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J. Non-Newtonian Fluid Mech. 103 (2002) 141– 148 Mass transfer in a laminar elongational flow of a drag reducing surfactant a a b, ∗ M.S. Boutoudj , A. Ouibrahim , C. Deslouis a Laboratoire d’Energétique de Mécanique et Matériaux Universitéde Boumerdès, de Boumerdès, Algeria b UPR 15 du CNRS, Université. P. et M. Curie, 4 ce Jussieu, 75252Paris Cédex 05, France Received 30 June 2001; received in revised form 18 October 2001 The mass transfer given by an electrochemical reaction towards a rotating disk in a solution of surfactant having drag reducing abilitie hibits a first order transition. Beyond this transition marked by a significant slump of the mass transfer rate, the flow es unsteady and is characterised by an hysteresis cycle. This phenomenon was earlier observed for viscoelastic dilute solutions of linear polymers of high molecular weight and is induced by the elongational strain prevailing in the flow around a rotating disk. © 2002 Elsevier Science B.V. s . : Surfactant; Mass transport; Drag reduction; Viscoelasticity; Electrochemistry 1. Introduction Some surfactant systems present the ability to reduce turbulent drag when the surfactant concentra- tion is above the critical micellar concentration (cmc) [1,2]. Quaternary ammonium associated to an appropriate counter-ion posses this property. The mechanism of drag reduction is generally ex ined in terms of elongational strains in cting with anisotropic elongated structures: this is the case of high polymer dilute solutions whi ay undergo conformational changes from statistical coils to fully elon- g