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Research sheds light on the underlying mechanics of soft filaments



  Better understanding of the soft artificial muscles
A thread is gripped at the upper end and stretched with a small amount by applying an axial load down to the lower end. The lower end is then twisted, keeping the axial load of the lower end constant. Once a critical amount of twist is inserted, the thread spontaneously curves into a magnifying glass. Credit: Nicholas Charles / Harvard SEAS

Artificial muscles will power the soft robots and wearable devices of the future. But one must understand more about the basic mechanics of these powerful structures in order to design and build new devices.


Now, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have discovered some of the basic physical properties of artificial muscle fibers.

"Thin soft fibers that can easily be stretched, bent, twisted or sheared are capable of extreme deformation that results in knot-like structures, like braids or loops, which can easily store or release energy," said L. Mahadevan, Professor of Applied Mathematics at Lola England de Valpin, Organism and Evolution Biology and Physics. "This has been used by a number of experimental groups recently to create prototypical artificial muscle fibers. But how the topology, geometry, and mechanics of these slender fibers combine during this process was not completely clear. Our study explains the theoretical principles that underlie this form of transformation and sheds light on the basic principles of design. "

'Soft fibers are the basic unit of musculature and could be used in everything from robotics to intelligent textiles that can respond to stimuli such as heat or humidity," said Nicola Charles, Ph.D. mathematics and the first author of the article. "The possibilities are endless if we can understand the system. Our work explains the complex morphology of soft, highly stretched and twisted fibers and provides guidance on the best designs. "

  Better understanding of soft artificial muscles
A thread is tightened at the upper end and pre-stretched by application of the axial load down to the lower end.After a critical amount of twist is inserted, the thread spontaneously curves into a form known as a solenoid Credit: Nicholas Charles / Harvard SEAS

The study was published in Physical Review Letters .

Soft fibers or filaments may be stretched, cut, bent or twisted. How these different actions interact to form nodes, braids and spirals is important for the design of soft actuators. Imagine stretching and twisting the rubber band as tightly as possible. As the twist becomes more tight and tight, part of the strap will pop out of the plane and begin to twist around itself in a coil or knot. These coils and contours, in the correct shape, can be harnessed to trigger the tangled fiber.

Researchers have found that different levels of tension and torsion lead to different types of complex non-planar shapes. They characterized which shapes lead to crooked loops, which to tight coils and which to a mixture of the two. They found that pre-stretching was important for the formation of coils, as these forms were the most stable when stretching, and modeled how such coils could be used to produce mechanical work.

"This study gives us a simple way to predict how soft the fibers will react to twisting and stretching," says Charles.

of hair, polymer dynamics and the dynamics of the lines of the magnetic field of the sun and other stars – said Mahadevan.


Studies show that twisting is hot, unwinding is cool


Provided by
Harvard John A. Paulson School of Engineering and Applied Sciences
Reference :
The study sheds light on the underlying mechanics of soft fibers (2019, November 15)
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from https://phys.org/news/2019-11-underlying-mechanics-soft-filaments.html

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