NIST’s fluid-tunable “metasurface” consists of copper structures and plastic tubing mounted on composite board. The presence of water in the tubing changes the resonant frequency at which the metasurface absorbs and stores energy. Photo: NIST |
Like an opera singer hitting a note that shatters a glass, a
signal at a particular resonant frequency can concentrate energy in a material
and change its properties. And as with 18th century “musical
glasses,” adding a little water can change the critical pitch. Echoing
both phenomena, researchers at the National Institute of Standards and
Technology (NIST) have demonstrated a unique fluid-tuned
“metasurface,” a concept that may be useful in biomedical sensors and
microwave-assisted chemistry.
A metasurface or metafilm is a two-dimensional version of a
metamaterial, popularized recently in technologies with seemingly unnatural
properties, such as the illusion of invisibility. Metamaterials have special
properties not found in nature, often because of a novel structure. NIST’s
metasurface is a small piece of composite circuit board studded with metal
patches in specific geometries and arrangements to create a structure that can
reflect, store, or transmit energy (that is, allow it to pass right through).
As described in a paper, NIST researchers used purified
water to tune the metasurface’s resonant frequency. They also calculated that
the metasurface could concentrate electric field strength in localized areas,
and thus might be used to heat fluids and promote microwave-assisted chemical
or biochemical reactions.
The metasurface’s behavior is due to interactions of 18
square copper frame structures, each 10 millimeters on a side. Computer
simulations help design the copper squares to respond to a specific frequency.
They are easily excited by microwaves, and each one can store energy in a
T-shaped gap in its midsection when the metasurface is in a resonant condition.
Fluid channels made of plastic tubing are bonded across the gaps. The sample is
placed in a waveguide, which directs the microwaves and acts like a
kaleidoscope, with walls that serve as mirrors and create the electrical
illusion that the metasurface extends to infinity.
Researchers tested the metasurface properties with and
without purified water in the fluid channels. The presence of water shifted the
resonant frequency from 3.75 to 3.60 gigahertz. At other frequencies, the
metasurface reflects or transmits energy. Researchers also calculated that the
metasurface, when in the resonant condition, could concentrate energy in the
gaps at least 100 times more than the waveguide alone.
Metasurface/fluid interactions might be useful in tunable
surfaces, sensing and process monitoring linked to changes in fluid flow, and
catalysis of chemical or biochemical reactions in fluid channels controlled by
changes in microwave frequency and power as well as fluid flow rates. NIST
researchers are also looking into the possibility of making metamaterial chips
or circuits to use for biomedical applications such as counting cells.
