Symmetric double bubbles in the Grushin plane

TL;DR

This paper studies the double bubble problem in the Grushin plane, proving existence and characterizing minimizers for equal volumes, revealing differences between vertical and horizontal interfaces, and conjecturing solutions in the general case.

Contribution

It introduces the first existence and characterization results for double bubbles in the anisotropic Grushin plane with equal volumes, including new insights into interface configurations.

Findings

Minimizers exist for both vertical and horizontal contact interfaces.

Angles at boundary intersections follow a 120-degree rule after coordinate change.

Horizontal interface minimizers are related to the Grushin isoperimetric set.

Abstract

We address the double bubble problem for the anisotropic Grushin perimeter $P_\alpha$, $\alpha\geq 0$, and the Lebesgue measure in $\mathbb R^2$, in the case of two equal volumes. We assume that the contact interface between the bubbles lays on either the vertical or the horizontal axis. Since no regularity theory is available in this setting, in both cases we first prove existence of minimizers via the direct method by symmetrization arguments and then characterize them in terms of the given area by first variation techniques. Angles at which minimal boundaries intersect satisfy the standard 120-degree rule up to a suitable change of coordinates. While for $\alpha=0$ the Grushin perimeter reduces to the Euclidean one and both minimizers coincide with the symmetric double bubble found in [Foisy et al., Pacific J. Math. (1993)], for $\alpha=1$ vertical interface minimizers have Grushin perimeter strictly greater than horizontal interface minimizers. As the latter ones are obtained by translating and dilating the Grushin isoperimetric set found in [Monti Morbidelli, J. Geom. Anal. (2004)], we conjecture that they solve the double bubble problem with no assumptions on the contact interface.