superhydrofoba

Superhydrophobic, or superhydrofoba in some languages, refers to surfaces that strongly repel water. It is typically defined by a static water contact angle exceeding 150 degrees and a low sliding or roll-off angle, often less than 10 degrees. The effect arises from a combination of micro- to nanoscale surface roughness and a surrounding layer of low-surface-energy material.

Two wetting regimes, Wenzel and Cassie-Baxter, describe how water interacts with roughness. In Cassie-Baxter state, air pockets trapped beneath the droplet reduce solid contact, leading to high apparent contact angles and easy droplet roll-off. In practice, durable superhydrophobic surfaces aim to maintain the Cassie-Baxter state, though transitions to Wenzel state can occur under pressure, contamination, or wear.

Natural examples include lotus leaves and some insect wings, which exhibit a lotus-effect self-cleaning property: dirt is carried away by rolling droplets. Artificial versions combine texturing and coatings, either by creating micro- or nano-scale roughness through etching, lithography, or nanoparticle deposition, and applying low-surface-energy chemicals such as fluorinated silanes, PDMS, or fluoropolymers.

Applications span self-cleaning surfaces, anti-icing, anti-fouling, corrosion protection, and microfluidics. However, durability and environmental concerns are important: roughness can wear away, and many long-lasting coatings rely on fluorinated compounds with ecological impacts. Performance may also degrade when oils or dirt contaminate the surface, or under high pressure where water can penetrate the roughness.

Measurement typically uses the water contact angle and the sliding angle; a contact angle above 150° and a low tilt angle indicate superhydrophobic behavior.

Ongoing research seeks more durable, omniphobic and scalable solutions, with applications in textiles, automotive and aerospace components, and energy devices.

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