Jumping drops on hydrophobic surfaces, controlling energy transfer by timed electric actuation.

Aqueous sessile drops are launched from a super-hydrophobic surface by electric actuation in an electrowetting configuration with a voltage pulse of variable duration. We show that the jump height, i.e. the amount of energy that is transferred from surface energy to the translational degree of freedom, depends not only on the applied voltage but also in a periodic manner on the duration of the actuation pulse. Specifically, we find that the jump height for a pulse of optimized duration is almost twice as high as the one obtained upon turning off the voltage after equilibration of the drop under electrowetting. Representing the drop by a simple oscillator, we establish a relation between the eigenfrequency of the drop and the optimum actuation time required for most efficient energy conversion. From a general perspective, our experiments illustrate a generic concept how timed actuation in combination with inertia can enhance the flexibility and efficiency of drop manipulation operations.