Unexpected drop of dynamical heterogeneities in colloidal suspensions approaching the jamming transition

Abstract

In dense colloidal suspensions, the spatial and temporal fluctuations in the dynamics of the constituent particles are closely related. But very close to the jamming transition—where the suspension becomes rigid—they are found to follow different trends. In supercooled molecular fluids or concentrated colloids and grains, the dynamics slow down markedly with no distinct structural changes as the glass1 or the jamming2 transition is approached. There is now ample evidence that structural relaxation in glassy systems can only occur through correlated rearrangements of particle ‘blobs’ of size ξ (refs 3, 4, 5, 6, 7), leading to dynamics that are heterogeneous both in time and in space. On approaching these transitions, ξ grows in glass-formers6,7,8, colloids3,4,9 and driven granular materials10 alike, strengthening the analogies between the glass and the jamming transitions and providing a possible explanation for the slowing down of the dynamics. However, little is known yet on the behaviour of dynamical heterogeneity very close to dynamical arrest. Here, we measure in colloids the maximum of a ‘dynamical susceptibility’, χ*, that quantifies the temporal fluctuations of the dynamics, the growth of which is usually associated with that of ξ (ref. 11). We find that χ* initially increases with particle volume fraction, but drops markedly very close to jamming. We show that this behaviour results from the competition between the growth of ξ and the reduced particle displacements associated with rearrangements in very dense suspensions, unveiling a richer-than-expected scenario.