The many faces of liquid gallium

APR 20, 2021

Controlling the material’s surface tension, oxide layer, and other properties leads to unique behaviors and applications.

DOI: 10.1063/PT.6.1.20210420a

Christine Middleton

4812/liquid-metal-fig-1.jpg — Collin Ladd

The periodic table hosts five metals that are liquid near room temperature. Although all are good conductors, working with them can be problematic. Francium is radioactive and has a half-life of just 22 minutes; rubidium and cesium are exceptionally reactive and prone to exploding or catching fire; and the most well-known, mercury, is famously toxic. That leaves just one liquid metal, gallium, for safe experimentation.

On page 30 of Physics Today‘s April 2021 issue, Michael Dickey of North Carolina State University notes that Ga’s propensity to form a surface oxide layer when exposed to oxygen has largely impeded its practical application. The layer prevents Ga from flowing freely and impedes electrochemical reactions. But as he goes on to explain, that layer can be viewed as an asset. Rather than thinking of Ga as a substitute for Hg, he and his research group have explored Ga’s inherent benefits. The videos below highlight just a few of their experimental discoveries.

To learn more about liquid Ga and its potential uses, see the article referenced above. And to see more videos of liquid Ga from the Dickey group, visit the team’s YouTube page .

Stretchable and self-healing electronics

Room-temperature liquid metals are ideal candidates for deformable electronics because they retain their conductive properties in the face of bending and stretching that would make normal metal wires unusable. The first video’s first clip demonstrates headphones with a stretchy wire. When Ga is encased in a polymer tube, the resulting wire’s stretchiness is limited only by that of the surrounding material. The metal can be recovered and reused by dissolving the polymer in a solvent. And if the polymer surrounding the liquid Ga is self-healing, then a cut wire can be repaired by simply pushing the two ends back together, as shown in the second clip.

Another way to make deformable conductors using liquid Ga is to disperse droplets of the metal in a liquid polymer such as PDMS; the video concludes with an example of that technique. If the material is used to fill a gap in a broken circuit, the metal droplets form chains that bridge the gap and restore conductivity to the circuit.

Clip 1 credit: S. Zhu et al., Adv. Func. Mater. 23, 2308 (2013); clip 2 credit: E. Palleau et al., Adv. Mater. 25, 1589 (2013); clip 3 and 4 credit: F. Krisnadi et al., Sci. Adv. 32, 2001642 (2020)

Tactile sensors

As with any conductor, a liquid-metal wire’s resistance depends on its geometry. But unlike solid wires, liquid metal ones can change shape when they’re surrounded by soft materials. Pushing on a liquid Ga conductor embedded in silicone changes the conductor’s shape and resistance and alters its local resistive heating.

If thermochromic pigments are incorporated into the silicone, as shown in this video, the heating corresponds to a change in color—or even switching among many colors if multiple pigments are used. The color change can even serve as a visual strain sensor that provides a warning when a stretched material is nearing its breaking point.

Video credit: Y. Jin et al., Nat. Commun. 10, 4187 (2019)

Surface tension and oxide layers

Liquid Ga has a high surface tension—nearly 10 times that of water. So when a droplet of the material breaks up, as shown in the next video’s first clip, it quickly re-forms. But applying a positive voltage causes an oxide layer to grow on the surface, effectively lowering the surface tension. The result: Falling liquid Ga that initially breaks into droplets suddenly holds together in a stream.

Applying a positive voltage to a droplet on a surface causes it to spread out. Under certain conditions it forms fingerlike protrusions and even fractals. A negative voltage reverses the oxide-layer growth and returns a flattened droplet to its spherical shape. Quickly doing so makes the droplet jump!

Clips 1 and 3 credit: C. E. Eaker et al., Phys. Rev. Lett. 119, 174502 (2017); clip 2 credit: M. Song et al., Proc. Natl. Acad. Sci. USA 117, 19026 (2020); clip 4 credit: M. Song et al., Appl. Phys. Lett. 118, 081601 (2021)

Micro and not-so-micro fluidics

A shell-like surface oxide layer forms on liquid Ga when it comes in contact with air, which allows three-dimensional liquid structures to hold their shapes. Researchers can exploit that feature to create microfluidic channels with almost arbitrary geometries, such as the spiral shown in this video. The channels drawn in liquid Ga are subsequently encased by a liquid polymer such as PDMS. Once the polymer is cured, the Ga can be pushed out to leave empty channels that are just a few hundred microns wide.

Applying a voltage across certain paths in a microfluidic system directs liquid Ga down selective paths. A positive voltage lowers the material’s surface tension and makes it easier for the material to flow; a negative voltage does the opposite.

Although liquid Ga’s surface oxide layer can bind to surfaces, that sticking can be prevented by first coating the surface with water. For example, at the end of the next video, water forms a slip layer that allows the metal to slide effortlessly through a pre-wetted tube in a pistonlike motion.

Clip 1 credit: video by Collin Ladd; C. Ladd et al., Adv. Mater. 25, 5081 (2013); clip 2 credit: D. Parekh et al., Lab Chip 16, 1812 (2016); clip 3 credit: S.-Y. Tang et al., Lab Chip 15, 3905 (2015); video copyright S.-Y. Tang, Y. Lin, I. Joshipura, K. Khoshmanesh, Michael Dickey, CC BY-NC 4.0; clip 4 credit: M. R. Khan et al., Appl. Mater. Interf. 6, 22467 (2014)