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

In supercooled molecular fluids or concentrated colloids and grains, the dynamics slow down markedly with no distinct structural changes as the glass(1) or the jamming(2) 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 xi (refs 3-7), leading to dynamics that are heterogeneous both in time and in space. On approaching these transitions, xi grows in glass-formers(6-8), colloids(3,4,9) and driven granular materials(10) 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', chi*, that quantifies the temporal fluctuations of the dynamics, the growth of which is usually associated with that of xi (ref. 11). We find that chi* 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 xi and the reduced particle displacements associated with rearrangements in very dense suspensions, unveiling a richer-than-expected scenario.