Forming 2D colloidal crystals (sometimes) with sedimented colloids

We use bright field microscopy to observe the phase behavior of the base layer in a gravitationally confined colloidal system of silica particles. After injecting the colloidal of particles into a chamber and giving the system sufficient amount of time for sedimentation, we observe those particles settling on the base of the chamber forming a static quasi-2D structure. We control two parameters of the system, namely the particle concentration and the Peclet number. The Peclet number measures the relative importance of the gravitational force over the thermal effects, and is positively related to the particle diameters. We find that the most ordered states are formed by heavy particles (with high Peclet numbers) at moderately high concentrations. Simulations on the gravitationally confined quasi-2D system are conducted simultaneously, aiming to understand the relation between the phase behavior and various factors, including the polydispersity and the initial state of the system.