Création de Lignes de Dislocation dans un Cholestérique par l'Application d'un Champ Magnétique

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Abstract
In a drop of cholesteric liquid crystal with a free surface there are no lines of discontinuity, analogous to those observed in a Grandjean wedge. We observe that the application of a magnetic field perpendicular to the helical axis gradually imposes on the surface molecules a single preferential direction with respect to the field. For a critical field HL the orientation of the surface molecules is everywhere the same, 45° to the field direction. A lattice of dislocation lines is formed, analogous to that observed in the Grandjean wedge geometry. At the transition an estimate of the wall energy gives the order of magnitude of the line energy.