Periodical shear instability of nematics : symmetry considerations

Abstract

We show that from symmetry considerations it is possible to predict the general shape of the threshold curves fixing the boundaries between the different flow regimes (uniform, roll or square) occurring in a homeotropic nematic submitted to a periodical shear. Calculations have been made for the case in which the transition is reduced to a break of symmetry characterized by the appearance of a roll set in the direction ψ. The variation of ψ at threshold when the shear is modified has also been obtained