Twirling elastica: Kinks, viscous drag, and torsional stress

Biological filaments such as DNA or bacterial flagella are typically curved in their natural states. To elucidate the interplay of viscous drag, twist ing, and bending in the overdamped dynamics of such filaments, we compute t he steady-state torsional stress and shape of a rotating rod with a kink. D rag deforms the rod, ultimately extending or folding it depending on the ki nk angle. For certain kink angles and kink locations, both states are possi ble at high rotation rates. The agreement between our macroscopic experimen ts and the theory is good, with no adjustable parameters.