Shear-thickening induced by Faraday waves in di- lute wormlike micelles

We investigate the effect of surface waves generated by the Faraday instability on a shear-thickening surfactant solution under vertical vibrations. We show that a prolonged oscillation of the surface above the instability onset leads to an increase of the fluid viscosity. This phenomenon is evidenced by comparing the time needed for the instability to develop in the fluid at rest and once the fluid has been shaken above onset: pre-shaking may delay the instability by two orders of magnitude. A simple model based on a time-dependent viscosity is proposed which accounts quantitatively for the experimental observations. Introduction. – In the last few decades, the coupling between microstructure and flow in complex fluids has been the subject of intense research effort [1]. In most cases the flow is a simple shear flow induced by a rheometer. Yet, another way to induce a controlled flow is by the means of hydrodynamic instabilities. In this paper the effect of the wellknown Faraday instability, by which the initially flat surface of a fluid layer submitted to vertical vibrations gives way to surface waves with characteristic wave number kc above some critical acceleration ac [2], is tested on a shear-thickening dilute solution of wormlike micelles [3]. Recently the Faraday instability in complex fluids has been the subject of various studies [4–11]. In most of these previous works [4–8], the authors compared the behaviour of a complex fluid to that of a Newtonian one close to the instability threshold. Differences were explained by invoking the linear viscoelastic properties of the samples. Far above the instability threshold, a stronger coupling between the flow induced by the instability and the complex fluid microstructure is expected. For instance, in clay suspensions, hysteresis and high amplitude fingerlike structures were shown to be linked to shear-thinning [9]. In shear-thickening colloidal suspensions, external perturbations may grow and get stabilized as “persistent holes” across the fluid surface [10]. In a dilute polymer solution, the flow– microstructure coupling can delay the transition to disordered states [11]. The present work shows that, far above the instability threshold, the surface waves become large enough to induce shear-thickening in dilute wormlike micelles. In the following we first present the complex fluid under study and the experimental setup. Then measurements of the set-up time of the Faraday instability, defined as the time needed for the surface waves to become large enough to be detected by our experimental setup, are presented. We show that the set-up time may increase by two orders of magnitude when the fluid is pre-shaken above the (a)E-mail: [email protected]