Dynamic Evolution and Scaling Law Deviation of Microdroplets in Millimeter-Scale Confined AC Electric Fields

In microfluidic chips, the geometric confinement arising from comparable droplet and electrode scales significantly distorts the local electric field, challenging the applicability of classical electrohydrodynamic theories. This study systematically investigates the dynamic evolution─deformation, oscillation, and breakup─of microdroplets within millimeter-scale confined AC electric fields. Crucially, experimental results reveal that the critical breakup field follows a scaling law of EC ∝ R–0.47, distinct from the classic Rayleigh prediction (R–0.5). This deviation is quantitatively attributed to the electric field distortion governed by the blockage ratio (χ). Based on this insight, a modified semiempirical prediction model is established, achieving high predictive accuracy (R2 = 0.997). Furthermore, this study investigates the effects of voltage, droplet size, surface tension, and viscosity. The mechanism of droplet oscillation is elucidated as being driven by a significant hydrodynamic phase lag (δ ≈ 80°) induced by high viscosity. This work not only reveals the failure mechanism of classical laws in confined geometries but also provides a robust theoretical foundation for the precise manipulation of droplets within microsystems.