title: Questioning the relationship between the χ4 susceptibility and the dynamical correlation length in a glass former.
abstract: Clusters of fast and slow correlated particles, identified as dynamical heterogeneities (DHs), constitute a central aspect of glassy dynamics. A key factor 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 experimental studies 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.
authors.affiliation: Laboratoire Matière et Systèmes Complexes, UMR CNRS 7057 & Université Paris Diderot, 10 rue A. Domon et L. Duquet, 75205 Paris Cedex 13, France. remy.colin@univ-paris-diderot.fr berengere.abou@univ-paris-diderot.fr.
authors.forename: Rémy
authors.initials: R
authors.lastname: Colin
authors.affiliation: Complex Assemblies of Soft Matter Laboratory, UMI CNRS 3254, Rhodia INC., 350 G. Patterson Blvd, Bristol, PA 19007, USA.
authors.forename: Ahmed M
authors.initials: AM
authors.lastname: Alsayed
authors.affiliation: Laboratoire Matière et Systèmes Complexes, UMR CNRS 7057 & Université Paris Diderot, 10 rue A. Domon et L. Duquet, 75205 Paris Cedex 13, France. remy.colin@univ-paris-diderot.fr berengere.abou@univ-paris-diderot.fr.
authors.forename: Cyprien
authors.initials: C
authors.lastname: Gay
authors.affiliation: Laboratoire Matière et Systèmes Complexes, UMR CNRS 7057 & Université Paris Diderot, 10 rue A. Domon et L. Duquet, 75205 Paris Cedex 13, France. remy.colin@univ-paris-diderot.fr berengere.abou@univ-paris-diderot.fr.
authors.forename: Bérengère
authors.initials: B
authors.lastname: Abou