An electro wetting on dielectrics - system utilizing two different dielectric layers
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Electrowetting on dielectrics is an established actuation principle in digital microfluidics. The Lippmann-Young equation describes the dependence of the contact angle to applied voltages. At high voltages the changes of the contact angle diminish and the contact angle saturates. Several theories try to explain that phenomenon. Amongst others, trapped charges and leak currents were proposed as the mechanisms behind the contact angle saturation. In this paper, we will combine these two theories to a common one and show that leak current and dielectric breakdown causes trapped charges and leads to a deviation of the Lippmann-Young equation and to a non-constant butoscillating contact angle atincreasing voltages. The thermodynamic derivation takes into account the electrostatic energy of a perfect capacitor, consisting of a homogeneous or a layered dielectric isolation. In our contribution, we show that isolation layers consisting of real materials with non-zero conductivities and, especially in the case of dielectric breakdown in one of these layers, display electrostatic energies different from that of an ideal capacitor. Thus, the contribution to the surface energies changes and the wetting contact angle differs from the Lippmann-Young prediction. It turns out, that the change of the surface energy of a solid to liquid interface differs depending on the order of the applied dielectric layers.