Texture and Wettability of Metallic Lotus Leaves (1511.01927v1)

Abstract: Superhydrophobic surfaces with the self-cleaning behavior of lotus leaves are sought for drag reduction and phase change heat transfer applications. These superrepellent surfaces have traditionally been fabricated by random or deterministic texturing of a hydrophobic material. Recently, superrepellent surfaces have also been made from hydrophilic materials, by deterministic texturing using photolithography, without low-surface energy coating. Here, we show that hydrophilic materials can also be made superrepellent to water by chemical texturing, a stochastic rather than deterministic process. These metallic surfaces are the first analog of lotus leaves, in terms of wettability, texture and repellency. A mechanistic model is also proposed to describe the influence of multiple tiers of roughness on wettability and repellency. This demonstrated ability to make hydrophilic materials superrepellent without deterministic structuring or additional coatings opens the way to large scale and robust manufacturing of superrepellent surfaces.

Texture and Wettability of Metallic Lotus Leaves

The paper "Texture and Wettability of Metallic Lotus Leaves" by C. Frankiewicz and D. Attinger, presents significant advancements in the creation of superrepellent surfaces using hydrophilic materials, specifically copper, through chemical texturing techniques. This work demonstrates a novel approach to fabricating surfaces analogous to natural lotus leaves, which possess remarkable water-repellent properties without requiring traditional low-surface-energy coatings.

Experimental Approach and Findings

The researchers employed chemical reactions to engineer copper surfaces with multiscale roughness to achieve superhydrophobic and superrepellent characteristics. The chemical processes involved the use of etchants like hydrogen peroxide and iron chloride combined with hydrochloric acid, followed by surface oxidation techniques. Three different surface treatments—E1, E2, and EA—were investigated using scanning electron microscopy to analyze the resulting multiscale textures, which mimic the roughness tiers seen in natural leaves such as rice, brassica, and lotus.

Critical measurements included contact angles and hysteresis angles, quantifying the surfaces' wettability and repellency. EA surfaces emerged as the most promising, achieving a remarkable static contact angle of 160° and hysteresis below 10°, indicative of strong repellency and low adhesion properties, similar to natural lotus leaves.

Mechanistic Insights

The mechanistic explanation suggests that multiple tiers of roughness contribute to the suspension of liquid atop these textures, preventing penetration into the surface roughness, which is essential for repellency. Using a theoretical model based on square waves, the researchers provided a quantitative framework to estimate the solid-liquid interaction parameters that define wettability.

Experiments demonstrated that the copper surfaces could resist water droplet penetration at impact velocities up to 2.1 m/s for EA, while natural leaves resisted even higher velocities. This indicates a replication of the ‘lotus effect’ on metallic surfaces, validating the necessity of corroborating models with experimental results.

Implications and Future Directions

The ability to fabricate superrepellent surfaces from hydrophilic materials such as copper using stochastic chemical texturing holds significant implications for industries requiring large-scale surface modifications, particularly in applications such as heat exchangers, computer heat sinks, and aerodynamic surfaces. The approach circumvents the limitations of previous deterministic techniques that were constrained by scalability and material restrictions.

Further research could explore optimizing chemical recipes for different materials, assessing the thermal and mechanical durability of these textured surfaces under operational conditions. Additionally, expansion into other metals or composite substrates could reveal new domains where superrepellent properties can enhance material performance.

Conclusion

This paper provides an insightful examination of the process of rendering hydrophilic materials superrepellent through chemical texturing. By advancing our understanding of multiscale roughness and its effects on wettability, Frankiewicz and Attinger open new avenues for the practical deployment of water-repellent technologies across diverse industrial applications. This step forwards promises robust, scalable methods for enhancing material surfaces, equipping them with protective and performance-enhancing features traditionally observed in natural biological systems.