Comparison of melting in three and two dimensions: Microscopy of colloidal spheres

Digital imaging is used to study the instantaneous particle positions and trajectories of highly charged monodisperse 0.3- m-diam polystyrene spheres in water suspension. We find that a three-dimensional fcc crystal near a smooth glass wall melts abruptly into a layered fluid as a function of the in-plane sphere density parallel to the wall. In contrast, a single two-dimensional layer of the identical colloid confined between two smooth glass walls exhibits a gradual two-stage melting transition in which there are separate divergences of translational and orientational order.