Liquid Metal Can Now Be Controlled Between Reflective and Scattering States

In a step towards electrically controllable mirrors or switchable light, researchers can now dynamically switch liquid metal surfaces between reflective and scattering states.

A liquid metal, like the name suggests, combines most properties of metal - electrical, optical, thermal - with the fluidity of a liquid. In the new study that potentially turns these materials into controllable mirrors, researchers created an electrically driven chemical reaction that allows for the change in the reflectivity of the liquid metal.

Researchers led by Yuji Oki of Kyushu University report their findings in the report "Dynamic control of reflective/diffusive optical surfaces on EGaIn liquid metal," appearing in the latest Optical Society (OSA) journal. They worked with the research team led by Michael D. Dickey from the North Carolina State University.

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Switching Liquid Metal Surfaces

The new study shows that this impressive control of liquid metal surfaces is achievable with only 1.4 V or roughly the same voltage used to power up an LED. Furthermore, the new technology could be implemented under ambient temperature and pressure conditions.

"In the immediate future this technology could be used to create tools for entertainment and artistic expression that have never been available before," said Oki in a news release from OSA. He adds that more development could lead to using this technology in something similar to 3D-printed electronically controlled optics from liquid metal surfaces. Additionally, this technology could be used in light-based health testing devices.

In their new controllable mirrors, researchers first created a reservoir with an embedded flow channel. Then, they used a so-called "push-pull method" in order to create optical surfaces, which is created by either pumping gallium-based liquid metal into the reservoir or sucking it out of it. This process created either convex, flat, or convex surfaces, each having its own different optical properties.

When researchers applied electricity, a chemical reaction reversibly oxidizes the liquid metal. This chemical reaction changes the volume of the liquid metal surfaces, creating multiple "small scratches" on its surface that cause light to scatter. However, when electricity is applied in the opposite direction, the liquid metal returns to its previous state, rearranging the liquid metal surface in a way that its scratches disappear, leaving a clear reflective surface. This makes for electrically controllable mirrors.

Conducting Electrochemical and Optical Characterization

Researchers then conducted an electrochemical and optical characterization of the different liquid metal surfaces generated by the application of electricity. They discovered that by changing the voltage applied on the surface from -800 millivolts to +800 millivolts, they could decrease the light intensity as the liquid metal surfaces switch from being a reflective surface into a scattering one. Additionally, electrochemical tests showed that a voltage change of about 1.4V was enough to recreate reduction-oxidation processes.

"We also found that under certain conditions the surface can be slightly oxidized and still maintain a smooth reflective surface," adds Oki. He further explains that by controlling this behavior, it is possible to create a wider range of optical surfaces. Their approach could lead to different applications requiring more advanced devices like optical elements and biochemical devices.