PhD. Student: Joeri de Valença Phone: 058-2843181

1. Effects of convection and membrane structure on the onset of overlimiting current in ion-exchange membranes PhD. Student: Joeri de Valença Phone: 058-2843181 Thesis advisor: Rob Lammertink E-mail: joeri.devalenca@wetsus.nl Supervisor: Amy Tsai URL: www.wetsus.nl Research group: SFI Research school: Supported by: Period: 2012-2016 Motivation Ion exchange membranes are widely used in industry and research. Classical ion-exchange (electrodialysis) membrane theory predicts a maximum ion transport (the so-called limiting current) through the membrane. The experimentally observed overlimiting current in ion-exchange membranes is still one of the most puzzling problems in the field of nanofluidics and membranes. Understanding the nature of overlimiting current regime (Fig. 1) and use this to fabricate membranes where the overlimiting regime is shifted towards lower voltages, is the ultimate aim of this research. Fig2. PIV setup. Plastic 5 µm particles are suspended in the fluid and will follow the fluid motion, i.e. convection patterns. Combining pulsed laser light and high speed camera gives real time velocity profiles and vortices sizes. The camera would stand perpendicular to this picture, like your eyes. Research objective Combining experiment and theory can help us to understand and describe the mechanism of convection and the parameter(s) responsible to trigger the onset of the overlimiting current regime. Our final goal is to add heterogeneous structures to the membrane surface which can enhance convection and therefore decrease the limiting transition region. That way membranes can operate under higher fluxes, implying a more effective mode of operation. Fig1. The typical current response on a gradually increasing voltage difference over the electrolyte-membrane-electrolyte system. Scans have been done at different electrolyte concentrations. Technological challenge The boundary layer at the interface of the electrolyte solution and the membrane dominates the overall transport resistance. Whereas at lower voltages current is limited by diffusion, at higher voltages convective transport takes over, resulting in current increase. Studying this process in more detail requires momentum and concentration profiles in the thin, 300 µm thick, boundary layer. Our approach is to use optical techniques like particle image velocimetry (PIV) and ion fluorescence spectroscopy in a special designed cell that allows PIV and current recordings simultaneously Fig3. (Left) A typical image of the suspended particles near the cation exchange membrane. (Right) A vector flow field showing the average fluid motion over 8 seconds at 3.5V.