Bubbles enable volumetric negative compressibility in metastable elastocapillary systems

Although coveted in applications, few materials expand when subject to compression or contract under decompression, i.e., exhibit negative com- pressibility. A key step to achieve such counterintuitive behaviour is the destabilisations of (meta)stable equilibria of the constituents. Here, we pro- pose a simple strategy to obtain negative compressibility exploiting capillary forces both to precompress the elastic material and to release such pre- compression by a threshold phenomenon – the reversible formation of a bubble in a hydrophobic flexible cavity. We demonstrate that the solid part of such metastable elastocapillary systems displays negative compressibility across different scales: hydrophobic microporous materials, proteins, and millimetre-sized laminae. This concept is applicable to fields such as porous materials, biomolecules, sensors and may be easily extended to create unex- pected material susceptibilities.