| Ultra low voltage electrowetting and beyond A.A.Kornyshev *, A.Kucernak *, C.W.Monroe *,#, M.Marinescu *, A.Sleightholme*,#, and M.Urbakh ** *Department of Chemistry, Imperial College London, SW7 2AZ UK #School of Chemistry, Tel Aviv University, Ramat Aviv, 69978 Israel **Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109-2136, USA Electrowetting – the use of electric fields to regulate the shape and dynamics of interfaces in contact with solids – has recently been applied in a number of current technologies, such as "liquid lenses" for mobile-phone cameras and microdisplays. These applications have reinvigorated the study of three-phase electrocapillarity. Working in the area of functionalized liquid-liquid interfaces our group has theoretically predicted and experimentally realized a novel electrowetting system, based on interfaces between two immiscible electrolytic solutions (ITIES). These interfaces are being used in a number of electrochemical applications, including phase-transfer catalysis and surface cleaning of metals, but their unique electrowetting properties have not been recognized before. We discovered a new, ITIES-based, electrowetting system with unique properties, which can change the shape of the lens at operating voltages, by less than ½Volt of variation of the electrode potential relative to the bulk, which is orders of magnitude smaller than those used in the existing electrowetting devices1-6. As we showed, ITIES are extremely well suited for electrowetting: applied potentials can be varied over a range of 1V, subject to electrochemical control, but this is sufficient to achieve contact angle variation, that is obtained in conventional electrowetting devices typically only at voltages >60 V; bulk and interfacial properties can be independently varied by adjusting the solution compositions, which gives additional opportunities for the modification of system properties. We presented several theoretical results that showed how charge is distributed within a droplet under an applied potential and how the contact angle between the droplet and a planar electrode depends on this potential. The theory hinges on two features that distinguish electrowetting with ITIES from other liquid/liquid configurations: both solutions screen charge and the droplet boundary is impermeable to ions. A linear theory has first been developed for small applied potentials1, later extended over a broader potential range.2-4 The latter rationalizes the puzzling contact-angle saturation phenomenon observed in many electrowetting systems. Experiments performed in our group5 using standard ITIES at sputtered gold electrode have qualitatively approved the theory. However, we detected a substantial contact angle hysteresis and pinning of the three-phase contact line on the atomically rough and, perhaps, energetically inhomogeneous sputtered-gold electrodes5. To overcome that difficulty, i.e. to make those ‘sticky’ surfaces ‘slippery’, we have used a method of microsecond voltage pulses, short enough not to start electrochemical reactions at the interface, but powerful enough to de-pin the contact line and move the shape of the surface, i.e. the contact line to its ‘final destination5. These experiments has revealed interesting electrowetting dynamics, that was worth of independent attention. We developed a successful theory of the system dynamic response to successive pulsing6, well reproducing experimental data, and rationalyzing the principles of the pulsing technique for hysteresis-free electrowetting devices on nonideal surfaces. This and several later developments will be overviewed at the conference. 1. C.W.Monroe, L.I.Daikhin, M.Urbakh, A.A.Kornyshev, “Principles of electrowetting with two immiscible electrolytic solutions,” J. Phys.: Condens. Matter 18 (2006) 2837–2869. 2. C. W. Monroe, L.I.Daikhin, M.Urbakh, A.A.Kornyshev, “Electrowetting with Electrolytes,” Phys. Rev. Lett. 97 (2006) 136102. 3. C.W. Monroe, M.Urbakh, A.A.Kornyshev. “The distinctive properties of electrowetting with ITIES”, J. Phys. Cond.Matter 19, #375113 (2007). 4. C.W. Monroe, M.Urbakh, A.A.Kornyshev, “Double-Layer Effects in Electrowetting with Two Conductive Liquids”, J. Electrochem. Soc. 156 , P21-P28 (2009). 5. A.A.Kornyshev, A.R.Kucernak, C.W.Monroe, A.E.S.Sleightholme, M.Urbakh, Ultra-low voltage electrowetting, J. Phys. Chem. C 114, 14885-14890 (2010). 6. M.Marinescu, T.Barnea, M.Urbakh and A.A Kornyshev, Electrowetting dynamics assisted by pulsing, J. Phys. Chem. C 114, 22558-22565 (2010). |
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