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By finely tuning the space between a single layer of nanoparticles, researchers developed a filter that changes from a mirror to a window. (Image credit Imperial College London)


Researchers at Imperial College London have developed a unique filter that can change from a reflective state to a clear state and back again in real-time- essentially a mirror that transforms into a window and back again on demand. Similar in the way Innovative Glass’ eGlass works by applying a low current to nanoparticles to alter their state of orientation, making clear glass instantly fog for privacy, like in spy movies.


Detailed in a recently released paper published in Nature, the researchers developed their Electrotunable Nanoplasmonic Liquid Mirror for applications that range from new sensors to super lenses and much more. Achieving this goal proved a challenge for materials scientists, considering those changes require precision control of the nanoparticles in said filter- in this case, gold nanoparticles.



To develop their ‘tunable mirror,' the researchers used a single layer of gold nanoparticles and localized them between two immiscible electrolyte solutions that don’t mix with each other. When a small current (±0.5 V) is applied, the tunable particle layer becomes dense or sparse, switching between a reflective state and transparent surface. More accurately, when the particles are closer together, they create a mirrored surface and when further apart, it becomes clear, allowing light to pass through.


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How the Electrotunable Nanoplasmonic Liquid Mirror is designed. (Image credit YouTube screenshot)


“It’s a really fine balance – for a long time, we could only get the nanoparticles to clump together when they assembled, rather than being accurately spaced out. But many models and experiments have brought us to the point where we can create a truly tuneable layer,” stated Professor Joshua Edel, co-author of the paper.


While the researchers were not the first to manipulate nanoparticle arrays, they were the first to develop a system that’s reversible- from one state to another with a precision level of uniformity. As demonstrated in the video above, the researchers showed that when a current was supplied, it reflected a 1-pound coin situated above the material. When that current was taken away, it revealed a ten-pound note below the array. It’s important to note that the filter is still under development, but it could lead to breakthroughs in chemical-based sensors, optical filters that can focus on certain light wavelengths or novelty mirrors that let you spy on people.   


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