RESEARCHERS HARNESS UNSTABLE RESPONSES TO BUILD NEW SOFT
ACTUATORS
(August 18, 2015) Instability in engineering is generally not a good thing. If
you’re building a skyscraper, minor instabilities could bring the whole structure
crashing down in a fraction of a second. But what if a quick change in shape is
exactly what you want?
Soft machines and robots are becoming more and more
functional, capable of moving, jumping, gripping an object, and even changing
color. The elements responsible for
their actuation motion are often soft, inflatable segments called fluidic
actuators. These actuators require large amounts of air or water to change
shape, making the machines slow, bulky and difficult to untether.
A team of researchers at the Harvard John A. Paulson School
of Engineering and Applied Science (SEAS) has engineered a new, soft actuator
that harnesses the power of instability to trigger instantaneous movement.
These new, soft
actuators harnesses the power of instability to trigger
instantaneous
movement (photo courtesy of the Bertoldi Group)
The research was led by Katia Bertoldi, the John L. Loeb
Associate Professor of the Natural Sciences, member of the Kavli Institute for
Bionano Science and Technology, and faculty associate of the Materials Research
Science and Engineering Center. The work is described in a paper in the Proceedings
of the National Academy of Sciences.
The actuator is inspired by a famous physics experiment in
which two balloons are inflated to different sizes and connected via a tube and
valve. When the valve is opened, air flows between the balloons. Instead of
equalizing in size, as one might expect, the larger balloon inflates more while
the smaller balloon deflates.
This unexpected behavior comes from the balloons’ non-linear
relationship between pressure and volume, meaning an increase in volume doesn’t
necessarily increase the pressure.
“When inflating a balloon, the first few blows are the
hardest but after reaching a critical pressure it becomes easier,” said
Johannes Overvelde, PhD student at SEAS and first author on the paper. “Similar
to the balloons, in our research we connect fluidic segments in such a way that
an interplay between their non-linear response results in unexpected behavior.
Certain combinations of these interconnected segments can result in fast moving
instabilities with negligible change in volume.”