Scientists create shape-shifting metal...
Liquid metal could be used to create morphing electronics
By Colin Jeffrey
September 21, 2014
Who could forget the scene in Terminator 2: Judgement Day
where the shape-shifting T-1000 reassembles itself from thousands of blobs of
molten metal? Researchers from North Carolina State University (NCSU) have
taken the first steps to such science fiction becoming reality by developing a
way to control the surface tension of liquid metals with the application of
very low voltages. This may offer opportunities in a new field of morphing
electronic circuits, self-healing electronics, or – one day – maybe even self-assembling
terminator-style robots.
The liquid metal used by the researchers was an alloy of
gallium and indium. Gallium is liquid just above room temperature at about 29°
C (84° F), while Indium has a much higher melting point at around 156° C (312°
F), yet when mixed together, they form an alloy that is liquid at room
temperature. In other words, a eutectic alloy – one that is composed of metals
with disparate melting points that, when combined, melt as a whole at a
specific temperature.
Another important aspect of this eutectic alloy, and one
that the researchers sought to exploit in their experiments, is its
exceptionally high surface tension of approximately 500 millinewtons per meter
(mN/m). The consequence of this is that a blob of this alloy resting on a
surface will tend to form an almost spherical ball and hold its shape if
undisturbed.
Researchers found that if they applied a small voltage
(less than one volt) in water to such a blob of the alloy, they were able
reduce the surface tension significantly, resulting in the molten metal
spreading and flattening out. When the voltage was removed, the high surface
tension returned, and the blob once more took on its spherical shape.
The amount of surface tension could also be varied
dependent upon that voltage, making the blob more or less viscous depending on
the electrical charge applied to it. In other words, the liquid metal could be
held in various states of liquidity from its original 500 mN/m right down to 2
mN/m, and anything in between.
The significance to this demonstration is that the metal
could be made to flow in and out of variously shaped capillaries or molds,
allowing the alloy to take on different contours. If these shapes were that of
an antenna, for example, then the metal could behave as a highly-variable or
tunable antenna capable of morphing its shape to receive or transmit a wide
range of different wavelengths all from the same component.
NCSU researchers have experimented with other versions of
shape-shifting antennas in the past, but this is the first time that they have
utilized electricity – rather than mechanical deformation – to alter the shape.
"The resulting changes in surface tension are among
the largest ever reported, which is remarkable considering it can be
manipulated by less than one volt," said Dr. Michael Dickey, an associate
professor of chemical and biomolecular engineering at NCSU and lead author of
the research. "We can use this technique to control the movement of liquid
metals, allowing us to change the shape of antennas and complete or break
circuits. It could also be used in microfluidic channels, MEMS, or photonic and
optical devices. Many materials form surface oxides, so the work could extend
beyond the liquid metals studied here."
This technique – a version of electrohydrodynamics in
which electric currents affect the movement of liquid metals – relies on an
oxide "skin" forming on the surface of the alloy when a voltage is
applied that behaves as a surfactant, lowering the surface tension between the
metal and the surrounding fluid.
This current research builds on experiments previously
carried out by the NCSU lab when they revealed a process for "3-D
printing" liquid metals where, unlike this experiment in water, they
created the oxide layer in open air to assist the liquid metal in retaining its
shape.
The research was recently published in the online journal
Proceedings of the National Academy of Sciences
Source: North Carolina State University
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