Liquid metal controlled by low voltage reversibly switches color and infrared response in nanoscale surfaces, enabling low power reflective displays, sensors, and optical security features.
(Nanowerk Spotlight) The moment sunlight hits your phone screen, the image dulls and your eyes strain. The device is still on and still bright, yet the display loses to the sky. The pixels have no way to push back against all that ambient light. They can only shine harder, draining the battery and still falling short. Anything that works by emitting its own light has this problem, from phones to outdoor signs to augmented reality glasses.
The obvious alternative is a surface that reflects and filters light instead of generating it. Nature already does this with butterfly wings and beetle shells, where tiny structural features bend and scatter light into vivid structural color without any pigments or power source. Engineers learned to copy that idea using nanoscale patterns etched into metals and films. These patterns can shape color with great precision and never fade, but once they are made, they cannot be changed. A painted wall can be repainted. A reflective nanostructure cannot be rewritten without starting over.
Other attempts at dynamic color control came from liquid crystals, electrochromic coatings, or materials that switch phase under heat. Each method works, but only by accepting limits in speed, brightness, durability, or energy cost. The common missing function across all of them is a simple, reversible way to make and remove a reflective layer just beneath the surface, close enough to affect light but without needing moving parts or high voltages.
Overview of the reconfigurable liquid metal (LM) nanophotonic platform. a) Schematic of the PDMS-based microfluidic system and the nanophotonic chip to be integrated; the zoomed-in inset shows a cross-sectional view of the chip containing arrays of Au nanoantennas. b) Illustration of how the microfluidic channels are sealed by the nanophotonic chip, with the corresponding reservoirs filled with LM and NaOH electrolyte, respectively. c) Schematic representation of LM movement inside the microfluidic channel under an applied electrical bias. The right-side inset shows the cross sectional view of the hybrid LM-insulator-metal (LMIM) resonators. A single LMIM resonator is highlighted by the dashed circle. d) Optical microscope image of an Au nanodisk array at ambience without LM contact (OFF state, middle) and image of the same array in contact with LM (ON state, right), revealing strong color contrast; the left panel shows the corresponding scanning electron microscopy (SEM) image of the Au nanodisks with ≈50 nm diameter and 125 nm array pitch. e) Measured reflectance spectra of the sample in (d) under the two operational conditions (ON/OFF). (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The researchers created a surface with nanoscale gold features on top and a reservoir of liquid metal below. When a small voltage is applied, the metal flows upward until it sits just nanometers under the gold, turning the surface into a mirror with a sharply defined color signature. Turn off the voltage and the metal retreats, leaving the surface in its unpowered state. The entire shift is controlled by surface chemistry, not heat or mechanics, and uses less power than a typical LED indicator light.
The experimental platform is built into a block of soft silicone, shaped to contain two fluid compartments connected by a narrow channel. One compartment holds a liquid alloy made from gallium and indium. The other holds a basic saltwater solution made from sodium hydroxide. Inside the connecting channel, the researchers placed a thin chip patterned with gold nanostructures on top of a few nanometers of insulating material. These structures are shaped as circles or ribbons, with sizes well below the wavelength of visible or infrared light. Electrodes allow a small voltage to be applied between the metal alloy and the salt solution.
In its resting state, the liquid metal stays in its reservoir. The gold nanostructures interact with light through a process known as plasmon resonance, where electrons in the metal oscillate together in response to light of certain colors. That effect can create weak color in isolated particles, but not the high contrast needed for visible devices or labels. To get more control, the gold needs a reflective layer nearby, close enough that incoming light can bounce between the two surfaces and create strong selective absorption.
That is where the liquid metal moves into place. A small positive voltage, around 1.5 to 3.0 volts, causes a thin oxide layer to form on the surface of the alloy. The oxide reduces surface tension, and the metal slides forward into the channel. As it moves, it rises to just beneath the layer that contains the gold nanostructures. A gap only a few nanometers thick separates the gold from the liquid metal. When light enters the surface, it reflects in a pattern controlled by this narrow spacing and by the size of the gold features. The surface shifts from a weak red or yellow tint to a high-contrast, stable color that depends on both shape and voltage.
Reversing the voltage removes the oxide, restoring surface tension and pulling the metal back. No heat pulse is needed, and the shift does not cause fatigue in the material. The power requirement remains low because the device does not rely on continuous currents or mechanical motion. Small bursts of voltage do all the work.
The team tested gold nanodisks ranging from 30 to 100 nanometers in diameter. With the mirror inactive, the arrays look similar under white light. When the mirror moves into place, each disk size produces a distinct reflected color. Small disks generate blue green tones. Medium disks shift toward yellow or red. Larger disks move into the near infrared, which the eye cannot see but sensors can detect. By printing patterns with two or more disk sizes, the researchers generated hidden images that appear only when the liquid mirror is active.
A university logo and a barcode design both showed sharp contrast in their active state and faded to near invisibility when the mirror was withdrawn. The effect works without a backlight and remains visible under bright ambient conditions. This suggests uses in optical tagging, information security, and low power signage.
The same architecture can shift light in the infrared. By replacing nanodisks with thin gold ribbons, the researchers tuned the resonance into the mid infrared. This region is useful for detecting water, carbon dioxide, and many biological molecules.
The study showed that when the device’s resonance matched the natural vibration frequency of water molecules in the electrolyte, the signal split into two peaks. That splitting indicates strong coupling between the optical response of the device and the vibrational mode of the molecule. Because the device can adjust its resonance with voltage alone, it could serve as a compact sensor that scans across molecular fingerprints without needing moving filters or thermal tuning.
The platform also works as a surface inspection tool. The team placed strips of titanium dioxide only 4.5 nanometers thick on the nanostructured chip. With the mirror off, the strips showed little contrast. With the mirror on, they altered the local spacing enough to change the reflected color, making them stand out clearly. Even leftover resist residue only 10 nanometers thick became visible in the activated state. This opens possibilities for detecting tiny quantities of material in manufacturing, medical diagnostics, or environmental testing.
Measured performance shows color shifts of up to 200 nanometers in wavelength and reflectance changes as high as 85 percent. The liquid metal flow determines the switching speed, which is slower than some solid-state optical modulators, but fast enough for labels, sensors, and displays that do not require rapid refresh. The power draw stays in the microamp range, supporting battery operation or even energy harvesting systems.
Some limitations remain. The electrolyte can evaporate, changing the oxidation process and the spacing over time. Enclosing the device or using a less volatile fluid could help. Gold works well for red and near infrared colors but does not reflect blue efficiently. Using aluminum or gallium for the nanostructures could expand the visible palette.
This study demonstrates a practical way to shift optical behavior using a soft, reconfigurable mirror positioned only nanometers from a patterned surface. It connects electrochemistry to photonics through fluid motion and does so with common materials and low voltage control. The result is a new class of programmable optical surfaces that can reveal images, detect chemicals, or tune color without heat, pigment, or wasteful illumination.
The work positions liquid metal as a viable tool for next generation reflective displays and on demand sensing devices across both visible and infrared wavelengths.
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Peter Q. Liu (The State University of New York, Buffalo)
, 0000-0002-3603-9885 corresponding author
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