A team of North Carolina State University (NCSU) researchers have created a reconfigurable liquid metal antenna controlled only by voltage. Liquid metal electronics have held the interest of the scientific community for years, but previous to this discovery these devices were not readily integrated into electronic systems because they required external pumps. This discovery advances the technology past this significant drawback.
Recently coauthor Michael Dickey's team within the NCSU Department of Chemical and Biomolecular Engineering found that they could cause the liquid metal to spread and contract by applying either positive or negative voltage. The team achieved this by bridging the interface between an electrolyte and the liquid metal with an electrical potential. It was this observation that inspired this latest discovery.
An antenna's critical properties like radiation pattern and operating frequency are determined by the length and shape of the conducting paths that create it. "Using a liquid metal-such as eutectic gallium and indium-that can change its shape allows us to modify antenna properties more dramatically than is possible with a fixed conductor," explained coauthor Jacob Adams, an assistant professor in NCSU's Department of Electrical and Computer Engineering.
The team used electrochemical reactions to change the antenna's operating frequency and elongate and shorten a filament of liquid metal to create the tunable antenna controlled by voltage only. To create a flow of metal into a capillary, the team applied a small amount of positive voltage. The corollary was also true; applying a small negative voltage causes the metal to withdraw from the capillary.
Adams explains that the direction of metal flow is dictated by differences in surface tension; positive voltage, "electrochemically deposits an oxide on the surface of the metal that lowers the surface tension, while a negative potential removes the oxide to increase the surface tension."
This allows the team to "remove or regenerate enough of the 'oxide skin' with an applied voltage to make the liquid metal flow into or out of the capillary. We call this 'electrochemically controlled capillarity,' which is much like an electrochemical pump for the liquid metal," Adams says.
The liquid metal technique greatly improves the range for the antenna's operating frequency. "Our antenna prototype using liquid metal can tune over a range of at least two times greater than systems using electronic switches."
The ability to use electrochemical signals to switch the antenna on and off from different wavelengths means that there are many potential applications, especially for mobile devices. Devices in our homes and everywhere else can now use these antennas, which have more than twice as much range.
"Mobile device sizes are continuing to shrink and the burgeoning Internet of Things will likely create an enormous demand for small wireless systems," Adams says. "And as the number of services that a device must be capable of supporting grows, so too will the number of frequency bands over which the antenna and RF front-end must operate. This combination will create a real antenna design challenge for mobile systems because antenna size and operating bandwidth tend to be conflicting tradeoffs."
Current antennas can't be miniaturized below a certain size and length because they contain fixed conductors. These fixed conductors can only be switched off and on between differently sized conductors. The well-known "death grip" dropped call issue with the iPhone 4 will be eliminated, because holding the phone by its bottom won't affect the improved, much smaller antenna inside. Liquid metal systems "yield a larger range of tuning than conventional reconfigurable antennas, and the same approach can be applied to other components such as tunable filters."
The researchers now want to gain more control over the liquid metal's shape. By starting to explore the tunable liquid metal's applied and fundamental elements they may one day soon be able to create almost any antenna shape they need. "There's still much to learn about the behavior of the surface oxides and their effect on the surface tension of the metal," Adams says. "And we're studying ways to further improve the efficiency and speed of reconfiguration."
"This would enable enormous flexibility in the electromagnetic properties of the antenna and allow a single adaptive antenna to perform many functions."
The article, "A reconfigurable liquid metal antenna driven by electrochemically controlled capillarity," is authored by M. Wang, C. Trlica, M.R. Khan, M.D. Dickey and J.J. Adams. It was published in the Journal of Applied Physics on May 19, 2015 and can be accessed here.
