Moving the focal plane approximately 0.2 m away from the mid-plane of the cell allows us to differentiate between spheres which are displaced upward and downward from the mid-plane (Figs. 2(g) and (h)). The dependence of the spheres' appearance on their vertical displacement can be calibrated [28,25]. For example, the integrated brightness of a sphere's image depends monotonically on its displacement from the focal plane; this is the measure we use. Not surprisingly, we find that the spheres in the triangular crystal layer do not lie rigidly in a single plane. The suspension is too dense to form a monolayer in equilibrium. Nor, moreover, are the vertical fluctuations of the layer purely random. Instead, we observe regions of correlated buckling whose structure is apparent in Fig. 9. Larger circles in this figure correspond to spheres which are closer to the viewing window. While the vertical displacement information is continuous, we have selected an arbitrary threshold in computing Fig. 9 to emphasize the buckled structure. The highlighted region displays a superlattice structure with ridges of displaced spheres running from the upper right corner to the lower left. Superimposed polygons help to demonstrate that the underlying lattice is still triangular, although distorted. This qualitative observation is supported by the observed 90 percent of the total field of view classified as 6-fold coordinated at this time in Fig. 5.
The same sort of buckling has been observed in equilibrium confined suspensions at phase boundaries such as that between the single triangular layer and the two-layer square crystal . Chou and Nelson  have analyzed this system and have identified three transverse phonon modes lying on the Brillouin zone boundary which can be responsible for buckling of the confined single layer. Of those, the one we see corresponds to the edge phonon. This also is observed in the equilibrium case . Its appearance in the Chou and Nelson theory is favored when the sphere-wall interaction is symmetric about the mid-plane of the system and is much stronger than gravity. The other corner phonon and edge phonon modes are asymmetric about the mid-plane and are favored by symmetry-breaking conditions.
Different regions of the crystal exhibit domains of differing buckling orientations, as can be seen in Fig. 9 when comparing the highlighted region to the region below. In addition to randomizing defects which suggest small admixtures of and phonon modes, buckling domains of the edge symmetry meet at four types of coherent domain walls which are comparable to twin boundaries. These appear schematically in Fig. 10. Each type of domain boundary appears at various times in our system. Boundaries need not follow crystallographic axes as drawn but tend to do so in our observations. Systematic studies of buckling domain wall motion is not possible in our nonequilibrium system.