Abstract
A lamellar lyotropic phase (aqueous solution of nonionic C12E6) is studied in the vicinity of a lamellar-to-isotropic phase transition. We use freeze fracture and electron microscopy to show that, as temperature increases: the concentration of screw dislocations increases, screws of the same sign build twist walls and screws belong to dislocation loops which cross layers. Within reach of the transition, the burst of the number of defects induces a disorganization of the lamellar phase into small blocks, disoriented one with respect to another and bounded by twist walls. To our knowledge this is the first experimental evidence of a defect-mediated phase transition.