The experimental realization of a two-dimensional colloidal model system

We present the technical details of an experimental method to realize a model system for two-dimensional (2D) phase transitions and the glass transition. The system consists of several hundred thousand colloidal superparamagnetic particles confined by gravity at a flat water-air interface of a pending water droplet where they are subjected to Brownian motion. The dipolar pair potential and, therefore, the system temperature are not only known precisely but also directly and instantaneously controllable via an external magnetic field H. In the case of a one-component system of monodisperse particles the system can crystallize upon application of H whereas in a two component system it undergoes a glass transition. Up to 10 000 particles are observed by video microscopy and image processing provides their trajectories on all relative length and time scales. The position of the interface is actively regulated thereby reducing surface fluctuations to less than 1 µm and the setup inclination is controlled to an accuracy of ±1 µrad. The sample quality being necessary to enable the experimental investigation of the 2D melting scenario, 2D crystallization, and the 2D glass transition, is discussed