Inserted: 1 mar 2011
We study a singular-limit problem arising in the modelling of chemical reactions. At finite $\epsilon>0$, the system is described by a Fokker-Planck convection-diffusion equation with a double-well convection potential. This potential is scaled by $1/\epsilon$, and in the limit $\epsilon$ goes to $0$, the solution concentrates onto the two wells, resulting into a limiting system that is a pair of ordinary differential equations for the density at the two wells.
This convergence has been proved in Peletier, Savaré, and Veneroni, SIAM Journal on Mathematical Analysis, 42(4):1805--1825, 2010, using the linear structure of the equation. In this paper we re-prove the result by using solely the Wasserstein gradient-flow structure of the system. In particular we make no use of the linearity, nor of the fact that it is a second-order system.
The first key step in this approach is a reformulation of the equation as the minimization of an action functional that captures the propety of being a curve of maximal slope in an integrated form. The second important step is a rescaling of space. Using only the Wasserstein gradient-flow structure, we prove that the sequence of rescaled solutions is pre-compact in an appropriate topology. We then prove a Gamma-convergence result for the functional in this topology, and we identify the limiting functional and the differential equation that it represents. A consequence of these results is that solutions of the $\epsilon$-problem converge to a solution of the limiting problem.
Keywords: singular perturbations, Gradient flows, Gamma convergence, Kramer's problem, Entropy dissipation, Chemical reactions, Curves of maximal slope