A STUDY OF PERIODIC TRANSITIONS IN SMECTIC-C LIQUID-CRYSTALS

We present a dynamical study of the instabilities that ensue when a re latively large magnetic field is applied suddenly across a sample of s mectic liquid crystal held between parallel plates. We consider the ca se when the smectic layers are initially aligned parallel to the bound ing plates, and the applied field is also parallel to the plates but n ormal to the initial average molecular orientation within the layers. Our linearized dynamical analysis for this situation confirms the pred iction of static theory that there is a threshold field beyond which a Freedericksz transition occurs. Additionally, however, it is shown he re that there is a second threshold beyond which a spatially periodic (rather than homogeneous) transition occurs, leading to a striped patt ern; such patterns are likely to be of concern in the construction of smectic display devices. For both types of instability a bulk velocity ('backflow') is induced. Both strong anchoring and weak anchoring are considered, and expressions are obtained for the threshold fields and for the growth rates in the various cases. Attention is focussed on t he smectic CM phase, though analogous results should be valid for more general smectic phases.