Length-constrained, length-penalised and free elastic flows of planar curves inside cones

Abstract

We study families of smooth, embedded, regular planar curves $ \alpha : \left [-1,1 \right ]\times \left [0,T \right )\to \mathbb{R}^{2}$ with generalised Neumann boundary conditions inside cones, satisfying three variants of the fourth-order nonlinear $L^2$- gradient flow for the elastic energy: (1) elastic flow with a length penalisation, (2) elastic flow with fixed length and (3) the unconstrained or `free' elastic flow. Assuming neither end of the evolving curve reaches the cone tip, existence of smooth solutions for all time given quite general initial data is well known, but classification of limiting shapes is generally not known. For cone angles not too large and with suitable smallness conditions on the $L^2$-norm of the first arc length derivative of curvature of the initial curve, we prove in cases (1) and (2) smooth exponential convergence of solutions in the $C^\infty$-topology to particular circular arcs, while in case (3), we show smooth convergence to an expanding self-similar arc.