2014 Nous étudions l'instabilité qui se produit quand un nématique homéotrope est soumis à un cisaillement elliptique alternatif. A cet effet, nous développons une méthode permettant de tenir compte de la dépendance en temps aussi bien du régime non perturbé que des perturbations apparaissant lors de l'instabilité. D'autre part, nous appliquons la méthode au cas des échantillons minces. Nous obtenons ainsi une description détaillée de la façon dont varie le seuil quand change la forme de l'ellipse de cisaillement.
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
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