitive to changes in the twist elastic constant, K-22, and use this threshold to evaluate the magnitude of K-22 for a variety of materials. These results show very good agreement with those obtained using the standard magnetic Freedericksz twist threshold experiment. This method does not require a complex experimental setup and has an accuracy of better than +/- 8%.