Field polarity effects on single and dual-droplet transport and evaporation in electric fields

High-voltage electric fields provide a low-energy, non-contact means of manipulating droplet dynamics, yet a systematic comparison of direct current (DC) and alternating current (AC) excitation, particularly for interacting droplet pairs, remains limited. In this study, a validated two-dimensional phase-field model in COMSOL Multiphysics is employed to simulate single and paired water droplets (normalized volume V* = 0.2–1.0) under uniform vertical fields ranging from E = 0.1–1 kV/mm. The coupled Navier–Stokes and Maxwell equations are solved to quantify droplet displacement, coalescence, and evaporation, and the model is benchmarked against experimental data with deviations below 3 %. The results show that DC excitation produces stronger displacement, faster evaporation, and earlier, more sustained coalescence, while AC requires higher field strengths and yields only intermittent merging with weaker transport effects. Importantly, the study identifies polarity-dependent thresholds: under DC fields, coalescence initiates at E_init ≈ 0.62 kV/mm and completes at E_comp = 0.77 kV/mm (V*=1), whereas under AC fields, coalescence initiates at ≈ 0.92 kV/mm and does not complete within the tested range. This systematic mapping of polarity-dependent thresholds represents the principal novelty of the work and provides a framework for interpreting electrohydrodynamic droplet behavior. The findings offer practical guidance for applications in digital microfluidics, droplet transport, and surface cooling.