[hal-01098350] Susceptibility χ4 fails to capture significant increase of dynamic correlation length near glass transition

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. Here, we investigate DHs in dense microgel suspensions using image correlation, 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. Our findings question the relevance of the broadly used χ4 in describing DHs.



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