Two for one in a colloidal glass

It is often said that glass is a frozen liquid. In a glass, the molecules are disordered like a liquid, but macroscopically, the sample no longer flows, behaving like a solid. While we can describe what a glass is, we still don’t fully understand the transition to the glassy state [1]. One way to learn about the glass transition is to study model systems. Perhaps the simplest experimental model system is a colloidal glass. Colloidal particles are small solid particles, suspended in a liquid and capable of rearranging via Brownian motion. The key control parameter is the volume fraction φ, the fraction of space occupied by the particles. When the volume fraction is sufficiently high, particles collide with their neighbors frequently and find it impossible to rearrange, a state called a colloidal glass. One advantage of using colloidal particles as a model system for all glasses is that they are large enough to be viewed directly with microscopy, so we can develop a microscopic picture of how particles rearrange at the glass transition [2, 3].