UC Berkeley

Biomimicry offers innovative sustainable solutions for many dire resource-based challenges. The Namib Desert beetle (sp. Stenocara) invites us to explore how we can collect fresh water more energy-efficiently. The beetle's unique back features with alternating hydrophobic-hydrophilic regions, aid its survival in a water scarce desert environment. We investigated the feasibility for enhanced condensation by patterning a zinc oxide (ZnO) surface to mimic the beetle's back. ZnO was selected as the material of choice given its non-toxicity and photoinduced hydrophilicity capability, combined with the ability to fabricate (super)hydrophobic ZnO surfaces with various thin film fabrication techniques.

The key innovation in our approach is developing a chemically homogeneous, non-insulating film on a metal substrate (practical engineering heat transfer material) that exhibits a hydrophobic-hydrophilic pattern. We successfully developed and implemented a scalable and practical technique to satisfy our main objective. Hydrophobic ZnO thin films were fabricated by electrodeposition on copper substrates and patterned with UV exposure to have both hydrophobic and hydrophilic regions. Even though statically the ZnO films were hydrophobic, they exhibited filmwise condensation instead of the more efficient dropwise condensation mode. This is attributed to the ZnO films being in a metastable Cassie-Baxter mode that transitioned to the wetting Wenzel mode during the dynamic and transient condensation state.