title: Dynamics of microvortices induced by ion concentration polarization.
abstract: We investigate the coupled dynamics of the local hydrodynamics and global electric response of an electrodialysis system, which consists of an electrolyte solution adjacent to a charge selective membrane under electric forcing. Under a dc electric current, counterions transport through the charged membrane while the passage of co-ions is restricted, thereby developing ion concentration polarization (ICP) or gradients. At sufficiently large currents, simultaneous measurements of voltage drop and flow field reveal several distinct dynamic regimes. Initially, the electrodialysis system displays a steady Ohmic voltage difference (ΔV_{ohm}), followed by a constant voltage jump (ΔV_{c}). Immediately after this voltage increase, microvortices set in and grow both in size and speed with time. After this growth, the resultant voltage levels off around a fixed value. The average vortex size and speed stabilize as well, while the individual vortices become unsteady and dynamic. These quantitative results reveal that microvortices set in with an excess voltage drop (above ΔV_{ohm}+ΔV_{c}) and sustain an approximately constant electrical conductivity, destroying the initial ICP with significantly low viscous dissipation.
authors.affiliation: Soft Matter, Fluidics and Interfaces, MESA+ Institute, University of Twente, 7500 AE Enschede, The Netherlands.|Wetsus, European Centre of Excellence for Sustainable Water Technology, Oostergoweg 9, 8911 MA Leeuwarden, The Netherlands.
authors.forename: Joeri C
authors.initials: JC
authors.lastname: de Valença
authors.affiliation: Wetsus, European Centre of Excellence for Sustainable Water Technology, Oostergoweg 9, 8911 MA Leeuwarden, The Netherlands.
authors.forename: R Martijn
authors.initials: RM
authors.lastname: Wagterveld
authors.affiliation: Soft Matter, Fluidics and Interfaces, MESA+ Institute, University of Twente, 7500 AE Enschede, The Netherlands.
authors.forename: Rob G H
authors.initials: RG
authors.lastname: Lammertink
authors.affiliation: Soft Matter, Fluidics and Interfaces, MESA+ Institute, University of Twente, 7500 AE Enschede, The Netherlands.|Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G8.
authors.forename: Peichun Amy
authors.initials: PA
authors.lastname: Tsai