Hydration Energy-Dependent Ion Intercalation on Graphite and the Asymmetric Electrowetting.

Ion intercalation in graphite is widely used in desalination, batteries, and graphene stripping; it has high value in the fields of industry and research. However, selective ion transport, particularly (de)hydration energy and the hydration shell effect on the intercalation of ions into the graphite interlayer spaces, is still unclear. Here, we report low-voltage ion intercalation as observed by electrowetting on highly oriented pyrolytic graphite of an aqueous drop containing various inorganic salts. The electrowetting response exhibits asymmetric behavior with no contact angle change for the negative polarity and a threshold voltage for the onset of the contact angle change for the positive polarity. To explain the asymmetric electrowetting behavior and quantitatively predict the threshold voltage, we developed a physical model based on the hydration shell energy and size of the ion that undergoes partial breaking/deformation during the co-intercalation into the spaces between graphite layers. Electrowetting experiments using ions with various hydration energies and hydration radii were performed to confirm the prediction of the model. Further, we show a strategy to make the electrowetting response of LiCl drops symmetric via tuning the hydration energy of the Li+ ions using a binary solvent of a glycerol-water mixture. This article will provide an understanding of the hydration (solvation) energy dependence intercalation mechanism in graphite for electrowetting, which underpins various processes such as ion battery applications and the graphene exfoliation process.

Mobility of Aqueous and Binary Mixture Drops on Lubricating Fluid-Coated Slippery Surfaces.

The mobility of liquid drops on lubricant-infused slippery surfaces depends strongly on various system parameters, for example, surface energy and roughness of the underlying solid surface and surface tension and viscosity of the test and the lubricating fluids. Here, we investigate lubricant-coated slippery surfaces fabricated on smooth hydrophobic solid surfaces and examine the influence of thickness and viscosity of the lubricating oil on the velocity of aqueous drops. We also investigate the effect of surface tension of the test liquid using a binary mixture of water and ethanol, on the apparent contact angle, which further affects their slip velocity. A theoretical model, based on various dissipative forces acting in different regions of the lubricating oil and a test drop, is also presented, which elucidates the dependence of drop velocity on lubricating oil viscosity and base radius of drops of test liquids.

Transition of interfacial capacitors in electrowetting on a graphite surface by ion intercalation.

The low voltage electrowetting response of a LiCl aqueous solution on a freshly cleaved surface of highly oriented pyrolytic graphite (HOPG) is presented. For applied voltages below 1 V, the energy stored in the electrical double layer (EDL) is insufficient to drive the spreading of the drop due to the pinning of the three phase contact line at the step edges. Electrochemical impedance spectroscopy shows a dramatic increase in capacitance above 1 V, which provides a sufficient electrowetting force for depinning the contact line, resulting in a subsequent decrease of the contact angle. The transition of the interfacial capacitance from the EDL to the many-fold high capacitance of the pseudocapacitor drives the electrowetting transition on the HOPG surface. The observed changes in the capacitances above 1 V are correlated with the cyclic voltammetry and atomic force microscopy results, which show that the Cl- ions intercalate into the graphite galleries upon acquiring sufficient energy to overcome the van der Waals attraction between the graphene layers through the side of the step edge of the basal planes. To the best of our knowledge, this is the first study on the voltage dependent intercalation mediated transition of interfacial capacitance driving the spreading of an aqueous electrolyte drop on the HOPG surface, which provides a fundamental understanding of the mechanism and opens up potential applications in microfluidics and charge storage technologies.

Electrowetting actuated microfluidic transport in surface grooves with triangular cross section.

Liquids show different static wetting morphologies in open triangular grooves depending upon the wedge angle (ψ) of the groove and the liquid contact angle (θ) with the substrate. Switching between different morphologies can be achieved either by varying the contact angle of the liquid or by changing the wedge angle of the groove. In the present work we manipulate the apparent contact angle of a liquid by electrowetting to switch between liquid morphologies, from droplet to filament, to achieve microfluidic transport of the liquid into open triangular grooves. The static length of liquid filaments in grooves is analyzed as a function of applied voltage for different applied ac frequencies. The dynamic advancement of the filament lengths in grooves is analyzed as a function of time for different applied voltages for two different liquids: first with contact angle greater than the wedge angle and second with contact angle smaller than the wedge angle. Later an exact electrical model is derived to explain the liquid transport in triangular grooves actuated by electrowetting which includes the precise geometry of the liquid morphology.

Approximately symmetric electrowetting on an oil-lubricated surface.

