Page 288 - Carbon Nanotube Fibres and Yarns

Carbon nanotube yarn-based actuators 277

Fig. 11.5 Torsional muscle by immersing part of a CNT yarn tethered at two ends [44].
(Source: J. Foroughi, G.M. Spinks, G.G. Wallace, J. Oh, M.E. Kozlov, S. Fang, T. Mirfakhrai,
J.D. Madden, M.K. Shin, S.J. Kim, Torsional carbon nanotube artificial muscles, Science 334
(6055) (2011) 494–497.)

current is cutoff and yarn torques in the immersed part is reduced. The
buildup of torque (and stored energy) in the unimmersed part continues
until the paddle finally stops. This is followed by the rotation of the paddle
in the opposite direction driven by releasing the energy stored in the un-
immersed part. Then, another cycle of paddle rotation is started by charging
the immersed part of the yarn [44].

11.3.3 Tensile actuators—Coiled CNT yarns
A spring-like coil formed from an extremely twisted yarn of responsive
fibers can generate tensile strokes when stimulated. We have discussed pre-
viously that the twist retraction of a yarn is related only to the twist angle
(angle of the surface fibers on a yarn). For a constant twist T, if the yarn di-
ameter (2r) increases due to fiber diameter expansion, the twist angle θ will
increase and the yarn will further retract in length. This yarn retraction will
reduce the length of the spring coil if the coil diameter D and rising angle
α (Fig. 11.6) are kept constant, resulting in a contraction of the spring coil.
Another part of the contraction of the spring coil is caused by the stiffen-
ing of the yarn when it expands its diameter when actuated by stimulus. This
leads to a rebalancing between the tension, torque, and bending moment of
the yarn in the coil and results in an increase of the coil diameter D. As the
actuation does not increase the yarn length (actually it decreases the yarn
length as explained above), the number of turns in the coil (i.e., the writhe)
will decrease as the coil diameter increases, leading to the shortening of the
spring coil, which contributes to the longitudinal contraction [22].