Frequency-dependent Transient Response of an Oscillating Electrically Actuated Droplet

The transient response of a millimeter-sized sessile droplet under electrical actuation is experimentally investigated. Under DC actuation, the droplet spreading rate increases as the applied voltage is increased due to the higher electrical forces induced. At sufficiently high DC voltages, competition between the electrical actuation force, droplet inertia, the retarding surface tension force and contact line friction leads to droplet oscillation. The time scale for the droplet to attain its maximum wetted diameter during step actuation is analyzed. Systematic experiments are conducted over a frequency range of 5 – 200 Hz and actuation voltages of 40 – 80 Vrms to determine the dependence of droplet oscillation on these parameters. The response of the droplet to different actuation frequencies and voltages is determined in terms of its contact angle and contact radius variation. The frequency of the driving force (equal to twice the frequency of the applied electrical signal) determines the mode of oscillation of the droplet which, together with its resonance characteristics, governs whether the droplet contact angle and contact radius vary in phase or out of phase with each other. In addition to the primary frequency response at the electrical forcing frequency, the droplet oscillation exhibits sub-harmonic oscillation at half the forcing frequency that is attributed to the parametric nature of the electrical force acting at the triple contact line of the droplet.