Anatomy of cage formation in a 2D glass-forming liquid

TL;DR

This study uses laser perturbations and video microscopy to investigate cage formation in a 2D colloidal glass-forming liquid, revealing that cage buildup is linked to the merging of rigid cooperative domains at the onset of glassy dynamics.

Contribution

It provides direct experimental evidence connecting cage formation to cooperative domain merging in a 2D colloidal system, elucidating the initial step of glass formation.

Findings

Maximum response at onset density indicates cage formation.

Domains with cooperative dynamics become more rigid and dominate particle behavior.

Cage formation is directly related to merging of these domains.

Abstract

The solidity of glassy materials is believed to be due to the cage formed around each particle by its neighbors, but in reality the details of cage-formation remain elusive [1-4]. This cage starts to be formed at the onset temperature/density at which the normal liquid begins to show the first signs of glassy dynamics. To study cage-formation we use here focused lasers to produce a local perturbation of the structure on the particle level in 2D colloidal suspensions and monitor by means of video microscopy the system's non-linear dynamic response. All observables we probed show a response which is non-monotonic as a function of the packing fraction, peaking at the onset density. Video microscopic images reveal that this maximum response is due to the buildup of domains with cooperative dynamics that become increasingly rigid and start to dominate the particle dynamics. This proof-of-concept from microrheological deformation demonstrates that in this glass-forming liquid cage formation is directly related to the merging of these domains, thus elucidating the first step in glass-formation [1, 5].