Dynamical buckling transitions of a confined colloidal monolayer

When a rigid two-dimensional triangular crystalline layer of colloidal spheres confined between two smooth repulsive walls is gradually given freedom to move out of plane, it buckles dynamically undergoing several Peierls transitions involving different soft phonon modes before forming a two layer crystal with square in-plane symmetry. We have mapped out the complex phase diagram of the buckling transitions as a function of sphere density and wall separation. Digital imaging is used to study the instantaneous particle positions and trajectories of the uniform, highly charged 0.3 μm diameter polystyrene spheres that comprise the crystalline layer in water suspension. Brownian motion of the spheres creates a true thermodynamic system with a real temperature, which is studied using video microscopy. We follow the collective dynamics of the system as well as individual particle motions and the motions and rearrangements of topological defects and domains. At sufficiently low sphere densities the system melts into a fluid. As the wall separation increases to the point of two layer formation we observe square symmetry in the fluid correlation volumes.