  The packing problem of long thin filaments that are injected into confined
spaces is of fundamental interest for physicists and biologists alike. How
linear threads pack and coil is well known only for the ideal case of rigid
containers, though. Here, we force long elastic rods into flexible spatial
confinement borne by an elastic shell to examine under which conditions
recently acquired knowledge on wire packing in rigid spheres breaks down. We
find that unlike in rigid cavities, friction plays a key role by giving rise to
the emergence of two distinct packing patterns. At low friction, the wire
densely coils into an ordered toroidal bundle with semi-ellipsoidal
cross-section, while at high friction, it packs into a highly disordered,
hierarchic structure. These two morphologies are shown to be separated by a
continuous phase transition. Our findings demonstrate the dramatic impact of
friction and confinement elasticity on filamentous packing and might drive
future research on such systems in physics, biology and even medical technology
toward including these mutually interacting effects.
