How to imitate natural spring-loaded snapping movement without losing
energy

Date:
September 3, 2020
Source:
University of Massachusetts Amherst
Summary:
Venus flytraps do it, trap-jaw ants do it, and now materials
scientists can do it, too - they discovered a way of efficiently
converting elastic energy in a spring to kinetic energy for
high-acceleration, extreme velocity movements as nature does it.



FULL STORY ========================================================================== Venus flytraps do it, trap-jaw ants do it, and now materials scientists at
the University of Massachusetts Amherst can do it, too -- they discovered
a way of efficiently converting elastic energy in a spring to kinetic
energy for high- acceleration, extreme velocity movements as nature
does it.


==========================================================================
In the physics of human-made and many natural systems, converting energy
from one form to another usually means losing a lot of that energy, say
first author Xudong Liang and senior researcher Alfred Crosby. "There
is always a high cost, and most of the energy in a conversion is lost,"
Crosby says. "But we have discovered at least one mechanism that helps significantly." Details are in Physical Review Letters.

Using high-speed imaging, Liang and Crosby measured in fine detail
the recoiling, or snapping, motion of elastic bands that can reach accelerations and velocities similar to many of the natural biological
systems that inspired them. By experimenting with different elastic band conformations, they discovered a mechanism for imitating ant and flytrap fast-motion, high-power impulse events with minimal energy loss.

Liang, who is now on the faculty at Binghamton University, and Crosby are
part of a group that includes roboticists and biologists led by former
UMass Amherst expert Sheila Patek, now at Duke University. She has studied
the mantis shrimp's extremely rapid raptorial appendage-snapping motion
for years. Their multi-institution team is supported by a U.S. Army Multidisciplinary University Research Initiative (MURI) grant funded by
the U. S. Army Research Laboratory and its Research Office.

In Liang's observations and experiments, he discovered the underlying conditions where energy is most conserved -- plus the fundamental physics
- - and presents what Crosby calls "some really beautiful theory and
equations" to support their conclusions. "Our research reveals that
internal geometric structures within a spring play a centrally important
role in enhancing the energy conversion process for high-power movements," Crosby notes.

The secret turned out to be adding strategically placed elliptical --
not circular -- holes to the elastic band, Liang says. "Maintaining
efficiency is not intuitive, it's very difficult to guess how to do it
before you experiment with it. But you can start to form a theory once
you see how the experiment goes over time. You can start to think about
how it works." He slowed the action to watch the snapping motion in a synthetic polymer that acts like a rubber band.

Liang discovered that the structural secret is in designing a pattern
of holes.

"With no holes everything just stretches," he notes. "But with holes,
some areas of the material will turn and collapse." When plain bands are stretched and recoiled, less than 70% of the stored energy is harnessed
for high-power movement, the rest is lost.

By contrast, adding pores transforms the bands into mechanical
meta-materials that create motion through rotation, Liang explains. He
and Crosby demonstrate that with meta-materials, more than 90% of the
stored energy is used to drive movement. "In physics, bending accomplishes
the same movement with less energy, so when you manipulate the pattern
of the pores you can design the band to bend internally; it becomes high-efficiency," Crosby adds.

"This shows that we can use structure to change properties in
materials. Others knew this was an interesting approach, but we moved
it forward, especially for high-speed movement and the conversion from
elastic energy to kinetic energy, or movement." The two hope this advance
will help roboticists on their MURI team and others with a performance
goal to help them design high-efficiency, rapid kinetic robotic systems.

Liang says, "Now we can hand over some of these structures and say,
'Here's how to design a spring for your robots.' We think the new theory
opens up a lot of new ideas and questions on how to look at the biology,
how the tissues are structured or their shells are configured to allow
rotation that we show is the key," he adds.


========================================================================== Story Source: Materials provided by
University_of_Massachusetts_Amherst. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Xudong Liang, Alfred J. Crosby. Programming Impulsive Deformation
with
Mechanical Metamaterials. Physical Review Letters, 2020; 125 (10)
DOI: 10.1103/PhysRevLett.125.108002 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200903171441.htm

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