Dislocation Effects on the Viscoelastic Properties of a Smectic A Liquid Crystal

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Abstract
We study the influence of defects on the viscoelastic properties of a semetic A liquid crystal (the octyloxycyanobiphenyl), by means of teh undulation instability technique under a dilative stress. We introduce layers edge dislocations by using a wedge geometry. In this geometry, we predict the Burgers vector of the edge dislocation to increase with the wedge angle, so that the periodic pattern of the edge dislocations keeps a maximum wave number, of the order of the undualtion instability wave number. We measure the undulation threshold and the short damping time τ of the elastic strongly with θ; for large θ this is well expalined by a recent model from Pershan and Prost. At low θ our values tend toward the penetration length of de Gennes. The damping time shows at large θ a decrease τ ∼ θ−1, well described by a simple model. At small θ, we observe large oscillation of τ for periodic values of θ. The observation is expalined by a geometrical coincidence of the wave length of the undulation instability and of the periodic pattern of edge dislocation in the wedge. These data are com[patible with our model for the Burger vector adjustment. Such a peiodic pattern of low Burgers vector is the smectic analog of the Grandjean-Cano lines in cholesterics.