Electrowetting: Contact angle saturation induced by non-local surface excess free energy

Contact-angle saturation limits the high-voltage response of electrowetting-on-dielectric systems, but it is not captured by the classical Young-Lippmann relation. Here, we develop a composition-dependent interfacial energy-landscape model in which the liquid-covered and gas-covered parts of the substrate are described by adsorption-layer states with composition-dependent free energies and capacitive responses. The Young-Lippmann behavior is recovered at low voltage, while saturation emerges when the gas-covered adsorption layer becomes liquid-rich and the electrical contributions to the solid-liquid and solid-gas interfacial free energies decrease synchronously. The model provides analytical expressions for the onset voltage, saturation voltage, and saturated contact angle and separates the voltage scale from the terminal interfacial-energy contrast. Comparison with experimental data shows that the normalized electrowetting response is consistent with the proposed interfacial transition pathway. The resulting energy landscape can also contain metastable states, giving rise to path-dependent behavior during voltage withdrawal. Together, these results indicate that variations in the underlying interfacial parameters can regulate both the terminal wetting state and the overall electrowetting response.