When electronics need to stretch, bend, or flex, like a wearable sensor or a shape-changing robot , liquid metal often carries the electricity that makes them work.
But researchers didn't fully understand how the metal conducted electricity through material, which made it difficult to control how electricity flowed in soft electronics.
"Our group and other researchers around the world have been using mechanical pressure to turn liquid metal composites into conductors for years, but we did not really understand what was happening inside the material," said Michael Bartlett, associate professor of mechanical engineering and John R. Jones III Faculty Fellow. "We wanted to uncover the physical mechanism that allows isolated liquid metal droplets to suddenly become an electrical network."
Now they have. Bartlett's latest project, connected to his NSF CAREER award , shows how liquid metal droplets embedded in soft materials connect to each other to form circuits, enabling soft materials to conduct electricity. The research, co-authored by Ph.D. candidates Wuzhou Zu and Hugh P. Grennan, was published in the prestigious journal Matter .
Why are liquid metals used in soft electronics?
Liquid metal composites have enormous potential for soft electronics because they combine the conductivity of a metal with the pliability of a soft material.
The team uses a metal alloy called eutectic gallium indium that is liquid at room temperature. At the microscopic level, it looks like a collection of spheres that resemble droplets. When the spheres touch, they merge to create pathways for electrical current to pass.
To contain those microscopic metal spheres in soft electronics, researchers use a material such as a polymer or rubber that holds the droplets in place. But that means the material that contains the metal droplets also gets in the spaces between them.
When that happens, how is electricity carried? How does the current move if the insulator material gets in the way?
How do liquid metals conduct electricity?
Bartlett's research team used microscopes and 3D X-ray imaging to observe what happened to the liquid metals under different circumstances. When they compressed the soft electronics, they got the answer they were looking for.
When pressed, the material between nearby metal droplets cracked, allowing the metal to flow between gaps. After the pressure was removed, metal remained in the cracks. This created the pathway through which electricity could flow, using the unique flexibility built into the material itself.
"We were able to watch the process happen in two dimensions and then reconstruct the pathways in three dimensions," Zu said. "That gave us a much clearer picture of how local cracks and liquid transport build into a larger conductive network."
The team also developed a mathematical model that predicts how much force is needed to make the material conductive. This prediction worked for many designs, from millimeter-sized droplets to droplets only a few dozen micrometers wide.
"By carefully observing what was going on during experiments, we could connect pathway formation to a fracture mechanics model," author Grennan said. "Now we can design liquid metal composites that activate at necessary forces and length scales for particular applications."
Where can liquid metals be used?
This work provides a new framework for designing how and when liquid metal composites become conductive, which could help manufacturers control where conductivity forms in stretchable sensors, wearable electronics, and soft robots.
"Knowing what governs how conductive networks form, we can use that knowledge to guide materials design and manufacturing," Bartlett said. "That opens the door to manufacturing soft electronic systems where conductivity is programmed in through the material architecture and the way the device is processed."
