The robots, just half a millimeter wide, are composed of microparticles. Confined between two liquids, they assemble themselves into star shapes when an alternating magnetic field is applied. Image: Argonne National Laboratory |
Alexey Snezhko and Igor Aronson, physicists at the U.S.
Department of Energy’s (DOE) Argonne National Laboratory, have coaxed microrobots
to do their bidding.
The robots, just half a millimeter wide, are composed of
microparticles. Confined between two liquids, they assemble themselves into
star shapes when an alternating magnetic field is applied. Snezhko and Aronson
can control the robots’ movement and even make them pick up, transport, and put
down other non-magnetic particles—potentially enabling fabrication of precisely
designed functional materials in ways not currently possible.
The discovery grew out of past
work with magnetic “snakes”. This time, however, Snezhko and
Aronson suspended the tiny ferromagnetic particles between two layers of immiscible,
or non-mixing, fluids.
Without a magnetic field, the particles drift aimlessly or
clamp together. But when an alternating magnetic field is applied perpendicular
to the liquid surface, they self-assemble into spiky circular shapes that the
scientists nicknamed “asters”, after the flower.
Left to their own devices, the asters don’t swim. “But
if you apply a second small magnetic field parallel to the surface, they begin
to move,” says Aronson. “The field breaks the symmetry of the asters’
hydrodynamic flow, and the asters begin to swim.”
By changing the magnetic field, the researchers discovered
they could remotely control the asters’ motion.
“We can make them open their jaws and close them,”
says Snezhko. “This gives us the opportunity to use these creatures as
mini-robots performing useful tasks. You can move them around and pick up and
drop objects.”
They soon discovered that the asters form in two
“flavors”; one’s flow circulates in toward the center of the aster,
and the other circulates outward. They swim in opposite directions based on
flavor. These properties are useful because scientists can play the flows
against one other to make the asters perform tasks.
For example, four asters positioned together act like a
miniature vacuum cleaner to collect free-floating particles.
The asters can pick up objects much larger than themselves;
in one video, an aster picks up a glass bead that weighs four times as much as
the aster itself.
“They can exert very small forces on objects, which is
a big challenge for robotics,” Aronson explains. “Gripping fragile
objects without smashing them has always been difficult for conventional
robots.”
The microrobots occupy a niche between laser-powered
manipulation and mechanical micromanipulators, the two previous techniques
developed for manipulation at the microscale. “You can grab microparticles
with lasers, but the force is much smaller,” Snezhko explained.
“These asters’ forces are more powerful, but they can handle items much
more delicately than mechanical micromanipulators can.”
The materials can even self-repair; if particles are lost,
the aster simply re-shuffles itself.
The research is a part of the ongoing effort, funded by the
DOE, to understand and design active self-assembled materials. These structures
can assemble, disassemble, and reassemble autonomously or on command and will
enable novel materials capable of multi-tasking and self-repair.
“For us, this is very exciting. This is a new paradigm
for reconfigurable self-assembled materials that can perform useful
functions,” Aronson says.
