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This paper presents detailed investigations on an electrostatic energy harvester using conductive droplet/marble rolling across a charged electret film. Both mercury droplets and ionic liquid marbles were used as the working medium. With a 1.2-mm mercury droplet rolling across the electret film of the prototype, a maximum output power was obtained at 0.18 μW and the peak value of the output voltage was 1.5 V. A semi-empirical model was developed to understand the output waveforms. Applied to the test data, it shows that the transducer short-circuit charge is mainly a function of droplet position in the pattern. Several factors influencing the output performance are discussed. Such droplet-based electrostatic energy harvesters are especially suitable for very low frequency vibration up to a few Hz.