Aluminum sinks. Its density — 2.7 grams per cubic centimeter — is nearly three times that of water. A solid aluminum tube submerged in a tank does what every physics textbook predicts: it descends and stays down.
Chunlei Guo's group at the University of Rochester etched the interior surface of aluminum tubes with laser-produced micro- and nanoscale pits. The texture makes the surface superhydrophobic — water molecules that contact it experience stronger cohesion to each other than adhesion to the metal. When the treated tube enters water, its interior captures and holds a pocket of air. The air cannot escape because the surface won't let water displace it.
The tube floats. It floats indefinitely. It floats after weeks of submersion in rough conditions. It floats when punched full of holes — the superhydrophobic interior surface re-traps air at each puncture boundary. Multiple tubes connected into rafts maintain buoyancy under conditions that would sink conventional structures.
The mechanism inverts the usual engineering of buoyancy. A ship floats because its hull excludes water from an enclosed volume, making the average density of hull-plus-air lower than water. The aluminum tube floats because its surface excludes water at the molecular level — not by enclosure but by refusal. The air isn't sealed in. It's held by the boundary condition itself.
Diving bell spiders do the same thing. Fire ants form floating rafts using their water-repellent exoskeletons. The strategy is ancient: buoyancy through surface chemistry, not structural containment. What the aluminum tube demonstrates is that the distinction between floating and sinking can be a surface property rather than a bulk one.