Continuous motion of an electrically actuated water droplet over a PDMS-coated surface

Electrically actuated linear motion of a water droplet over PDMS-coated single active electrode is analyzed from detailed experiments and modeling. In an experiment, continuous motion of the droplet is achieved when it is located over an active electrode with a horizontal ground wire placed just above in an open-electrowetting-on-dielectric configuration. Using a CCD camera, the instantaneous centroid position of the droplet is determined and its velocity is inferred by numerical differentiation. The edge-detected image is also used to determine the advancing and receding contact angles of the moving drop relative to the substrate. Motion of 2, 6, and 10 µl water droplets for voltages in the range of 170–270 V DC is examined to investigate the effect of drop volume and voltage on drop deformation and velocity. The motion of the droplet is initiated by Young-Lippmann spreading at the three-phase contact line, followed by a nonuniform electric force field distributed between the active electrode and the ground wire localized at the droplet-air interface. Simulations carried out using COMSOL© Multiphysics with full coupling between the electric field and hydrodynamics are in conformity with experiments. A contact angle model with pinning and friction leads to close agreement between simulations and drop motion over a bare PDMS layer, particularly in terms of the relevant timescales. When contact line friction is neglected, the fully coupled numerical solution shows a good match with experimentally determined drop movement over a silicone oil-coated PDMS layer. Over both surfaces, continuous motion of the water droplet is seen to be achieved in three stages, namely, initial spreading, acceleration, and attainment of constant speed.