Read out Wake-up is treacherous, because it's easy to spot from the air. Some military ships even pull tows of up to one kilometer behind them. Now, the US Navy has patented a technique that makes the wake almost invisible: Acoustic wave nets catch small bubbles in the water, reports the New Scientist.

"If there are no bubbles in the wake, you can not see it, " explains Robert Kuklinski of the Naval Undersea Warfare Center in Rhode Island, who developed the new technology. As a ship moves through the waves, it "plows" air bubbles underneath the water surface, which are swirled in the strong circular currents of the ship's propeller. Large bubbles then rise relatively quickly to the surface, reports Kulinski. The small bubbles, however, keep relatively long in the water. The light breaks in these bubbles: the wake becomes visible.

Kuklinski's technique relies on bringing all the bubbles back to the surface directly behind the ship. For this he sends acoustic waves of one megahertz into the water, which interfere with each other and form a three-dimensional network of different pressure zones. High-pressure areas press the tiny air bubbles of approximately 0.2 millimeters in diameter into low-pressure pockets. There they unite to larger bubbles, with about 1.5 millimeters in diameter. Such bubbles have enough buoyancy to migrate to the water surface.

Kuklinski is now working to transfer his method from simulation to "real life": "My experiments in large water tanks have worked amazingly well. It took maybe a minute for the tank to clear. "The high frequency of the acoustic waves should be harmless to humans and marine animals.

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