Correlating hydrophobicity to surface chemistry for low-frequency vibration energy-harvesting applications

The relationship between hydrophobicity and surface chemistry is crucial for optimizing materials used in vibration energy harvesting (VEH) applications, where environmental resilience and charge transfer efficiency are essential. Aluminum alloys, commonly used in the automotive, aerospace, and energy sectors, naturally develop an oxide layer that offers limited corrosion resistance in humid and saline environments. A promising strategy to improve performance and durability consists of modifying the wettability of aluminum surfaces .

In this study, we produced highly hydrophobic aluminum surfaces using a one-step etching method, yielding microstructured roughening of the surfaces that facilitates air trapping, enhancing hydrophobic behaviour. Contact Angle Goniometry (CA), Scanning Electron Microscopy (SEM), and X-ray Photoelectron Spectroscopy (XPS) were employed to correlate surface wettability with sub-micrometer-scale morphology and chemical composition. We found that surface hydrophobicity is governed by the interplay between hierarchical micro/nanostructures and the chemical composition of the outermost layers. We used the optimized aluminum surfaces for a portable VEH device, specifically leveraging Reverse Electrowetting on Dielectric (REWoD) technology, that efficiently harvests energy from low-frequency vibrations (<10 Hz) typical of human motion. We realized the device using off-the-shelf polyacrylamide (PAAm) hydrogels loaded with saline solutions using a heat treatment that extends the hydrogel drying times significantly. The laboratory prototype generated an average power of ∼1.55 μW at 7 Hz, achieving a power density of 9 nW/μl.