A new discrete monotonicity formula with application to a two-phase free boundary problem in dimension two

Abstract: We continue the analysis of the two-phase free boundary problems initiated in \cite{DK}, where we studied the linear growth of minimizers in a Bernoulli type free boundary problem at the non-flat points and the related regularity of free boundary. There, we also defined the functional $$\phi_p(r,u,x_0)=\frac1{r^4}\int_{B_r(x_0)}\frac{|\nabla u^+(x)|^p}{|x-x_0|^{N-2}}dx\int_{B_r(x_0)}\frac{|\nabla u^-(x)|^p}{|x-x_0|^{N-2}}dx$$ where $x_0$ is a free boundary point, i.e. $x_0\in\partial{u>0}$ and $u$ is a minimizer of the functional $$J(u):=\int_{\Omega}|\nabla u|^p +\lambda_+^p\,\chi_{{u>0}} +\lambda_-^p\,\chi_{{u\le 0}}, $$ for some bounded smooth domain $\Omega\subset {\mathbb R}^N$ and positive constants $\lambda_\pm$ with $\Lambda:=\lambda_+^p-\lambda^p_->0$. Here we show the discrete monotonicity of $\phi_p(r,u,x_0)$ in two spatial dimensions at non-flat points, when $p$ is sufficiently close to 2, and then establish the linear growth. A new feature of our approach is the anisotropic scaling argument discussed in Section 4.