Liquid metals take shape

Ask people what comes to mind when they hear the term “liquid metal,” and many of them say mercury—or they recall a particular shape-shifting villain from the “Terminator” movies.

Not so for Michael D. Dickey.

It’s not that the North Carolina State University (NCSU) chemical engineer has a gripe with element number 80 or with the Arnold Schwarzenegger films. In fact, he enjoyed the action series and even made his mother sit through the one that introduced the android assassin made from a fictitious “mimetic polyalloy.”

Rather, “liquid metal” makes Dickey think of real metals—mainly gallium and a few of its alloys. Although gallium needs to be heated slightly to melt, these alloys melt below room temperature. The NCSU researcher is one of a small but growing number of scientists exploring the unusual properties of these materials, which they say could be useful for making flexible, stretchable electronics.

Stretchy circuits embedded in comfortable contour-hugging athletic wear or implanted in the body could lead to new applications in sports, medicine, and robotics. Liquid metals are also being studied for use in other areas including microfluidic devices.

To hear Dickey and other liquid metal researchers describe the properties of gallium and its alloys, mainly ones containing indium and tin, you’d think that the description was lifted from a sci-fi screenplay.

With a melting point of around 30 °C, gallium can liquefy in your hand, Dickey says, yet it’s difficult to freeze because it supercools. That means that as the temperature of a gallium droplet slowly falls below its melting point, not much happens to the liquid metal—if left unperturbed. But if you poke it, the liquid quickly solidifies because your poke creates crystal nucleation sites, he says. The same holds true for the alloys.

Other sci-fi properties of gallium include expanding when it freezes—something most metals don’t do—and remaining liquid over a huge temperature range—more than 2,000 °C. As a result, the metal has nearly zero vapor pressure, which means it doesn’t evaporate even under vacuum. That behavior is usually associated with thick, viscous liquids. Yet at room temperature, gallium’s viscosity is only about twice that of water.

But gallium’s most striking behavior stems from its surface chemistry. On exposure to air, gallium and its alloys, which aren’t toxic like mercury, spontaneously form a nanometers-thin oxide skin mainly composed of Ga₂O₃. The skin mechanically stabilizes the liquid metals, enabling researchers to use a syringe or nozzle to draw arbitrary, freestanding patterns that retain their shape and remain in place in a way that would be impossible for ordinary liquids. If the pattern is jostled, the skin breaks and the metal flows momentarily until the skin re-forms around the liquid.

In some ways, the patterning process is like using a frosting dispenser to decorate a cake with complex three-dimensional designs—except that the frosting in this case is metallic. As such, the design is electrically conductive, which is what enables liquid metals to be used in circuitry.

Pastry chefs learn from trial and error which dispenser tip to use, how fast or slow to dispense the frosting, and how to control other parameters to get the best cake-decorating results. Liquid metal researchers, such as Purdue University mechanical engineer Rebecca K. Kramer, learn how to pattern with gallium alloys in much the same way.

Two years ago, Kramer’s group worked out some of the key relationships among nozzle diameter, liquid metal flow rate, tip-to-surface distance, and other variables in a study focused on an alloy known as eGaIn (pronounced “e-gain”). The material is a eutectic mixture of gallium and indium in a 3-to-1 ratio by weight with a melting point of 15.5 °C, several degrees below room temperature.

By using a pump system and stationary tip together with a computer-controlled motorized stage, the team drew patterns of eGaIn on glass and polydimethylsiloxane (PDMS). Then, the researchers used microscopy to examine the interface between the naturally forming oxide shell and the substrate surfaces. They found that the patterns were freestanding and stable because the oxide’s tendency to adhere to the substrate is stronger than the alloy’s drive to wet, or spread across, it. From there, they optimized their printing parameters.

The group also showed that the setup could be used to form functioning circuits known as strain gauges. These devices measure the extent to which a material is deformed by monitoring the decrease in electrical conductance caused by deformation. The team encapsulated eGaIn-based strain gauges in the polymer PDMS and showed that as the PDMS and circuit components were stretched, the electrical signal tracked the deformation (Adv. Funct. Mater. 2014, DOI: 10.1002/adfm.201303220).

Not only does the oxide skin on gallium-based liquid metals give the materials the ability to hold a shape, it also enables them to be reconfigured into new shapes—even extreme ones—which remain stable, all while maintaining electrical conductance. That bit of magic happens because when a liquid metal droplet is stretched, the oxide skin breaks, allowing the metal to briefly flow before the skin re-forms.

Dickey’s group took advantage of that property to devise a room-temperature process for making custom wires on the fly. The team squirts a few tens of microliters of eGaIn onto a stretchy polymer. Thanks to the adhesion between the two materials, as the researchers then stretch the polymer, the liquid metal stretches with it.

The process, which does not require special equipment or much force, can produce wires with diameters as small as 10 μm in a variety of shapes. The wires can remain encased in polymers that are flexible or rigid, depending on chemical treatment, or they can be exposed and yet remain stable and freestanding because of the oxide shell (Extreme Mech. Lett. 2016, DOI: 10.1016/j.eml.2016.03.010).

