Statistical mechanics of thin spherical shells

TL;DR

This paper investigates how thermal fluctuations influence the mechanical stability of thin spherical shells, revealing a size-dependent negative surface tension that can cause large shells to collapse unless stabilized by external pressure.

Contribution

It introduces a renormalization group approach for spherical shells, uncovering novel fluctuation-induced phenomena and stability criteria not previously understood in flat membranes.

Findings

Small shells remain stable under thermal fluctuations.

Large shells are prone to collapse due to fluctuation-induced negative surface tension.

Outward osmotic pressure can stabilize large shells, with size scaling non-linearly with pressure.

Abstract

We explore how thermal fluctuations affect the mechanics of thin amorphous spherical shells. In flat membranes with a shear modulus, thermal fluctuations increase the bending rigidity and reduce the in-plane elastic moduli in a scale-dependent fashion. This is still true for spherical shells. However, the additional coupling between the shell curvature, the local in-plane stretching modes and the local out-of-plane undulations, leads to novel phenomena. In spherical shells thermal fluctuations produce a radius-dependent negative effective surface tension, equivalent to applying an inward external pressure. By adapting renormalization group calculations to allow for a spherical background curvature, we show that while small spherical shells are stable, sufficiently large shells are crushed by this thermally generated "pressure". Such shells can be stabilized by an outward osmotic pressure, but the effective shell size grows non-linearly with increasing outward pressure, with the same universal power law exponent that characterizes the response of fluctuating flat membranes to a uniform tension.