The edge fracture occurrence on lyotropic liquid crystals

In the present work, the fracture behavior of the lyotropic liquid crystals during rheological measurements on the cone-and-plate geometry was studied . Three kinds of experiments were performed in order to produce different a lignment patterns on the samples under investigation. The results obtained showed that the fracture occurrence depends strongly on the previous histor y of the sample. If the velocity profile is suddenly changed by a small amo unt, the system may relax most of the stress jump in a viscous manner inste ad of an elastic one. which may induce the fracture initiation. The thresho ld shear rate for the edge fracture to occur depends upon the type and degr ee of the sample alignment. A compromise between the initiation and the pro pagation steps was observed. The presence of the orientation defects is nec essary for the fracture to initiate but a great number of them will not con tribute to decrease the fracture shear rate. (C) 2000 Elsevier Science B.V. All rights reserved.