Impact of sedimentation dynamics on colloidal 2D phase behavior

Prior work has used colloidal monolayers to study 2D phase behaviors as a model 2D hard disk system. We use gravitational confinement of colloidal SiO2 particles to form monolayers; our particular interest is how these monolayers form as the particles sediment to the bottom of our sample chamber. In particular, we wish to characterize how hexagonal ordering depends on area fraction, sedimentation dynamics, and particle diffusivity. Variation of particle concentration influences the resulting monolayer area fraction. Variation of particle diameter influences the Peclét number Pe, a measure of the relative importance of gravitational potential energy and thermal energy on particle behavior. The key idea is that larger particles sediment faster and diffuse slower (larger Pe), allowing less time for particles to rearrange into efficiently packed structures as the monolayer forms, and leading to more disordered monolayers. We find that hexagonal ordering requires some time to occur, and if sedimentation occurs too quickly, the monolayer can be trapped in a disordered non-equilibrium state.