Questioning the relationship between the χ4 susceptibility and the dynamical correlation length in a glass former
Résumé
Clusters of fast and slow correlated particles, identified as dynamical heterogeneities (DHs), con-
stitute a central aspect of glassy dynamics. A key ingredient of the glass transition scenario is a
significant increase of the cluster size ξ4 as the transition is approached. In need of easy-to-compute
tools to measure ξ4 , the dynamical susceptibility χ4 was introduced recently, and used in various ex-
perimental works to probe DHs. Here, we investigate DHs in dense microgel suspensions using image
correlation analysis, and compute both χ4 and the four-point correlation function G4 . The spatial
decrease of G4 provides a direct access to ξ4 , which is found to grow significantly with increasing
volume fraction. However, this increase is not captured by χ4 . We show that the assumptions that
validate the connection between χ4 and ξ4 are not fulfilled in our experiments.
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