(475a) Electrokinetic Vortices Due to Anomalous Polarization at a Nanoslot Entrance

Electro-osmotic micro-vortices are known to develop at the entrance of an ion-selective nanoslot or a nanoporous membrane. These vortices are driven by electric body forces on the space charges in the extended polarized layer just outside the nanoslot. We have been using these vortices to trap and concentrate nanocolloids, cells and molecules at the stagnation points of these vortex pairs to enhance the sensitivity and speed of biosensors by orders of magnitude. Here, we discuss several fundamental mechanisms, verified by confocal imaging, impedance and cyclic voltametry characterization of the ion current and theories, behind the formation of these vortices. The breakdown of (velocity) streamline-(electric) fieldline similaritude in nanoslots with transverse dimensions much smaller than the microchamber width is shown to develop a single entrance vortex pair, whose vorticity axis is parallel to the entrance and the substrate, due to a mismatch in the viscous length scales at the entrance[1]. A micro-instability, first predicted by Isaak Rubinstein, is shown to produce a vortex array with vorticity axis perpendicular to the substrate[2]. This hydrodynamic instability develops during the formation of the depletion layer due to a coupling of ion convection in the depletion zone and the length of the polarized region at the nanoslot end of the depletion layer. The self-similar t^1/2 depletion front dynamics has a distinct Warburg signature in the low-frequency AC impedance spectrum, which is used to scrutinize the onset of the instability. This instability is shown to be sensitive to the gap height (effective viscosity of the Hele-Shaw microslot flow) outside the nanoslot. Yet another kind of vortices develops when the width of the nanoslot becomes much smaller than the electrode width such that the nanoslot becomes a point source with a finite depletion length and a polarized region with a predetermined dimension[3]. A phase diagram is constructed to delineate all these electrokinetic vortices at a nanslot entrance due to anomalous polarization.

[1] Yossifon, G., Chang, Y.-C. and Chang, H.-C., Phys Rev Lett, 103, 154502(2009). [2] Yossifon, G. and Chang, H.-C., Phys Rev Lett., 101, 254501(2008). [3] Yossifon, G., Mushenheim, P., Chang, Y.-C. and Chang, H.-C., Phys Rev E,81,046301(2010).