 |
| Silicone beads embedded in a water-based gel (photograph is ~2 cm across). |
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.
No comments:
Post a Comment