Materials

Ethanol was purchased from Sinopharm Chemical Reagent Co., Ltd. Pure water was provided by the water purification systems (Direct-Q8UV-R, Merk Millipore, Germany). Human hair was provided by the author Qian-Ru Xiao, who has no objection to the hair being used in this study.

Preparation and characterization of the twisted hair fibers

Long black hair with a diameter of 90-100 μm was cleaned and used in this study. The length of the hair fibers ranges from 40 to 70 cm. For preparation of the twisted hair fiber, a raw hair fiber was placed vertically with a weight hanging on its bottom end. Twist was inserted at the top end of the hair fiber by a motor in the counter-clockwise direction. Meanwhile, the weight at the bottom was tethered to avoid twist release during the twist insertion. After twisting, the fiber was folded at its middle point and formed a torque-balanced two-plied structure. Then the single twisted hair filament was attached to a folded paper with an observation window and observed with a macro lens. ImageJ was used to measure the helical angles.