Finite-Size Effects on the Closest Packing of Hard Spheres

We study closely packed crystalline structures formed by slow lateral compression of a colloidal suspension of hard spheres in a thin wedge. In addition to the known sequence of structural transitions, a buckling mechanism was recently proposed to maximize the packing fraction $\Phi$ between one and two layers. We here confirm this prediction experimentally and present the first evidence that for more than two layers, buckling, in this case of prism shaped arrays of particles, also takes place. This efficient mechanism may enhance $\Phi$ by up to 4% and dominates in major regions of the phase diagram.