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Electroosmosis with surfactants : from one interface to a macroscopic foam
Effects of surface-active materials on free surface flows have long been reported (since Franklin’s observation of wave damping) and are encountered in many applications involving for example liquid foams or sprays. Nowadays, gas/liquid materials with a controlled micro or nanostructure are used to design new technological systems for tissues engineering, optics, phononic crystals, and lab on chip applications. Building these micro structured materials require a fine control of liquid flow at the nanoscales in the vicinity of gas/liquid interface. In this work, we therefore experimentally investigate how an electrically driven flow can be generated in these conditions.
Application of an electric field to a neutral system (such as a surfactant solution) would not induce a liquid flow unless charge separation occurs. Indeed, in the case of ionic surfactants, a part of them are known to adsorb on the interfaces, and then the generation and the magnitude of an electroosmotic flow is possible and will depend on the molecular details of the surface. Although many studies have focussed on surfactant adsorption at equilibrium [1], many questions on the effect of the flow on the surfactant repartition (and vice et versa) still remains. How does the flow affect adsorption ? Is there a Marangoni stress at the interface ? What is the exact hydrodynamic boundary condition ?
We then build an experiment allowing to probe simultaneously the surfactant repartition at the interface, by investigating the non linear optical response of the system (second harmonic generation or SHG technique) [2], together with the measurement of the electroosmotic flow underneath using particle tracking under a fast confocal microscope [3]. This first coupled measurement allowed us to prove that the surfactant layer remains homogeneously distributed, then immobile and that the interface remains stress-free. This result is for the first time an experimental proof that no force acts on the charged layer in EO phenomena as theoretically expected [4]. In a second part, we will present the influence of the system deformability on the magnitude of EO flow. We will consider in particular the case of soap films [5] and macroscopic foam systems, where capillary recoil counteracts the pressure induced by the flow.
[1] J. Schulze-Schlarmann, N. Buchavzov, and C. Stubenrauch, Soft Matter 2, 584 (2006).
[2] G. Martin-Gassin, E. Benichou, G. Bachelier, I. Russier-Antoine, C. Jonin, and P.-F. Brevet, The Journal of Physical Chemistry C 112, 12958 (2008).
[3] Blanc, Bonhomme, Benichou, Brevet, Ybert and Biance, electroosmosis near stress free interface, in revision.
[4] L. Bocquet and E. Charlaix, Chemical Society Reviews 39, 1073 (2010).
[5] O. Bonhomme, O. Liot, A.-L. Biance, and L. Bocquet, Phys. Rev. Lett. 110, 054502 (2013).
Collaborators : Oriane Bonhomme, Baptiste Blanc, Christophe Ybert, Emmanuel Benichou, Pierre-François Brevet.