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

Abstract

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 ξ (refs 3-7), leading to dynamics that are heterogeneous both in time and in space. On approaching these transitions, ξ 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', 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.Dynamical heterogeneity is a key ingredient in many of the most advanced attempts to understand and rationalize the glass and the jamming transitions. The recent observation of a critical-like growth of temporal and spatial dynamical fluctuations in a two-dimensional athermal system approaching jamming 10 , similar to that hypothesized for glass-formers 12 , has raised hope that the glass and the jamming transitions may be unified, calling at the same time for further, tighter experimental verifications. Here, we investigate temporal dynamical heterogeneity in a three-dimensional thermal system, concentrated colloidal suspensions close to the maximum packing fraction. Temporal and spatial dynamical heterogeneity are usually closely related: the former can be quantified by a 'four-point dynamical susceptibility' χ 4 (the variance of a time-resolved correlation function describing the system relaxation), the amplitude of which is proportional to ξ 3 (refs 11,13,14). Surprisingly, we find that very close to jamming, temporal and spatial dynamical heterogeneity decouple: whereas ξ continuously grows with volume fraction, the amplitude of temporal fluctuations drops sharply close to the maximum packing fraction. These findings challenge current scenarios where the slowing down of the dynamics on approaching jamming is accompanied by enhanced temporal fluctuations of the dynamics.The dynamics of colloidal hard spheres slows down markedly close to ϕ = ϕ g ≈ 0.58; beyond ϕ g , ultraslow relaxations on short length scales are still observed 3 , until dynamics freeze at the maximum (random) packing fraction, ϕ max . We study concentrated suspensions of polyvinyl chloride xenospheres 15 suspended in dioctyl p...

Beyond simple exponential correlation functions and equilibrium dynamics in x-ray photon correlation spectroscopy

“…The positions of the peak τ * as a function of Q and as a function of waiting time are shown in figures 5 and 6, respectively, illustrating this close correspondence between the maximum of the temporal heterogeneity and the correlation time. Similar correspondence has been observed for χ (Q, t) for several systems [31], [37]- [39], [49,73] and also for χ 4 , where the peak position is considered a measure of the persistence of spatial heterogeneities [63]- [66]. While the position of the peak in χ(Q, t) unsurprisingly follows this common trend, the magnitude of χ (Q, t) for the sheared depletion gel is significantly smaller than the variance characterizing the hyper-diffusive dynamics in dilute gels measured with DLS [31], reflecting the relatively smooth evolution in dynamics observed in figure 3.…”

Monte Carlo simulations of densely-packed athermal polymers in the bulk and under confinement

“…Due to its great importance in many fields the unique characteristics of the jammed state have been studied extensively over the last years (Ballesta et al, 2008;Corwin et al, 2005;Donev et al, 2004aDonev et al, , 2004bDonev et al, , 2005aGoodrich et al, 2012;Keys et al, 2007;Lu et al, 2008;O'Hern et al, 2002O'Hern et al, , 2003Zexin et al, 2009). While initial studies focused understandably on dense packing of uniform spheres research has since been extended to objects with complex, anisotropic shapes (Donev et al, 2006;Frenkel et al, 1988;Haji-Akbari et al, 2013, 2011a, 2011bJiao et al, 2009;Jiao and Torquato, 2011;Torquato and Jiao, 2009;Veerman and Frenkel, 1991;Xu et al, 2005;Zeravcic et al, 2009).…”

Shear-induced fragmentation of laponite suspensions

“…Although this picture remains appealing, it is clear that a complete understanding and modelling of glassy-like phenomena involved in materials with microstructures as diverse as submicron hard spheres, micronsized grains, short-range attractive particles, or highly charged platelets, are still out of reach [3]. Most recent experiments performed to investigate the structure and the dynamics of SGM at rest have relied on single or multiple light scattering techniques and have focused on ageing properties [4] or on the presence of dynamical heterogeneities [5,6,7]. Together, ageing and dynamical heterogeneities call for a spatiotemporal modelling of SGM that is still lacking.…”