Experimental Studies of Melting and Hexatic Order in Two-Dimensional Colloidal Suspensions

Abstract

Suspensions of monodisperse colloidal particles are a rich model experimental system for understanding the fundamental mechanism for melting of two-dimensional crystals, and for searching for the elusive hexatic phase, with order intermediate between that of a perfect crystal and disordered fluid. By performing imaging experiments on colloidal systems, one can surmount a number of problems that have plagued either past experiments on atomic scale systems or computer simulations. The colloid experiments share some useful features with computer simulations: In particular, the colloids can be made very rigidly two-dimensional, so that promotion to a second layer may not occur. The substrate potential can be made atomically smooth so that the coupling to underlying lattice symmetry is nonexistent or, on the other hand, modulated with any strength and symmetry. The particle-particle and particle-wall interactions can be adjusted from short- to long-range. The interparticle separation, collision time, and particle diameter can be chosen for optimum video imaging with visible light so that one can obtain direct visual evidence of lattice defects, their dynamics, and of bond-orientational order in real space in the system on relevant timescales. Compared to simulations, however, the colloid imaging experiments have some very important advantages: the system size can be very large, so that boundary conditions and sample size can be effectively eliminated as problems, and the system can be monitored for easily 107 collisions, so that time for attainment of equilibrium may be reached.