Torsional actuation performance of a self-balanced, 2-ply cotton yarn muscle. (a) Rotation angle and rotation speed as a function of time for three cycles of delivering/removing water fog. (b) Maximum rotation angle for 500 cycles of water-fog induced torsional actuation. (c) Forward and reverse maximum rotation speeds for 500 cycles of water-fog induced torsional actuation. (d) Maximum rotation angle and maximum rotation speed as a function of isobaric stress. The ambient relative humidity was 40%, and room temperature was 25 °C. The water fog flux was 0.25 g⋅s⁻¹⋅m⁻². The initial inserted twist was 1200 turns/m. The paddle weight in (a)–(c) was 0.47 g. The initial non-twisted cotton yarn was 25 cm in length and 140 μm in diameter.