Ultrasounds dance the 'moonwalk' in new metamaterial

Silicone beads embedded in a water-based gel (photograph is ~2 cm across). Credit: © CRPP

Metamaterials have extraordinary properties when it comes to diverting and controlling waves, especially sound and light: for instance, they can make an object invisible, or increase the resolving power of a lens. Now, researchers at the Centre de Recherche Paul Pascal (CNRS) and the Institut de Mécanique et d'Ingénierie de Bordeaux (CNRS/Université de Bordeaux/Bordeaux INP/Arts et Métiers ParisTech) have developed the first three-dimensional metamaterials by combining physico-chemical formulation and microfluidics technology. This is a new generation of soft metamaterials that are easier to shape. In their experiment, the researchers got ultrasonic oscillations to move backwards while the energy carried by the wave moved forwards. Their work opens up new prospects, especially for high-resolution imaging (ultrasonography). It is published on 15 December 2014 in the journal Nature Materials.

Since the 2000s, the international scientific community has seen interest in metamaterials and their extraordinary properties grow exponentially. A metamaterial is a medium in which the phase velocity of light or sound waves can be negative (the material is said to have a negative refractive index).. In such a medium, the phase of the wave (the successive oscillations) and the energy carried by this same wave move in opposite directions. This property is not found in any natural homogeneous medium.

To obtain a metamaterial, it is necessary to make a heterogeneous medium that contains a large number of inclusions (known as microresonators). The usual way is to use micromechanical methods (etching, deposition, etc) to machine solid supports that will have the properties of metamaterials in one or two dimensions. However, this method cannot be used to work with soft matter at the micrometer scales required for ultrasounds, and the materials obtained remain limited to one or two dimensions.

In this study, the researchers developed a new type of metamaterial, in the fluid phase, formed of porous silicone microbeads embedded in a water-based gel. This metafluid is the first three-dimensional metamaterial to work at ultrasonic frequencies. In addition, due to its fluid nature, it can be made using physico-chemical processes and microfluidics technologies, which are much easier to implement than micromechanical methods.

One of the properties of porous media is that sound travels through them at very low speed (a few tens of meters per second) compared to water (1500 meters per second). Due to this sharp contrast, the whole suspension has the properties of a metamaterial provided the bead concentration is sufficient: when the researchers studied the propagation of ultrasonic waves through this medium, they directly measured a negative refractive index. Within such a metafluid, the energy carried by the wave travels from the emitter to the receiver, as expected, whereas the oscillations appear to move backwards in the opposite direction, rather like a dancer doing the 'moonwalk'.

These results open the way to numerous applications ranging from high-resolution ultrasound imaging to sound insulation and stealth in underwater acoustics. In addition, the soft-matter physico-chemical techniques used to make this metamaterial makes it possible to produce fluid or flexible materials with adaptable shapes, potentially at the industrial scale.

Source: CNRS (Délégation Paris Michel-Ange)

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Smart window that tints and powers itself invented

NTU Prof Sun Xiaowei holding his smart window invention that can self-tint and also functions as a battery. Credit: Image courtesy of Nanyang Technological University

Nanyang Technological University (NTU) scientists have developed a smart window which can darken or brighten without the need for an external power source.

This unique self-tinting window requires zero electricity to operate and is also a rechargeable battery. The window's stored energy can be used for other purposes, such as to light up low-powered electronics like a light emitting diode (LED).

Currently, the window solutions in the market are either using permanent tinting which cannot brighten at night or are windows that can change its light transmission properties only with an external power source.

The NTU smart window however can be turned into a cool blue tint in bright daylight, cutting light penetration by about half, and then reverts back to clear glass at night or as required.

This breakthrough research led by NTU Professor Sun Xiaowei, was published recently in Nature Communications.

How it works

The trick to making the self-powered smart window is a new technology developed by Prof Sun's team from NTU's School of Electrical and Electronic Engineering.

"Our new smart electrochromic window is bi-functional; it is also a transparent battery," Prof Sun explained. "It charges up and turns blue when there is oxygen present in the electrolyte -- in other words, it breathes."

The NTU smart window contains liquid electrolyte placed in between two glass sheets coated with indium tin oxide (ITO), commonly used as transparent conductive coatings for television displays. One sheet is coated with an additional layer of a pigment known as Prussian Blue and the other one is attached to a thin strip of aluminium foil. The Prussian Blue gives the glass a blue tint when it is fully charged.

The two glass sheets are connected by typical electrical cables. When the electrical circuit between them is broken, a chemical reaction starts between Prussian Blue and the dissolved oxygen in the electrolyte, turning the glass blue. To turn off the blue tint, the electrical circuit is closed to discharge the battery, turning the Prussian Blue into a colourless Prussian White.

Such an innovative technology can adjust the amount of sunlight coming into buildings in the day, which promises significant savings on cooling and lighting costs.

"Our technology is very attractive as a zero-sum consumption smart window. Buildings owners and even common households can reap energy savings right from the outset and over the long term. Developers who are looking at constructing environmentally-friendly green buildings will find our technology attractive for their building plans," said Prof Sun.

Prof Sun is an electrical engineering expert whose other innovations include various solar technologies, glass-free 3D technologies, next-generation lightings and displays.

The NTU team, consisting of five researchers, is now enhancing their invention and is looking forward to collaborating with industry partners to commercialise their technology.

Source: Nanyang Technological University

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