  A collection of thin structures buckle, bend, and bump into each-other when
confined. This contact can lead to the formation of patterns: hair will
self-organize in curls; DNA strands will layer into cell nuclei; paper, when
crumpled, will fold in on itself, forming a maze of interleaved sheets. This
pattern formation changes how densely the structures can pack, as well as the
mechanical properties of the system. How and when these patterns form, as well
as the force required to pack these structures is not currently understood.
Here we study the emergence of order in a canonical example of packing in
slender-structures, i.e. a system of parallel growing elastic beams. Using
experiments, simulations, and simple theory from statistical mechanics, we
predict the amount of growth (or, equivalently, the amount of compression) of
the beams that will guarantee a global system order, which depends only on the
initial geometry of the system. Furthermore, we find that the compressive
stiffness and stored bending energy of this meta-material is directly
proportional to the number of beams that are geometrically frustrated at any
given point. We expect these results to elucidate the mechanisms leading to
pattern formation in these kinds of systems, and to provide a new mechanical
meta-material, with a tunable resistance to compressive force.
