We present a visualization of the predicted instability in ionic conduction from a binary electrolyte into a charge selective solid. This instability develops when a voltage greater than critical is applied to a thin layer of copper sulfate flanked by a copper anode and a cation selective membrane. The current-voltage dependence exhibits a saturation at the limiting current. With a further increase of voltage, the current increases, marking the transition to the overlimiting conductance. This transition is mediated by the appearing vortical flow that increases with the applied voltage. DOI: 10.1103/PhysRevLett.101.236101 PACS numbers: 82.45.Gj, 47.20.Ma, 82.40.Ck Microscale fluid flows commonly arise when a dc current passes through the diffusion layers (DL) of binary ionic solutions adjacent to charge selective solids, such as electrodes [1], ion exchange granules [2] or membranes [3], and arrays of nanochannels [4]. Under conditions of extreme diffusion limitation (concentration polarization (CP) near the limiting current [5]), these flows provide an additional ionic transport mechanism. This mechanism is essential for the operation of nanofluidic preconcentrators [4] and overlimiting electrodialysis [6,7]. On short length scales and in the absence of free interfaces, these flows are not driven by gravity or surface tension. Instead, they are driven by the electric force acting upon the space charge of the nanometers-thick interfacial electric double layer (EDL). Slip-like fluid flow induced by this force is known as electro-osmosis (EO).There are two regimes of EO that correspond to the different states of the EDL and are controlled by the nonequilibrium voltage drop (overvoltage) across it [8]. These are the quasiequilibrium regime [9,10] and the nonequilibrium EO [2,8,11]. While both regimes result from the action of a tangential electric field upon the space charge of the EDL, the first relates to the charge of quasiequilibrium EDL, whereas the second relates to the extended space charge of nonequilibrium EDL. The nonequilibrium EDL develops in the course of CP near the limiting current.According to a recent theory [8], a novel critical instability of quiescent ionic conduction related to the extended charge EO stands behind the overlimiting conductance through a planar ion exchange membrane. During 1D conduction through a planar layer, an electrolyte concentration gradient forms. The related electric force does not impair the mechanical equilibrium in the system, which remains stable as long as the EDL retains its quasiequilibrium structure. As voltage increases, the system moves away from quasiequilibrium, and an extended space charge develops in the EDL. EO slip related to this extended space charge renders the quiescent conduction unstable [8]. This instability of 1D ionic conduction is reminiscent of instabilities in 1D thermal conduction, such as the RayleighBenard and Marangoni instabilities. While reports of the underlying extended space charge EO [2] and possibly its related flow patterns [1] ...

We investigate the spreading at variable rate of a water drop on a smooth hydrophobic substrate in an ambient oil bath driven by electrowetting. We find that a thin film of oil is entrapped under the drop. Its thickness is described by an extension of the Landau-Levich law of dip coating that includes the electrostatic pressure contribution. Once trapped, the thin film becomes unstable under the competing effects of the electrostatic pressure and surface tension and dewets into microscopic droplets, in agreement with a linear stability analysis. Our results recommend electrowetting as an efficient experimental approach to the fundamental problem of dynamic wetting in the presence of a tunable substrate-liquid interaction. Apart from technological applications, EW has also proven to be a very useful tool for studying fundamental problems in wetting and thin film hydrodynamics, where the contact angle is often a crucial parameter that is difficult if not impossible to vary experimentally without changing other important aspects of the system. Examples include wetting of complex surfaces [3,4], capillary pinch-off and microdroplet generation [5][6][7], and deposition [8]. Frequently, electrowetting experiments are performed in an ambient oil bath in order to minimize both the evaporation of liquid and contact angle hysteresis. It has been indicated by several authors [9][10][11][12][13] that thin layers of the ambient oil might form between the drop and substrate in such a twophase configuration. Quilliet and Berge [9] found theoretically that the balance between electrical forces and the disjoining pressure should give rise to an equilibrium thickness of the films of approximately 10 -20 nm for typical values of the applied voltage. However, despite the importance of these layers-for instance for the reduction of contact angle hysteresis, but also for the protection of the surfaces from adsorption of biomolecules [10,14]their properties and formation mechanism remained elusive in previous experimental studies [11].In the present Letter we study the dynamics of moving contact lines in EW systems with a two-phase configuration, as just described. We show that a layer of oil is indeed entrapped under the drop with an initial thickness that is determined by the hydrodynamics of the moving contact line rather than by equilibrium properties. To describe the entrapment process we extend the Landau-Levich [15] treatment of dynamic wetting by an additional electrostatic pressure contribution, a topic that attracted considerable attention in the recent wetting literature [16 -18]. Following the entrapment, the oil film turns out to be unstable and breaks up into a number of smaller oil droplets. The size distribution of these droplets is described by a linear stability analysis of the thin film in the lubrication approximation, taking into account the balance between surface tension and electrostatic pressure. The problem thus combines two aspects: the entrapment process itself and the subsequent time evolution of the entra...

We studied the flow fields generated inside sessile drops that oscillate periodically between states of high and low contact angle under the influence of alternating electric fields of variable frequency and amplitude. Following the motion of dye patches, we show that the number of oscillation cycles required to achieve mixing scales logarithmically with the Péclet number as expected for chaotic mixing. High speed movies reveal an asymmetry of the drop shape between the spreading and receding phase of the oscillations. This results in net internal flow fields that we characterize by tracing the motion of colloidal seed particles. The strength and frequency dependence of the flow are explained in terms of Stokes drift driven by capillary waves that emanate from the oscillating contact line.

Drops sandwiched between two substrates are often found in lab-on-chip devices based on digital microfluidics. We excite azimuthal oscillations of such drops by periodically modulating the contact line via ac electrowetting. By tuning the frequency of the applied voltage, several shape modes can be selected one by one. The frequency of the oscillations is half the frequency of the contact angle modulation by electrowetting, indicating a parametric excitation. The drop response to sinusoidal driving deviates substantially from sinusoidal behavior in a "stop and go" fashion. Although our simple theoretical model describes the observed behavior qualitatively, the resonances appear at lower frequencies than expected. Moreover, the oscillations produce a nonperiodic fluid transport within the drop with a typical velocity of 1 mm/s. In digital microfluidic devices, where the typical drop size is less than 1 mm, this flow can result in very fast mixing on the spot.