AN ELECTRO WETTING ON DIELECTRICS - SYSTEM UTILIZING TWO DIFFERENT DIELECTRIC LAYERS MOTIVATION Contact angle saturation in EWOD systems is well documented. The typical two dielectric layer structure of EWOD experiments as well as physical behavior before and after the dielectric breakdown have to be considered EWOD SYSTEM Figure 1. a) An increase of the applied voltage (black line) increases the voltage in layer 1 (red line). b) The electric field can be unequal in the two different dielectric layers Contact angel θ(V): DIELECTRIC BREAKDOWN Figure 2. a) The voltage in layer 1 (red line) exceeds the breakdown limit V bd,1. Layer 1 becomes conducting and the total current density is limited be the dielectric properties of layer 2. The voltage across layer 1 drops to the termination level V 1,term. b) The norm of the electric field is highest at the edges. Therefore a dielectric breakdown is most likely to happen there. Contact angel θ(V): EWOD SATURATION Figure 3. a) The voltage in layer 1 (red line) is confined between the breakdown limit V bd,1.and the termination level V 1,term. b) Caused by the dielectric breakdown in one of the layers, charges are trapped at the interface of the layers. Figure 3. CONTACT ANGLE SATURATION Contact angle after dielectric breakdown: MODEL PREDICTIONS The saturation voltage can be different even if the hydrophobic surface and the fluid are the same the contact angle at the saturation voltage can be different even if the total capacitance is the same. the contact angle at saturation adopts is at a local minimum at saturation. A further increase of the voltage can lead to a larger contact angle. an increased voltage will lead to repeated breakdowns which lead to a contact angle oscillating over the voltage PUBLISHED EXPERIMENTS Fig. 4 Left: EWOD Experiment featuring a 1µm nano-composite dielectric layer consisting of 50vol% and 75vol% BaTiO3 for the black and the blue data points, respectively. Image source: Kilaru, M. K., Heikenfeld, J., Lin, G., and Mark, J. E. (2007). Strong charge trapping and bistable electrowetting on nanocomposite fluoropolymer: BaTiO3 dielectrics. Applied Physics Letters, 90 Right: EWOD Experiment featuring a 260 ±10nm Teflon AF1600 layer. Image source: Nanayakkara, Y.S., Moon, H., Payagala, Th., Wijeratne, A.B., Crank, J. A., Sharma, P. S. and Armstrong, D.W. (2008) Fundamental Study on Electrowetting by Traditional and Multifunctional Ionic Liquids: Possible Use in Electrowetting on Dielectric-Based Microfluidic Applications Anal. Chem., 80 (20), pp 7690–7698 NanayakkaraMoonPayagalaWijeratneCrankSharmaArmstrong CONCLUSIONS AND OUTLOOK  The predicted small oscillations of the contact angle at a further increase the voltage after the onset of the saturation effects are displayed in several publications  The proposed model explains the origin and the depths of trapped charges  The seeming limitation to a two layer system is not necessarily given. Due to non-uniform temperature distributions, regions with increased conductivity are more likely to experience a local dielectric breakdown.  To test the proposed model, all electric current densities and temperatures have to be measured during an EWOD experiment with several thickenesses of the same two dielectric layers Aknowledgements The authors gratefully acknowledge the financial support from the Austrian Center of Competence in Mechatronics (ACCM). Thomas Lederer 1, Bernhard Jakoby 1, Wolfgang Hilber 1 1 Institute for Microelectronics and Microsensors- Johannes Kepler University Linz, Austria