A newly
fabricated material does more than just hold up under pressure. Unlike many
ordinary objects that shrink when squeezed, the metamaterial — a synthetic
structure designed to exhibit properties not typically found in natural
materials — expands at higher pressures.
This
counterintuitive material is made up of a grid of hollow 3-D crosses — shaped
like six-way pipe fittings — mere micrometers across. When surrounding pressure
of air, water or some other substance increases, the crosses’ circular surfaces
bow inward. Because of the way these crosses are connected with levers, that
warping forces the crosses to rotate and push away from each other, causing the
whole structure to expand, says study coauthor Jingyuan Qu, a physicist at
Karlsruhe Institute of Technology in Germany.
The researchers
were “very clever about how they connected this quite complex set of structural
elements,” says Michael Haberman, a mechanical engineer at the University of
Texas at Austin, who wasn’t involved in the work.
Qu and
colleagues fashioned a microcube of their
metamaterial, described in a paper accepted to Physical Review
X, from a plasticlike substance, using a microversion of 3-D printing. When the
researchers placed the material inside a gas chamber and cranked up the air
pressure from one bar (about the atmospheric pressure at sea level) to five
bars, the cube’s volume increased by about 3 percent.
Until
now, researchers have only described such pressure-expanding metamaterials in mathematical
models or computer simulations, says Joseph Grima, a materials scientist at the
University of Malta in Msida not involved in the work. The new metamaterial
provides “much-needed proof” that this type of stuff can actually be
fabricated, he says.
Adjusting the
thickness of the crosses’ surfaces could make this new metamaterial more or
less expandable: The thicker it is, the less the structure expands. A
metamaterial fine-tuned to stay the same size under a wide range of pressures
could be used to build equipment that withstands the crushing pressures of the
deep sea or the vacuum of outer space.
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