Experimentally observing the Gardner transition in a 3D colloidal glass

As a glassy suspension is compressed towards jamming it can undergo an additional phase transition known as the Gardner transition. This fundamental phase transition corresponds to the emergence of a marginal glass phase where energy basins split into a hierarchy of marginally stable subbasins. The Gardner transition has recently been found for mean-field systems in infinite dimensions but evidence from simulations and experiments in three dimensions is limited. Here, we investigate the Gardner transition by directly observing the marginal states in a binary glass of silica microspheres. We use a custom-built compression cell mounted on a confocal microscope to induce shifts between marginally stable configurations by repeatedly compressing the glass. We image and track the silica particles over time to measure the glass cage size and the subcage spacing between marginal configurations. We further investigate the diverging timescale, susceptibility, and length scale associated with fluctuations in the cage size and spacing at the Gardner transition. These experiments will provide a foundation for further exploration of the marginal glass phase and are an important step in relating glass formation to jamming.