As the most widely used insulator materials in the electrowetting (EW) systems, amorphous fluoropolymers (AFs) provide excellent hydrophobicity, dielectric properties and chemical inertness; however, they suffer from charge trapping during electrowetting with water and the consequent asymmetric phenomenon. In this study, an ultra-thin oil-lubricated AF surface was proposed to release the charge trapping in the dielectric layer and further suppress the polarity-dependent asymmetry during electrowetting. The negative spontaneously trapped charges gathering on the dielectric/water interface with aging time were characterized by various measurements and calculations, which explained the polarity dependence of the asymmetric electrowetting. Approximately symmetric EW curves withstanding water aging were obtained for the oil-lubricated AF surface, confirming the blocking effect on charge trapping induced by the lubricated surface. The improved reversibility of EW with low contact angle hysteresis brought by the oil-lubricated surface was also demonstrated. This study reveals the mechanism behind the asymmetric EW phenomenon and offers an attractive oil-lubricated EW material system for suppressing the charge trapping on the dielectric/water interface, which can significantly improve the manipulation of the EW devices.

Wettability Manipulation by Interface-Localized Liquid Dielectrophoresis: Fundamentals and Applications.

Electric field-based smart wetting manipulation is one of the extensively used techniques in modern surface science and engineering, especially in microfluidics and optofluidics applications. Liquid dielectrophoresis (LDEP) is a technique involving the manipulation of dielectric liquid motion via the polarization effect using a non-homogeneous electric field. The LDEP technique was mainly dedicated to the actuation of dielectric and aqueous liquids in microfluidics systems. Recently, a new concept called dielectrowetting was demonstrated by which the wettability of a dielectric liquid droplet can be reversibly manipulated via a highly localized LDEP force at the three-phase contact line of the droplet. Although dielectrowetting is principally very different from electrowetting on dielectrics (EWOD), it has the capability to spread a dielectric droplet into a thin liquid film with the application of sufficiently high voltage, overcoming the contact-angle saturation encountered in EWOD. The strength of dielectrowetting depends on the ratio of the penetration depth of the electric field inside the dielectric liquid and the difference between the dielectric constants of the liquid and its ambient medium. Since the introduction of the dielectrowetting technique, significant progress in the field encompassing various real-life applications was demonstrated in recent decades. In this paper, we review and discuss the governing forces and basic principles of LDEP, the mechanism of interface localization of LDEP for dielectrowetting, related phenomenon, and their recent applications, with an outlook on the future research.

Facile silicone oil-coated hydrophobic surface for surface enhanced Raman spectroscopy of antibiotics.

Surface-enhanced Raman scattering (SERS) technique has emerged as a potentially powerful tool for the detection of trace amounts of environmental contamination and pollutants such as various antibiotics and their active metabolites in the surface aquatic ecosystem (drinking water). In this study, we report the detection method for ciprofloxacin and norfloxacin analytes, two largely used antibiotics in the world, at a very low detection concentration based on the enrichment and efficient delivery of analytes after the evaporation of the solvent on slippery-SERS substrates. The slippery-SERS substrates were fabricated in a very efficient and cost effective way by simply spin-coating the silicone oil onto the widely used glass slides followed by annealing. The analyte particles with gold nanorods (GNRs) were efficiently delivered to the active site by evaporating the aqueous solvent on the slippery surface via the suppression of the coffee ring effect caused by the smooth contraction motion of the base contact radius of the droplet without any pinning. Thus, the detection limits of ciprofloxacin and norfloxacin analytes were reduced to 0.01 ppm, which is the lowest limit of detection achieved by any SERS technique. Finally, this study suggests that the fabricated silicone oil-coated slippery surface and GNRs based combinational approach for the SERS detection technique might be a powerful strategy for the reliable detection of the aqueous pollutant analytes even at very low concentrations.

Tunable superoleophobicity via harnessing the surface chemistry of UV responsive titania coatings.

Superoleophobic surfaces exhibiting tunable wettability are prepared by the combination of simple spray coating of Ultra Violet (UV) responsive titania nanoparticles and a low surface energy coating of a self-assembled monolayer (SAM) of 1H,1H,2H,2H-perflurodecyltrichlorosilane (PFDTS). Spray coating creates random micron-sized roughness with reentrant geometry, a necessary requirement for the superoleophobic surface, and a porous network at the nanometer size level, confirmed by the field emission scanning electron microscope (FE-SEM) images. By employing the rough surface and a low surface energy monolayer, the substrates possess superhydrophobicity with a water (γ = 72 mN m-1) contact angle of 163° and superoleophobicity with a decane (γ = 23 mN m-1) contact angle of 144°. Wettability of these surfaces is completely reversed to the superoleophilic state upon 6 h of UV irradiation. A quantitative X-ray photoelectron spectroscopy (XPS) analysis has confirmed the mechanism of decomposition of PFDTS molecules on the superoleophilic surfaces via interaction with the defect Ti3+ states of titania upon UV exposure. Furthermore, the superoleophobicity is restored to complete the transition cycle by changing the surface chemistry of the UV exposed surface via annealing and regrafting of the PFDTS monolayer.