  We theoretically study a vibrating pair of parallel electrodes bridged by a
(deformed) liquid droplet, which is a recently developed microfluidic device to
harvest vibrational energy. The device can operate with various liquids,
including liquid metals, electrolytes, as well as ionic liquids. We numerically
solve the Young-Laplace equation for all droplet shapes during a vibration
period, from which the time-dependent capacitance follows that serves as input
for an equivalent circuit model. We first investigate two existing energy
harvesters (with a constant and a vanishing bias potential), for which we
explain an open issue related to their optimal electrode separations, which is
as small as possible or as large as possible in the two cases, respectively.
Then we propose a new engine with a time-dependent bias voltage, with which the
harvested work and the power can be increased by orders of magnitude at low
vibration frequencies and by factors 2-5 at high frequencies, where frequencies
are to be compared to the inverse RC-time of the circuit.
