M. Cloitre ^*, T. Hall ^**, C. Mata ^**, D.D. Joseph ^**,

*Laboratoire Mixte CNRS-ELF ATOCHEM, 95 Rue Danton, 92303, Levallois-Perret, France

**University of Minnesota, AEM, 107 Akennan Hall, 110 Union Street, Minneapolis, MN 55455, USA

Received 5 May 1998

Abstract

It has been proposed recently that the combined action of inerlia and non-linear viscoelasticity may be the origin of very iculiar behaviors with dramatic changes of flow type. Two exarnples are the problem of delayed die swell and the orientation : elongated parlicles sedimenling in solutions of wormlike micelles. These solutions give rise to weIl defmed viscoelastic operties which can be tuned precisely by changing the surfactant weight fraction. Our analysis does not rely on any lostitutive equation and our results strongly support the interpretation that delayed die swell and sedimentation of long Klies in wormlike micellar solutions are ruled by a change of flow type from subcritical to supercritical. @ 1998 Elsevier ience B.V. AIl rights reserved.

Keywords~ Delayed-die swell; Viscoelastic; Sedimentation; Elongated parlicles; Wormlike micellar solutions