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We show that the nonlinear piezoelectric response of polyvinylidene fluoride (PVDF) exhibits quite different behavior compared with the piezoceramics. Instead of increasing linearly with the stress amplitude Tac, d31 coefficient raises with Tac2. Furthermore, the nonlinear effect does not set in until the strain increases beyond 0.3%, indicating very strong barriers to the domain wall motions in PVDF. The electromechanical coupling factor k31 also exhibits an increase with stress. Furthermore, the piezopolymer can withstand more than 3% strain without degrading the piezoelectric responses.

Aquatic debris monitoring is of great importance to human health, aquatic habitats and water transport. In this paper, we first introduce the prototype of an aquatic sensor node equipped with an embedded camera sensor. Based on this sensing platform, we propose a fast and accurate debris detection algorithm. Our method is specifically designed based on compressive sensing theory to give full consideration to the unique challenges in aquatic environments, such as waves, swaying reflections, and tight energy budget. To upload debris images, we use an efficient sparse recovery algorithm in which only a few linear measurements need to be transmitted for image reconstruction. Besides, we implement the host software and test the debris detection algorithm on realistically deployed aquatic sensor nodes. The experimental results demonstrate that our approach is reliable and feasible for debris detection using camera sensors in aquatic environments.

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