Abstract
The dynamic response of a polyvinylidene fluoride (PVDF) cantilever beam under excitation of water droplet impact is investigated by developing an electromechanical model. In the model, the governing equations of beam motion and output voltage are derived in the theoretical way, such that the voltage across the PVDF layer and the cantilever deflection can be predicted. The motion of the beam is described by the multi-mode vibration model through which more accurate results can be obtained. The predicted results of the model are validated by the experiment. Combined with the experiment and the model, the effect of surface wettability on droplet-substrate interaction mechanisms is investigated, which provides an insight into the improvement of mechanical-to-electrical energy conversion efficiency in raindrop energy harvesting (REH) applications. Results show: (1) the droplet splash on a super-hydrophobic beam surface has a positive effect on voltage generation. The splash limit that affects the reaction force of the impacting droplet is experimentally determined and greatly dominant by the Weber number. (2) Small-scaled droplets in splash regime allow generating higher voltage output from a super-hydrophobic beam surface than from an untreated hydrophilic beam surface. (3) Tests of successive droplet impacts also show that a super-hydrophobic surface performs better over a hydrophilic surface by producing constant peak voltage and higher electrical energy harvested. In this case, the voltage measured from the hydrophilic surface decreases gradually as the water layer is accumulated. Overall, the electromechanical behaviors of a super-hydrophobic PVDF cantilever sensor can be well predicted by the model which shows a great potential in energy harvesting by maximizing the inelastic collision upon droplet-substrate interactions.
Highlights
As the raindrop energy harvesting (REH) applications are being extensively concerned, harvesters based on piezoelectric beams have shown great potential in energy scavenging
Since the first model developed by Guigon et al [1,2] based on the shock theory combined with momentum conservation, droplet impact on piezoelectric beams is considered as an inelastic collision
The dynamic response of the simulated results obtained by the multi-mode model developed are compared and validated the by cantilever excited by impacting droplets can be systematically studied whereby the impact in-laboratory impact experiments
Summary
As the raindrop energy harvesting (REH) applications are being extensively concerned, harvesters based on piezoelectric beams have shown great potential in energy scavenging. To model the electromechanical behaviors of the harvester, most previous studies only considered the first-mode vibration of the beam where the lumped parameter approach were usually applied [28]. Following the Euler-Bernoulli assumption model proposed by Inman and ErturkIt [32,33], Doria et al [6,34] recently developed a multi-mode model for a novel PZT cantilever harvester equipped with a tip spoon It showed that the sequence of phenomena related to the impact on the liquid surface leads to more energy harvested and the simulated results fitted well with the experiments. A multi-mode electromechanical model of PVDF cantilever is developed following the distributed parameter approach It is validated by experiments for a wide range of droplet sizes (i.e., 2.4 mm~4.6 mm) and impact velocities (i.e., 0.9 m/s~3.4 m/s).
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