Page 285 - Carbon Nanotube Fibres and Yarns

274 Carbon Nanotube Fibers and Yarns

Fig. 11.1 Snarls generated in excessively twisted yarns. (A) Side-snarl and (B) longitudi-
nal snarl taking the shape of a spring-like cylindrical coil [22]. (Source: X. Yang, W. Wang,
M. Miao, Moisture-responsive natural fiber coil-structured artificial muscles, ACS Appl.
Mater. Interfaces 10 (38) (2018) 32256–32264.)

straight yarn to jump to form a snarl due to the insertion of excessive twist.
After the onset, an indefinitely long cylindrical yarn coil can be formed by
continuing insertion of twist to the yarn.
The cylindrical snarl, or spring-like coil, is found to be particularly use-
ful as a contractile actuator, often referred to as “artificial muscle.” In knot
theory terminology, each turn of the yarn coil represents (1−sinα) unit of a
“writhe” [39], where α is the rising angle of the coil helix. The sum of the
twist in the yarn and the writhe in the coil is known as the linking number
of the twisted yarn coil.
11.2.4 Plied yarns

In a two-ply yarn, the axes of the two single yarns follow two separate he-
lical paths with the same helix angle determined by the two-ply twist and
the diameter of the singles, as shown in Fig. 11.2. For a torque-free two-ply
yarn, the ply twist is in the opposite direction to the singles twist and at a
ratio of 1/ 2 in terms of twist turns per meter [40].

Fig. 11.2 Schematic of a two-ply yarn.