Buckling instabilities of a confined colloid crystal layer.

A model predicting the structure of repulsive, spherically symmetric, monodisperse particles confined between two walls is presented. When plate separations are small, only one layer of particles can be confined; however, when the plate separation is increased, multiple layers will eventually form. We study the buckling transition of a single flat layer as the double layer state develops. Experimental realizations of this model are suspensions of stabilized colloidal particles squeezed between glass plates. By expanding the thermodynamic potential about a flat state of N confined colloidal particles, we derive a free energy as a functional of in-plane and out-of-plane displacements. As the gap separation increases, certain out-of-plane modes soften. The wavevectors of these first buckling instabilities correspond to three different ordered structures. Landau theory predicts that the symmetry of these phases allows for second order phase transitions. This possibility exists even in the presence of gravity or plate asymmetry. These transitions lead to critical behavior and phases with the symmetry of the three-state and four-state Potts models, the X-Y model with 6-fold anisotropy, and the Heisenberg model with cubic interactions. Experimental detection of these structures is discussed. PACS numbers: 82.70Dd, 46.30.Lx, 63.75.+z Typeset using REVTEX 1