How does sensitivity to dynamical heterogeneity in supercooled colloidal liquids depend on tracer size?

Few experimental investigations have examined the dependence of probe length scale on the evolution of dynamical heterogeneity in supercooled liquids. In this study, we use fluorescent tracer probes, which are smaller than that of the constituent unlabeled particles in the matrix suspension, to investigate the tracer size sensitivity to the onset of dynamical heterogeneity in model hard-sphere colloidal suspensions close to the glass transition. The dynamics of poly(methyl methacrylate) (PMMA) tracer particles of radii a(S) = 137 nm embedded into constituent PMMA particles of a(L) = 212 and 637 nm with bulk volume fraction phi varied from 0.50 to 0.57 is examined using fluorescence correlation spectroscopy (FCS). Upon increasing phi for the largest tracer size ratio explored, f (= a(S)/a(L)) = 0.646, the onset of dynamic heterogeneity is observed from the broad distribution of characteristic decay times of the tracers, which is extracted from the measured FCS autocorrelation functions. The strong coupling between tracer size and its sensitivity to dynamical heterogeneity is reflected in the dynamic susceptibility, where dynamic correlation lengths are immeasurable for the smallest size ratio and show a marked increase to five tracer particle diameters for the largest size ratio.