Taming Electrowetting Using Highly Concentrated Aqueous Solutions

Abstract

Wetting of carbon surfaces is one of the most widespread, yet poorly understood, physical phenomena. Control over wetting properties underpins the operation of aqueous energy-storage devices and carbon-based filtration systems. Electrowetting, the variation in the contact angle with an applied potential, is the most straightforward way of introducing control over wetting. Here, we study electrowetting directly on graphitic surfaces with the use of aqueous electrolytes to show that reversible control of wetting can be achieved and quantitatively understood using models of the interfacial capacitance. We manifest that the use of highly concentrated aqueous electrolytes induces a fully symmetric and reversible wetting behavior without degradation of the substrate within the unprecedented potential window of 2.8 V. We demonstrate where the classical “Young–Lippmann” models apply, and break down, and discuss reasons for the latter, establishing relations among the applied bias, the electrolyte concentration, and the resultant contact angle. The approach is extended to electrowetting at the liquid|liquid interface, where a concentrated aqueous electrolyte drives reversibly the electrowetting response of an insulating organic phase with a significantly decreased potential threshold. In summary, this study highlights the beneficial effect of highly concentrated aqueous electrolytes on the electrowettability of carbon surfaces, being directly related to the performance of carbon-based aqueous energy-storage systems and electronic and microfluidic devices.

Anion Intercalation into Graphite Drives Surface Wetting

“…These conditions require that the physicochemical changes induced by the application of potential bias change the energetics of the system in a reversible way that also promotes electrowetting. 35,45 In our case, anion intercalation alters the energetics of the interface by largely lowering the interfacial surface tension, while simultaneously increasing the in-plane conductivity of the outermost graphene layers on HOPG. Both Journal of the American Chemical Society phenomena are responsible for the observed significant enhancement of electrowetting at E > E pzc .…”

“…At this point, it is worth highlighting the fact that the enhancement of electrowetting due to the intercalation of anions into HOPG demonstrates that, in strong contrast to what is well-established in EWOD technology, charge transfer processes (or charge injection using EWOD terminology) can have a beneficial effect in the case of electrowetting directly on conductors when certain conditions are satisfied. These conditions require that the physicochemical changes induced by the application of potential bias change the energetics of the system in a reversible way that also promotes electrowetting. , In our case, anion intercalation alters the energetics of the interface by largely lowering the interfacial surface tension, while simultaneously increasing the in-plane conductivity of the outermost graphene layers on HOPG. Both phenomena are responsible for the observed significant enhancement of electrowetting at E > E pzc .…”

Dielectric-free electrowetting on graphene

“…The setup used for the electrowetting experiments has been previously described in detail. 16,17 CVD graphene of varied layer thickness on Si/SiO 2 substrates served as working electrodes (WE).…”

“…The characteristic times of the droplet's motion used to determine the timescales were taken to be 90% of the corresponding equilibrium contact angles. Contact angle values were extracted from the recorded images using a custom-made edge detection algorithm in MATLAB ® (MathWorks Inc., Natick, MA, USA) (for details see 16 ). All electrochemical experiments were performed on an Autolab PGSTAT302N potentiostat from Metrohm, operated using Nova 1.11.2 software.…”

“…Based on the derived data, it appears that the use of highly concentrated electrolytes suppresses the effect of impurities as well as that of the underlying electrochemical reactions on electrowetting using graphene, in a similar way with what is reported for graphite. 16 Additionally, the high electrolyte concentration leads to increased electrostatic interactions between the electrolyte ions and graphene that influences the potential dependent solid|liquid interfacial surface tension and therefore affects electrowetting.…”