Discrete Contact Angles and Electric Field Singularity in Electrowetting: A Multi-Scale Complex Potential Analysis

Abstract

This study constructed a multi-scale theoretical framework to resolve the electric field singularity at the Triple Contact Point in electrowetting. Utilizing conformal transformation and complex analysis, we established the structure for both the global potential and local field solutions, complementing the analysis with numerical methods. Our primary finding is that the contact angle $\theta$ is not continuously adjustable but is restricted to a discrete set of values, constrained by the characteristic exponent $\lambda$. Analysis of the complex potential established $\text{Re}[\lambda] \ge 1$ as the critical condition for a non-singular electric field; conversely, singular solutions ($\text{Re}[\lambda] < 1$) are localized exclusively in the acute-angle regime ($\theta < \pi/2$). The high-order solution region exhibits a degeneracy phenomenon at specific angles, implying the local field structure is geometrically stable and universally applicable for a wide range of permittivity ratios $k$. Furthermore, we determined that the onset of electric field oscillation requires the simultaneous satisfaction of two critical conditions: the geometry must approach a flat boundary ($\theta \to \pi$) and the dielectric ratio must approach homogeneity ($k \to 1$). These findings provide a solid theoretical basis for designing non-singular electric fields and mitigating the common contact angle saturation phenomenon.