Structure, mechanical, and actuation properties of yarns with different twist density. (a) Measured and calculated surface bias angles of a 140-μm-diameter cotton yarn as a function of inserted twist density. (b) Stress–strain curves of a 140-μm-diameter cotton yarn at different inserted twist. (c) Breaking stress and failure strain obtained from (b). (d) Stress–strain curves of a 140-μm-diameter cotton yarn with 800 turns/m of inserted twist before and after exposure to 100% relative humidity. (e) Torsional rotation angle as a function of time for a self-balanced, 2-ply, single filament cotton yarn muscle with different inserted twist on exposure and removing the water fog. (f) Maximum rotation angle, maximum rotational speed, and the ratio of the maximum rotational speed in the forward process to that in the reverse process for a self-balanced, 2-ply, single filament cotton yarn muscle with different inserted twist on exposure to water fog. The ambient relative humidity is 40%, and room temperature is 25 °C. The loaded paddle during torsional actuation for (e) and (f) is 0.47 g. The flux of water fog in (e) and (f) is 0.25 g⋅s⁻¹⋅m⁻².