Thin 'Bubble' Coatings Could Hide Submarines from Sonar

Sailors aboard the USS Topeka (SSN 754) prepare the mooring lines as the submarine enters port on Nov. 24, 2004.

Bubble-filled rubbery coatings may one day help make submarines virtually undetectable to sonar, researchers say.

To avoid detection by sonar,military submarines are often covered with sound-absorbing tiles called anechoic coatings. These perforated rubber tiles are typically about 1 inch (2.5 centimeters) thick.

In the past decade, research has suggested that the same degree of stealth could be provided by much thinner coatings filled with vacant cavities. When hit by sound waves, empty spaces in an elastic material can oscillate in size, "so it will dissipate a lot of energy," said lead study author Valentin Leroy, a physicist at the Université Paris Diderot in France.

However, figuring out how to optimize such materials for stealth applications previously involved time-consuming simulations. To simplify the problem, Leroy and his colleagues modeled the empty spaces in the elastic material as spherical bubbles, with each giving off a springy response to a sound wave that depended on its size and the elasticity of the surrounding material. This simplification helped them derive an equation that could optimize the material's sound absorption to a given sound frequency.

The researchers designed a "bubble meta-screen," a soft layer of silicone rubber that is only 230 microns thick, which is a little more than twice the average width of a human hair. The bubbles inside were cylinders measuring 13 microns high and 24 microns wide, and separated from each other by 50 microns.

In underwater experiments, the scientists bombarded a meta-screen placed on a slab of steel with ultrasonic frequencies of sound. They found that the meta-screen dissipated more than 91 percent of the incoming sound energy and reflected less than 3 percent of the sound energy. For comparison, the bare steel block reflected 88 percent of the sound energy.

"We have a simple analytical expression whose predictions are in a very good agreement with numerical simulations and real experiments," Leroy told Live Science. "I find it exciting and beautiful." 

To make submarines invisible to the sound frequencies used in sonar, larger bubbles are needed. Still, the researchers predicted that a 0.16-inch-thick (4 millimeters) film with 0.08-inch (2 millimeters) bubbles could absorb more than 99 percent of the energy from sonar, cutting down reflected sound waves by more than 10,000-fold, or about 100 times better than was previously assumed possible.

However, despite the possibilities, "making these samples will probably be tough," Leroy cautioned.

The scientists detailed their findings online Jan. 6 in the journal Physical Review B.

HIV, Syphilis Tests? There's an App for That

There are gizmos that let your smartphone read credit cards, sync with your fitness wristband and even function as a TV remote control. Now you can add "run an HIV test" to the list.

The dongle connects to a smartphone via audiojack.

A device invented by biomedical engineers at Columbia University turns a smartphone into a lab that can test human blood for the virus that causes AIDS or the bacteria that cause syphilis. The device is a dongle that attaches to the headphone jack, and requires no separate batteries. An app on the phone reads the results.

The dongle contains a lab on a chip. It consists of a one-time-use cassette — which has tiny channels as thin as a human hair — and a pump, which is operated by a mechanical button and draws blood from an inlet through the channels.

Once the blood is inside the device, it meets chemicals that react with markers for HIV and syphilis. This kind of test is called an enzyme-linked immunosorbent assay (ELISA), and is considered one of the best methods for diagnosing diseases, said Samuel Sia, an associate professor of biomedical engineering at Columbia, who led the research. 

The blood changes the color and opacity of the chemicals (formally speaking, the solutions' optical depth changes). Then, LED lights shine through the mixture to a set of photocells, which read the change in the color and opacity and send the data to the app. The whole process takes 15 minutes.

The device requires little power because the pump is hand-activated — the person who wants to conduct the blood test presses a plunger to draw the blood. The current to run the LEDs comes from the phone's audio signal, according the researchers' report of their device, which is published today (Feb. 4) in the journal Science Translational Medicine.

The test results can be read by anyone with little prior training in lab technique necessary, the researchers said.

The researchers got the idea for the device when examining the costs and the logistical difficulties of getting equipment for HIV testing to rural areas or developing countries. Lab-on-a-chip devices have become more common in the last several years, but few are designed for use by people who don't have a lot of trianing, and the devices themselves tend to be expensive and customized.

"People [developing such devices] were not focused on usability," Sia said. "If you have a test that takes 20 steps and a laboratory staff, that's not going to make an impact on society."

Although sophisticated lab technology is scant in the developing world, smartphones are being adopted quickly. The research firm Informa UK projects that the number of smartphone connections, a close proxy for users, in Africa will grow to 204 million in 2015, from 154 million in 2014.

That kind of growth makes smartphones a natural target for the kind of technological development involved in the blood-testing device, the researchers said.