Dickey notes that the wires could be used for stretchable electronics or made as needed for repairing electrical connections.“Our goal is not to replace conventional cabling,” he says, “but rather to provide an alternative method for making conductive traces that takes advantage of the unique properties of liquid metal.”

Several researchers are also exploiting the surface oxide on gallium-based liquid metals to convert nonconducting polymers to conducting ones. Established methods for causing that transformation rely on doping polymers with carbon black, carbon nanotubes, or metal particles. But those additives can make polymers stiff or brittle. Adding liquid metals, in contrast, enables the plastic films to stretch—and maintain conductance while doing so.

Playing with this attribute, Carnegie Mellon University (CMU) mechanical engineer Carmel Majidi took it to an extreme. In a recent study, Majidi and teammate Andrew Fassler manually mixed a thin piece of PDMS and a gallium-indium-tin alloy known as galinstan (68.5% Ga, 21.5% In, 10% Sn, by weight) until the metal droplets were reduced to micrometer size and embedded in the polymer. At that point, the composite was nonconducting. But by drawing on it with a ballpoint pen or other fine tip, the CMU researchers caused the embedded droplets to flow and coalesce. The writing process formed conductive patterns, but only where the researchers pressed the tip into the polymer.

In a demonstration, the team showed that conductive traces made via the fast and simple method remain highly conductive even after repeatedly stretching the polymer. Other materials studied for use in flexible electronics, such as conductive inks made from solid metal particles, cannot withstand repeated stretching. The CMU team also showed that light-emitting diodes (LEDs) attached along the conductive traces shone brightly when the group supplied power to the patterned material (Adv. Mater. 2015, DOI: 10.1002/adma.201405256).

To have a good shot at being scaled to commercial levels, fabrication methods involving gallium alloys should be automated, for example via ink-jet printing. But liquid metals are not suitable for ink-jet printing. They tend to be corrosive to other metals, and the surface oxide can easily clog ink-jet nozzles, Purdue’s Kramer says.

One way around that problem is to form dispersions of liquid metal nanoparticles by sonicating the alloy in a volatile solvent such as ethanol. In the dispersed form, liquid metals can be ink-jet printed easily because the dispersion properties are dictated by the solvent rather than the liquid metal. Kramer’s group used that approach to pattern stretchy nitrile gloves with arrays of strain gauges and associated circuit elements made from eGaIn nanoparticles that coalesce during printing, forming electrically conductive patterns (Adv. Mater. 2015, DOI: 10.1002/adma.201404790).

Most of the research on liquid metals focuses on eGaIn and galinstan and their use in stretchable electronics. But those aren’t the only liquid metals or applications being studied. A research group led by Stéphanie P. Lacour of the Swiss Federal Institute of Technology (ETH), Lausanne, makes stretchable electronics from hybrid solid-liquid thin metal films that include gold.

Unlike most groups, which use liquid deposition methods, Lacour’s group uses vapor deposition techniques to keep the procedures compatible with standard semiconductor fabrication. To make the films, the researchers sputter gold onto PDMS and evaporate gallium onto the gold. The result is a stretchy two-phase film consisting of a continuous network of solid AuGa₂ clusters interspersed with liquid gallium and microscopic regions of stand-alone liquid gallium droplets.

The ETH Lausanne team used the biphasic material to fashion stretchable complex electronics, including devices with stacked layers of LEDs and wearable sensors that tracked the subtle motions of fingers (Adv. Mater. 2016, DOI: 10.1002/adma.201506234).

But stretchable electronics aren’t the only application researchers are eyeing for liquid metals. After focusing on stretchable electronics for the past several years, Dickey’s group recently turned to microfluidics. Instead of incorporating liquid metal into a product to give it special properties, as is done in electronics research, Dickey’s group uses eGaIn as a synthesis template, then gets rid of it.

Specifically, they form complex 3-D patterns with eGaIn, coat them with monomer solution, and let it polymerize, thus forming a shell around the patterns. Then the researchers use an acid or electrochemical treatment to withdraw and recover the eGaIn, leaving behind empty microfluidic channels. The technique, which is compatible with 3-D printing technology but would be difficult to do with most other 3-D-printable materials, can be used to integrate multilayer electronic devices and replicate the microvasculature found in living organisms (Lab Chip 2016, DOI: 10.1039/c6lc00198j).

Liquid metal researchers know the joys and challenges of studying these not-very-well-known materials. “The beauty of working with them is that they’re incredibly versatile,” CMU’s Majidi says. “You can coax them into forming all sorts of useful shapes, patterns, and geometries.” But they have shortcomings. “They can be difficult to process,” Kramer stresses. “The combination of their surface tension, viscosity, and density makes them incompatible with most standard liquid-processing techniques.”

Nonetheless, their popularity has been growing steeply in recent years as more and more researchers get into the game. Dickey predicts that trend will continue. “When I give talks on liquid metals, people are enthusiastic and full of questions. But when I put a polymer-liquid metal sample in their hands and let them play with it, they are completely blown away.”