Hexagonal packing in gravitationally confined quasi-two-dimensional colloidal silica systems

Colloids are made from small solid particles in a liquid. When the particles are heavier than the liquid, they will eventually sink to the bottom of the liquid. We use this to form a monolayer of particles at the bottom of a microscope sample chamber. At high enough particle concentration, this monolayer organizes into a hexagonally packed crystal. However, the structure is influenced by the interplay between gravity (causing the sedimentation) and diffusion (helping particles organize into a crystal). We use high-density silica colloid particles of different sizes to study how the competition between thermal fluctuations and gravitational sedimentation affects the final structure of the layer.