The Scaling Limit of Polymer Pinning Dynamics and a One Dimensional Stefan Freezing Problem

We consider the stochastic evolution of a 1 + 1-dimensional interface (or polymer) in the presence of a substrate. This stochastic process is a dynamical version of the homogeneous pinning model. We start from a configuration far from equilibrium: a polymer with a non-trivial macroscopic height profile, and look at the evolution of a space-time rescaled interface. In two cases, we prove that this rescaled interface has a scaling limit on the diffusive scale (space rescaled by L in both dimensions and time rescaled by L ² where L denotes the length of the interface) which we describe. When the interaction with the substrate is such that the system is unpinned at equilibrium, then the scaling limit of the height profile is given by the solution of the heat equation with Dirichlet boundary condition ; when the attraction to the substrate is infinite, the scaling limit is given by a free-boundary problem which belongs to the class of Stefan problems with contracting boundary, also referred to as Stefan freezing problems. In addition, we prove the existence and regularity of the solution to this problem until a maximal time, where the boundaries collide. Our result provides a new rigorous link between Stefan problems and Statistical Mechanics.