  Colloidal gels formed by strongly attractive particles at low particle volume
fractions are composed of space-spanning networks of uniformly sized clusters.
We study the thermal fluctuations of the clusters using differential dynamic
microscopy by decomposing them into two modes of dynamics, and link them to the
macroscopic viscoelasticity via rheometry. The first mode, dominant at early
times, represents the localized, elastic fluctuations of individual clusters.
The second mode, pronounced at late times, reflects the collective,
viscoelastic dynamics facilitated by the connectivity of the clusters. By
mixing two types of particles of distinct attraction strengths in different
proportions, we control the transition time at which the collective mode starts
to dominate, and hence tune the frequency dependence of the linear viscoelastic
moduli of the binary gels.
