Electrowetting has become one of the most widely used tools to manipulate tiny amount of liquids on surfaces. Applications range from lab-on-a-chip devices to adjustable lenses or new types of electronic displays. In the present article, we review the recent progress in this rapidly growing field including both fundamental and applied aspects. We compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high. We discuss in detail the origin of the electrostatic forces that induce both the contact angle reduction as well as the motion of entire droplets. We examine the limitations of the electrowetting equation and present a variety of recent extensions to the theory that account for distortions of the liquid surface due to local electric fields, for the finite penetration depth of electric fields into the liquid, as well as for finite conductivity effects in the presence of AC voltage. The most prominent failure of the electrowetting equation, namely the saturation of the contact angle at high voltage, is discussed in a separate section. Recent work in this direction indicates that a variety of distinct physical effects -rather than a unique one -is responsible for the saturation phenomenon, depending on experimental details. In the presence of suitable electrode patterns or topographic structures on the substrate surface, variations of the contact angle can not only give rise to continuous changes of the droplet shape, but also to discontinuous morphological transitions between distinct liquid morphologies. The dynamics of electrowetting are discussed briefly. Finally, we give an overview of recent work aimed at commercial applications, in particular in the fields of adjustable lenses, display technology, fiber optics, and biotechnology-related microfluidic devices.3

We investigated the influence of the relative humidity on amplitude and phase of the cantilever oscillation while operating an atomic force microscope (AFM) in the tapping mode. If the free oscillation amplitude A0 exceeds a certain critical amplitude Ac, the amplitude- and phase-distance curves show a transition from a regime with a net attractive force between tip and sample to a net repulsive regime. For hydrophilic tip and sample this critical amplitude Ac is found to increase with increasing relative humidity. In contrast, no such dependence was found for hydrophobic samples. Numerical simulations show that this behavior can be explained by assuming the intermittent formation and rupture of a capillary neck in each oscillation cycle of the AFM cantilever

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] ...

The distribution of ions and charge at solid-water interfaces plays an essential role in a wide range of processes in biology, geology and technology. While theoretical models of the solid-electrolyte interface date back to the early 20th century, a detailed picture of the structure of the electric double layer has remained elusive, largely because of experimental techniques have not allowed direct observation of the behaviour of ions, i.e. with subnanometer resolution. We have made use of recent advances in high-resolution Atomic Force Microscopy to reveal, with atomic level precision, the ordered adsorption of the mono- and divalent ions that are common in natural environments to heterogeneous gibbsite/silica surfaces in contact with aqueous electrolytes. Complemented by density functional theory, our experiments produce a detailed picture of the formation of surface phases by templated adsorption of cations, anions and water, stabilized by hydrogen bonding.

The impingement of drops onto solid surfaces 1,2 plays a crucial role in a variety of processes, including inkjet printing, fog harvesting, anti-icing, dropwise condensation and spray coating 3-6. Recent e orts in understanding and controlling drop impact behaviour focused on superhydrophobic surfaces with specific surface structures enabling drop bouncing with reduced contact time 7,8. Here, we report a di erent universal bouncing mechanism that occurs on both wetting and non-wetting flat surfaces for both high and low surface tension liquids. Using high-speed multiple-wavelength interferometry 9 , we show that this bouncing mechanism is based on the continuous presence of an air film for moderate drop impact velocities. This submicrometre 'air cushion' slows down the incoming drop and reverses its momentum. Viscous forces in the air film play a key role in this process: they provide transient stability of the air cushion against squeeze-out, mediate momentum transfer, and contribute a substantial part of the energy dissipation during bouncing. The role of ambient air in drop impact and other dynamic wetting phenomena has long been neglected. Only recently, observations such as the suppression of splashing in drop impact at reduced ambient pressure 10 , the generation of splashes by superhydrophobic spheres falling into a liquid bath 11 , and the entrainment of air by fast-moving contact lines 12 highlighted the relevance of this rather viscous ambient medium. For drop impact, theoretical studies 13-16 suggested that splashing might be related to the presence of a thin lubricating air layer between the impacting drop and the substrate. Subsequent experiments confirmed the transient formation of an air layer with (sub)micrometre thickness 9,17-20. However, it turned out that the air film collapses on a microsecond timescale for typical impact speeds of splashing experiments (of the order of m s −1). However, as we report in this Letter, the air film remains intact if the initial impact speed is reduced to less than ν ∼ 0.5 m s −1. In this case, the drop rebounds without ever directly touching the surface. We release liquid drops of water, glycerol, silicone oil and various other organic liquids (Supplementary Table 1) of millimetric size (R = 0.52. .. 1.03 mm) from a height of several millimetres to a few centimetres to fall onto carefully cleaned and dust-free surfaces of variable wettability (Methods). On their first impact the drops have initial Weber numbers We = ρRν 2 /σ = 0.64. .. 4.3 (ρ: liquid density; R: drop radius; σ : surface tension). For all liquids studied, side-view images taken with a high-speed video camera (Fig. 1a) show that the drops bounce provided that the impact speed is not too high. For water, the maximum impact speed is 0.48 m s −1 , corresponding to We max ≈ 4. Throughout the entire bouncing process, the apparent contact angle observed in side-view images remains close to 180 • , both for clean glass substrates with an equilibrium contact