  Assemblies of purely repulsive and frictionless particles, such as emulsions
or hard spheres, display very curious properties near their jamming transition,
which occurs at the random close packing for mono-disperse spheres. Although
such systems do not contain the long and cross-linked polymeric chains
characterizing a rubber, they behave macroscopically in a similar way: the
shear modulus $G$ can become infinitely smaller than the bulk modulus $B$.
After reviewing recent theoretical results on the structure of such packing (in
particular their coordination) I will propose an explanation for the observed
scaling of the elastic moduli, and explain why the arguments both apply to soft
and hard particles.
