Engineers from the University of Nebraska-Lincoln and China’s Xi’an Jiaotong University have dropped the mic and made a splash by designing a device with unparalleled sensitivity to the low-frequency sound waves that can propagate miles beneath the ocean’s surface.
The team developed its device by embedding a network of silver nanoparticles in hydrogel – a polymer-based material consisting of about 90 percent water – and sandwiching that gel between two electrodes. As reported in the journal Nature Communications, the resulting device measured roughly 6,000 times more sensitive to acoustic waves than did a state-of-the-art counterpart that features silver nanowires.
When picking up the low-frequency sound waves that persist even at great depths, the team’s device produced a signal roughly 30 decibels stronger than commercial counterparts, the study reported. Its nanoparticle network also allowed the device to effectively detect airflow and touch.
The design could bolster SONAR applications ranging from communication to navigation to defense, said co-author Li Tan, associate professor of mechanical and materials engineering at Nebraska.
Because water dissipates most of the electromagnetic waves that enable satellite-based GPS, submarines and other submersibles often rely on low-frequency bandwidths. But many existing underwater microphones are made from ceramics, Tan said, which generally absorb and register only about 20 percent of the low-frequency acoustic waves traveling through water. The other 80 percent reflects from a ceramic, resulting in a weaker signal.
“When you emit low-frequency sound, it may be from hundreds or thousands of miles away,” Tan said. “So when it arrives at a location, it’s already very weak. If you reflect 80 percent of it, there’s (almost) nothing to measure. We want something that has 100 percent reception.”
Those reflected waves can subsequently be picked up by others who might be listening, a problem for vessels designed with stealth in mind. The reduced signal also poses a challenge to the navigation and communication essential for underwater exploration, Tan said. According to the National Oceanic and Atmospheric Administration, humans have explored a mere five percent of the world’s oceans. A 2012 study estimated that the other 95 percent hides at least several hundred thousand undiscovered species.
Though the water content of the team’s hydrogel allows it to register most of the acoustic waves that strike it, its sensitivity also stems partly from a surface-level phenomenon known as an electrical double layer. This double layer of oppositely charged atoms naturally assembles between a conductive electrode on the hydrogel’s surface and sodium chloride dissolved into the hydrogel itself.
The two charged layers are themselves separated by a neutral layer of molecules just nanometers in width, a boundary much narrower than those found in traditional capacitors. This essentially gives the device extra room to build and hold an electric charge thousands of times larger than it otherwise could. Tan and his colleagues demonstrated the ability to tune this capacitance by adjusting the concentration of sodium chloride in the hydrogel.
But Tan and his colleagues faced a problem: Water and its cousin hydrogel respond only to pressures greater than those usually produced by sound waves. So the team devised a multistep process for embedding the hydrogel with a network of branching nanoparticles.
When the device received acoustic waves at a 90-degree angle, they compressed the hydrogel just enough to enlarge the distance between its branches and trap more of the electric charges conducted by electrodes on either side of the hydrogel. When acoustic waves instead struck the hydrogel at a more acute angle, the distance between branches closed and reduced its ability to hold the charges.
These variations make the device responsive to a relatively broad range of frequencies and sensitive to acoustic waves coming from multiple directions, Tan said. And the natural conductivity of the hydrogel – especially when compared with ceramics, which are insulators – helps the device more efficiently transform those waves into an electronic signal.
Tan led the study with Qin Zhou, assistant professor of mechanical and materials engineering at the University of Nebraska-Lincoln, alongside Yong Mei Chen and Yang Gao of Xi’an Jiaotong University. They were joined by Nebraska’s Jingfeng Song, postdoctoral researcher in physics and astronomy; Shumin Li, doctoral student in mechanical and materials engineering; Christian Elowsky, assistant professor of practice of agronomy and horticulture; You “Joe” Zhou, research professor of veterinary medicine and biomedical sciences; and Stephen Ducharme, professor of physics and astronomy.
The team’s research was supported in part by the National Science Foundation and the J.A. Woollam Co.