Large mass rigidity for a liquid drop model in 2D with kernels of finite moments

TL;DR

This paper proves that for a generalized liquid drop model with a nonlocal kernel of finite moments, the optimal shape for large mass is a disk, extending classical isoperimetric results to nonlocal energies.

Contribution

It establishes the uniqueness and convexity of minimizers for a nonlocal isoperimetric problem in 2D and higher dimensions, generalizing Gamow's liquid drop model with kernels of finite moments.

Findings

Minimizers are convex and nearly circular in 2D for small epsilon.

The unit ball is the unique nearly spherical minimizer in higher dimensions.

Existence of a critical mass beyond which disks are the unique minimizers.

Abstract

Motivated by Gamow's liquid drop model in the large mass regime, we consider an isoperimetric problem in which the standard perimeter $P(E)$ is replaced by $P(E)-\gamma P_\varepsilon(E)$, with $0<\gamma<1$ and $P_\varepsilon$ a nonlocal energy such that $P_\varepsilon(E)\to P(E)$ as $\varepsilon$ vanishes. We prove that unit area minimizers are disks for $\varepsilon>0$ small enough. More precisely, we first show that in dimension $2$, minimizers are necessarily convex, provided that $\varepsilon$ is small enough. In turn, this implies that minimizers have nearly circular boundaries, that is, their boundary is a small Lipschitz perturbation of the circle. Then, using a Fuglede-type argument, we prove that (in arbitrary dimension $n\geq 2$) the unit ball in $\mathbb{R}^n$ is the unique unit-volume minimizer of the problem among centered nearly spherical sets. As a consequence, up to translations, the unit disk is the unique minimizer. This isoperimetric problem is equivalent to a generalization of the liquid drop model for the atomic nucleus introduced by Gamow, where the nonlocal repulsive potential is given by a radial, sufficiently integrable kernel. In that formulation, our main result states that if the first moment of the kernel is smaller than an explicit threshold, there exists a critical mass $m_0$ such that for any $m>m_0$, the disk is the unique minimizer of area $m$ up to translations. This is in sharp contrast with the usual case of Riesz kernels, where the problem does not admit minimizers above a critical mass.