  Jamming is a fundamental transition that governs the mechanical behavior of
particulate media, including sand, foam and dense suspensions but also
biological tissues: Upon compression, particulate media can change from freely
flowing to a disordered solid. Jamming has previously been conceived as a bulk
phenomenon involving particle motions in fixed geometries. In a diverse class
of soft materials, however, solidification can take place in a deformable
geometry, such as on the surface of a fluid droplet or in the formation of a
bijel. In these systems the nature and dynamics of jamming remains unknown.
Here we propose and study a scenario we call metric jamming that aims to
capture the complex interactions between shape and particles. Unlike classical
jamming processes that exhibit discrete mechanical transitions, surprisingly we
find that metric jammed states possess mechanical properties that are
continuously tunable between those of classical jammed and conventional elastic
media. New types of vibrational mode that couple particulate and surface
degrees of freedom are also observed. Our work lays the groundwork for a
unified understanding of jamming in deformable geometries, to exploit jamming
for the control and stabilization of shape in self-assembly processes, and
provides new tools to interrogate solidification processes in deformable media
more generally.
