Superhydrophobicity

Superhydrophobicity is a surface property in which water is strongly repelled. In the source dossier, it is the property that makes an etched aluminum tube trap a stable air bubble and float. The dossier describes the mechanism in two parts: a hierarchical micro/nanoscale surface texture and a low surface energy that together cause the surface to "strongly repel water and remain dry."

How it is described in this source

The University of Rochester press item states that etching the interior of aluminum tubes creates micro- and nano-pits on the surface, which turns it superhydrophobic. When the treated tube enters water, the superhydrophobic surface traps a stable bubble of air inside the tube and prevents the tube from getting waterlogged and sinking. The press item compares this to the way diving bell spiders trap an air bubble to stay buoyant underwater, and to fire ants forming floating rafts with their hydrophobic bodies.

The dossier does not report a measured water contact angle, contact-angle hysteresis, or sliding angle for the tube surfaces. Those are the standard quantitative descriptors of superhydrophobicity, and their absence is a limitation of the material supplied here.

Related surface behavior in the same dossier

The same laboratory's patent US10876193B2 claims the inverse behavior — superwicking/superwetting (superhydrophilic) surfaces — using the same micro-rough and nano-rough feature scales. The coexistence of both wetting extremes in one laboratory's patent portfolio is a reminder that the surface texture alone does not determine wetting; surface chemistry and treatment conditions matter as well.

Related pages

• Air-Trapping Buoyancy (Plastron-Based Flotation) — the mechanism by which superhydrophobicity produces flotation in this source.
• Hierarchical Micro/Nanoscale Surface Texturing — the fabrication principle.
• water, chemistry, physics — parent topic areas.