Dynamics of droplet spreading on a flexible substrate under electrowetting-on-dielectric

This study systematically investigates the electrowetting-on-dielectric (EWOD)-induced spreading dynamics of water droplets on flexible polydimethylsiloxane dielectric substrates with varying thicknesses, flexibility, and surface modifications. By conducting experimental and theoretical analysis, the influential factors (applied voltage, dielectric layer thickness, substrate flexibility, and surface wettability) that regulate the transition between intermittent (overdamped) and continuous (underdamped) droplet spreading modes were elucidated. The experimental results demonstrate that increasing voltage accelerates droplet spreading and excites capillary waves; thicker dielectric films reduce interfacial capacitance and enhance pinning, thereby elevating the onset and critical voltages. Substrate flexibility decisively modifies droplet dynamics: softer substrates exhibit stronger hysteresis, with the dissipation of partial interfacial wave energy and suppression of capillary oscillations, resulting in higher onset and critical voltages. Hydrophobic Teflon coatings decrease surface energy and hysteresis, facilitating depinning and lowering the critical voltage. Under high-voltage conditions (≥175 V), droplets on all tested substrates consistently follow a universal four-stage evolution—rapid spreading, capillary wave-induced pinning, recovery spreading, and final stabilization. This highlights the robustness of the spreading dynamics under strong electrowetting forces. A theoretical model based on Newton's second law was developed to predict the onset and critical voltages, and the predictions showed good agreement with experimental results. These findings provide a meaningful perspective for investigating EWOD-driven droplet dynamics on flexible substrates and offer new insights for the design of microfluidic, thermal management, and flexible electronic systems.