Bistable helices

We extend elasticity theory of filaments to encompass systems, such as bact erial flagella, that display competition between two helical structures of opposite chirality. A general, fully intrinsic formulation of the dynamics of bend and twist degrees of freedom is developed using the natural frame o f space curves, spanning from the inviscid limit to the viscously overdampe d regime applicable to cellular biology. Aspects of front propagation found in flagella are discussed.