title: Helical vortex formation in three-dimensional electrochemical systems with ion-selective membranes.
abstract: The rate of electric-field-driven transport across ion-selective membranes can exceed the limit predicted by Nernst (the limiting current), and encouraging this "overlimiting" phenomenon can improve efficiency in many electrochemical systems. Overlimiting behavior is the result of electroconvectively induced vortex formation near membrane surfaces, a conclusion supported so far by two-dimensional (2D) theory and numerical simulation, as well as experiments. In this paper we show that the third dimension plays a critical role in overlimiting behavior. In particular, the vortex pattern in shear flow through wider channels is helical rather than planar, a surprising result first observed in three-dimensional (3D) simulation and then verified experimentally. We present a complete experimental and numerical characterization of a device exhibiting this recently discovered 3D electrokinetic instability, and show that the number of parallel helical vortices is a jump-discontinuous function of width, as is the overlimiting current and overlimiting conductance. In addition, we show that overlimiting occurs at lower fields in wider channels, because the associated helical vortices are more readily triggered than the planar vortices associated with narrow channels (effective 2D systems). These unexpected width dependencies arise in realistic electrochemical desalination systems, and have important ramifications for design optimization.
authors.affiliation: Research Laboratory of Electronics, Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.|Singapore-MIT Alliance for Research and Technology (SMART), Singapore 138602.|Department of Ship Engineering and Fluid Mechanics, Hanoi University of Science and Technology, No1 DaiCoViet, Hanoi, Vietnam.
authors.forename: Sang V
authors.initials: SV
authors.lastname: Pham
authors.affiliation: Research Laboratory of Electronics, Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.|Pohang Universities of Science and Technology, Pohang, Gyeoungbuk 790784, Republic of Korea.
authors.forename: Hyuckjin
authors.initials: H
authors.lastname: Kwon
authors.affiliation: Research Laboratory of Electronics, Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Bumjoo
authors.initials: B
authors.lastname: Kim
authors.affiliation: Research Laboratory of Electronics, Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Jacob K
authors.initials: JK
authors.lastname: White
authors.affiliation: Pohang Universities of Science and Technology, Pohang, Gyeoungbuk 790784, Republic of Korea.
authors.forename: Geunbae
authors.initials: G
authors.lastname: Lim
authors.affiliation: Research Laboratory of Electronics, Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.|Singapore-MIT Alliance for Research and Technology (SMART), Singapore 138602.|Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Jongyoon
authors.initials: J
authors.lastname: Han