Résumé
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].