Differences between Robin and Neumann eigenvalues

Let $\Omega\subset \mathbb R^2$ be a bounded planar domain, with piecewise smooth boundary $\partial \Omega$. For $\sigma>0$, we consider the Robin boundary value problem $$-\Delta f =\lambda f, \qquad \frac{\partial f}{\partial n} + \sigma f = 0 \mbox{ on } \partial \Omega$$ where $\frac{\partial f}{\partial n}$ is the derivative in the direction of the outward pointing normal to $\partial \Omega$. Let $0<\lambda^\sigma_0\leq \lambda^\sigma_1\leq \dots$ be the corresponding eigenvalues. The purpose of this paper is to study the Robin-Neumann gaps $$d_n(\sigma):=\lambda_n^\sigma-\lambda_n^0 .$$ For a wide class of planar domains we show that there is a limiting mean value, equal to $2{\rm length}(\partial\Omega)/{\rm area}(\Omega)\cdot \sigma$ and in the smooth case, give an upper bound of $d_n(\sigma)\leq C(\Omega ) n^{1/3}\sigma$ and a uniform lower bound. For ergodic billiards we show that along a density-one subsequence, the gaps converge to the mean value. We obtain further properties for rectangles, where we have a uniform upper bound, and for disks, where we improve the general upper bound.