Physics World - a pair of physicists in the US have proposed a new type of material that lets water flow around an object as if it were not there at all. The design, which has yet to be built, could boost the energy efficiency of ships and submarines – and even prevent them from being detected. "
Urzhumov has a couple of ideas in mind. One is a piezoelectric pump, which consists of a small crystal that bends when a voltage is applied across it. Another is an electro-osmotic pump, in which a voltage across a membrane creates a pressure difference, forcing water through. "Electro-osmotic micro-pumps have a much lower flow rate, so they may [only] be used to build a proof-of-principle, scaled-down, slow-moving prototype," Urzhumov says. "Piezoelectric micro-pumps are the most likely candidates."
If Urzhumov and Smith's fluid-flow cloak were built, the researchers predict that one advantage would be efficiency. As a vessel moves, it drags nearby water with it, displacing more mass than it strictly has to. On the other hand, if the vessel were propelled only by the active metamaterial, then it would displace only the minimum water necessary.
Another advantage is silence: the turbulent wake of a vessel is where a lot of its acoustic noise is generated. By killing the wake, the metamaterial should make a vessel quieter. "Acoustic noise is definitely used by defence [agencies] for detection purposes," says Urzhumov.
Sebastien Guenneau, a physicist at Liverpool University who helped develop the water-wave cloak in 2008, says the fluid-flow cloak could have "tremendous potential applications in aeronautics", reducing the disturbed flow around boats, submarines and even aircraft. "There are obvious applications in civil engineering, but I guess the military would be interested too," he adds.
Urzhumov estimates that the cloak on a 10-centimeter-wide sphere could be anywhere from 1 centimeter to 10 centimeters thick. "Generally, thicker cloaks are easier to fabricate, but they also weigh more, so it's a tradeoff that engineers will decide on," he says.
At the moment, Urzhumov is setting his sights a bit lower. In the model described in an upcoming issue of Physical Review Letters, the computer simulation studied a fully submerged, bullet-sized vessel that travels at crawling speed, just a few millimeters per second. Yet even this has applications as the
However, he and Smith suggest that a different sort of cloak, made specifically to reduce the drag rather than the entire wake, might be easier to make and could be scaled up to fuel-efficient dream boats.