Single‐domain cobalt dot arrayswith high magnetic particle density, patterned over large areas (e.g., 10 cm diameter wafers) are fabricated by self‐assembled block copolymer lithography, using a polystyrene–poly(ferrocenyldimethylsilane) copolymer as a template. By varying the copolymer type and etching conditions the magnetic properties can be tuned. The Figure shows a typical array of Co dots with tungsten caps obtained via this procedure.

The integration of mass transport control by means of membrane functionality into microfluidic devices has shown substantial growth over the last 10 years. Many different examples of mass transport control have been reported, demonstrating the versatile use of membranes. This review provides an overview of the developments in this area of research. Furthermore, it aims to bridge the fields of microfabrication and membrane science from a membrane point-of-view. First the basic terminology of membrane science will be discussed. Then the integration of membrane characteristics on-chip will be categorized based on the used fabrication method. Subsequently, applications in various fields will be reviewed. Considerations for the use of membranes will be discussed and a checklist with selection criteria will be provided that can serve as a starting point for those researchers interested in applying membrane-technology on-chip. Finally, opportunities for microfluidics based on proven membrane technology will be outlined. A special focus in this review is made on the membrane properties of polydimethylsiloxane (PDMS), since this material is frequently used nowadays in master replication.

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] ...

When placed on rough hydrophobic surfaces, water droplets of diameter larger than a few millimeters can easily form pearls, as they are in the Cassie-Baxter state with air pockets trapped underneath the droplet. Intriguingly, a natural evaporating process can drive such a Fakir drop into a completely wetting (Wenzel) state. Our microscopic observations with simultaneous side and bottom views of evaporating droplets upon transparent hydrophobic microstructures elucidate the water-filling dynamics and suggest the mechanism of this evaporation-triggered transition. For the present material the wetting transition occurs when the water droplet size decreases to a few hundreds of micrometers in radius. We present a general global energy argument which estimates the interfacial energies depending on the drop size and can account for the critical radius for the transition.