Pentamode metamaterials almost behave like fluids. Their manufacture opens new possibilities in transformation acoustics. Source: CFN, KIT |
A
research team lead by Professor Martin Wegener at the Karlsruhe
Institute of Technology (KIT) has succeeded in realizing a new material
class through the manufacturing of a stable crystalline metafluid, a
pentamode metamaterial. Using new nanostructuring methods, these
materials can now be realized for the first time with any conceivable
mechanical properties. The researchers will present their results in the
cover story of the May issue of Applied Physics Letters.
The
Rubicon was crossed, so to speak, at the DFG Center for Functional
Nanostructures (CFN) and at the Institute of Applied Physics (AP) in
Karlsruhe during the past few months. Eventually, numerous
three-dimensional transformation acoustics ideas, for example
inaudibility cloaks, acoustic prisms or new loudspeaker concepts, could
become reality in the near future.
So
far, pentamodes, proposed in 1995 by Graeme Milton and Andrej Cherkaev,
have been purely theoretical: The mechanical behavior of materials such
as gold or water is expressed in terms of compression and shear
parameters. Whereas the phenomenon that water, for example, can hardly
be compressed in a cylinder is described through the compression
parameter, the fact that it can be stirred in all directions using a
spoon is expressed through the shear parameters.
The
word penta is derived from ancient Greek and means “five”. In the case
of water, the five shear parameters equal zero, and only one parameter,
compression, differs from that value. In terms of parameters, the ideal
state of a pentamode metamaterial corresponds to the state of water,
which is why that material is referred to as a metafluid. Theoretically,
any conceivable mechanical properties whatsoever can be obtained by
varying the relevant parameters.
The stable four-leg structure (shown in orange) is the basic element of the pentamode metamaterial. It is arranged in the form of a three-dimensional adamantine crystal such that the resulting material as a whole can be formed. (Source: CFN, KIT) |
“Realizing
a pentamode metamaterial is about as difficult as trying to build a
scaffold from pins that must not touch but at their tips,” first author
Dr. Muamer Kadic explains. “The Karlsruhe prototype has been
manufactured from a polymer. The mechanical behavior of the material is
determined by the acuteness and length of the individual “sugar loaves”.
On the one hand, we must be capable of designing small sugar loaves in
the nanometer range and connect them to one another at the right angle.
On the other hand, the entire structure must eventually become as large
as possible. Since the material itself contributes only little more than
one percent to the respective volume, the composite obtained is
extremely light.
“To
obtain similar 3D results, as in transformation optics, transformation
acoustics is exclusively dependent on metamaterials. In view of this,
this first manufacture of our pentamode metamaterial is a quite
significant success,” adds Tiemo Bückmann, who is about to receive his
diploma at the Institute of Applied Physics and is responsible for
realizing the structures of the new material by means of dip-in laser
writing, a method that has been derived from direct laser writing
developed by the Nanoscribe company.
In
recent years, a Professor at the Institute of Applied Physics and CFN
coordinator, Martin Wegener and his collaborators, have developed direct
laser writing and, based on that method, established optical
lithography of three-dimensional nanostructures. Numerous achievements
of Wegener’s group in transformation optics e.g., the first
three-dimensional cloak of invisibility in the range of visible light
have been due to that technique.
