Modica type estimates and curvature results for overdetermined elliptic problems

In this paper, we establish a Modica type estimate on bounded solutions to the overdetermined elliptic problem \begin{equation*} \begin{cases} Δu+f(u) =0& \mbox{in $\mathrm{\Omega }$, }\ u>0 &\mbox{in $\mathrm{\Omega }$, } u=0 &\mbox{on $\mathrm{\partial }\mathrm{\Omega }$, } \partial_ν u=-κ&\mbox{on $\mathrm{\partial }\mathrm{\Omega }$, } \end{cases} \end{equation*} where $\mathrm{\Omega }\subset {\mathbb{R}}^n,n\ge 2$. As we will see, the presence of the boundary changes the usual form of the Modica estimate for entire solutions. We will also discuss the equality case. From such estimates we will deduce information about the curvature of $\mathrm{\partial }\mathrm{\Omega }$ under a certain condition on $\kappa$ and $f$. The proof uses the maximum principle together with scaling arguments and a careful passage to the limit in the arguments by contradiction.