title: Self-organized arrays of dislocations in thin smectic liquid crystal films.
abstract: Combining optical microscopy, synchrotron X-ray diffraction and ellipsometry, we studied the internal structure of linear defect domains (oily streaks) in films of a smectic liquid crystal 8CB with thicknesses in the range of 100-300 nm. These films are confined between air and a rubbed PVA polymer substrate which imposes hybrid anchoring conditions (normal and unidirectional planar, respectively). We show how the presence or absence of dislocations controls the structure of highly deformed thin smectic films. Each domain contains smectic layers curved in the shape of flattened hemicylinders to satisfy both anchoring conditions, together with grain boundaries whose size and shape are controlled by the presence of dislocation lines. A flat grain boundary normal to the interface connects neighboring hemicylinders, while a rotating grain boundary (RGB) is located near the axis of curvature of the cylinders. The RGB shape appears such that dislocation lines are concentrated at its summit close to the air interface. The smectic layers reach the polymer substrate via a transition region where the smectic layer orientation satisfies the planar anchoring conditions over the entire polymer substrate and whose thickness does not depend on that of the film. The strength of planar anchoring appears to be high, larger than 10(-2) mJ m(-2), compensating for the high energy cost of creating an additional 2D defect between a horizontal smectic layer and perpendicular ones of the transition region. This 2D defect may be melted, in order to avoid the creation of a transition region structure composed of a large number of dislocations. As a result, linear defect domains can be considered as arrays of oriented defects, straight dislocations of various Burger vectors, whose location is now known, and 2D nematic defects. The possibility of easy variation between the present structure with a moderate amount of dislocations and a structure with a large number of dislocations is also demonstrated.
authors.affiliation: CNRS UMR 7588, Sorbonne Universités, UPMC Univ Paris 06, Institut des NanoSciences de Paris (INSP), 4 place Jussieu, 75005 Paris, France. emmanuelle.lacaze@insp.jussieu.fr.
authors.forename: Delphine
authors.initials: D
authors.lastname: Coursault
authors.forename: Bruno
authors.initials: B
authors.lastname: Zappone
authors.forename: Alessandro
authors.initials: A
authors.lastname: Coati
authors.forename: Athmane
authors.initials: A
authors.lastname: Boulaoued
authors.forename: Laurent
authors.initials: L
authors.lastname: Pelliser
authors.forename: Denis
authors.initials: D
authors.lastname: Limagne
authors.forename: Nathalie
authors.initials: N
authors.lastname: Boudet
authors.forename: Bicher Haj
authors.initials: BH
authors.lastname: Ibrahim
authors.forename: Antonello
authors.initials: A
authors.lastname: de Martino
authors.forename: Michel
authors.initials: M
authors.lastname: Alba
authors.forename: Michel
authors.initials: M
authors.lastname: Goldmann
authors.forename: Yves
authors.initials: Y
authors.lastname: Garreau
authors.forename: Bruno
authors.initials: B
authors.lastname: Gallas
authors.forename: Emmanuelle
authors.initials: E
authors.lastname: Lacaze
mesh_terms: D001713:Biphenyl Compounds / Q000737:chemistry*; D003198:Computer Simulation; D050866:Liquid Crystals / Q000737:chemistry* / Q000648:ultrastructure*; D008956:Models, Chemical; D009570:Nitriles / Q000737:chemistry*; D044367:Phase Transition; D011142:Polyvinyl Alcohol / Q000737:chemistry; D014961:X-Ray Diffraction
chemical_list: C495061:4'-octyl-4-cyanobiphenyl; D001713:Biphenyl Compounds; D009570:Nitriles; D011142:Polyvinyl Alcohol