Ultra-long-range dynamic correlations in a microscopic model for aging gels

Abstract

We use large-scale computer simulations to explore the nonequilibrium aging dynamics in a microscopic model for colloidal gels. We find that gelation resulting from a kinetically-arrested phase separation is accompanied by 'anomalous' particle dynamics revealed by superdiffusive particle motion and compressed exponential relaxation of time correlation functions. Spatio-temporal analysis of the dynamics reveals intermittent heterogeneities producing spatial correlations over extremely large length scales. Our study is the first microscopically-resolved model reproducing all features of the spontaneous aging dynamics observed experimentally in soft materials.Understanding the complex mechanisms underlying the formation and stability of colloidal gels remains a challenge, despite the diversity of existing applications exploiting their mechanical properties. Gels are lowdensity structures forming percolating networks, with bonds that are either permanent (chemical gels) or transient (physical gels) [1][2][3][4][5][6]. A prevalent method to form physical gels follows a nonequilibrium route by quenching a homogeneous fluid into a phase coexistence region [1,[7][8][9][10][11][12], which generates bicontinuous structures. When the dense phase forms an amorphous solid, the phase separation is kinetically hindered [13][14][15][16] and a gel forms. The microscopic dynamical and structural properties of these nonequilibrium gels evolve slowly with time.This aging dynamics has been the subject of many experimental studies, which established that aging in colloidal gels is 'anomalous' [17,18], i.e. it differs qualitatively from the aging observed in conventional glassy materials, such as polymer and colloidal glasses [19,20]. Scattering experiments [21][22][23][24][25] report that time correlation functions are described by compressed exponential relaxations. Such behaviour differs from the (exponential) diffusive dynamics in simple liquids or the (stretched exponential) decay observed in glassy fluids [26]. In addition, compressed exponentials are seen to emerge only for large enough displacements, with the relaxation timescale τ (q) crossing over from τ ∼ q −2 , characteristics of diffusion, to τ ∼ q −1 , characteristic of ballistic dynamics, with decreasing the scattering wavevector q. Finally, spatiallyresolved dynamic measurements revealed the existence of long-ranged correlations extending up to the system size [27,28], again contrasting with the much smaller-ranged dynamic heterogeneity observed in glassy materials [29]. Such peculiar behaviour is hypothesized to follow from the infrequent release of 'internal stresses' that relax the fractal network [18], but this interpretation remains to be confirmed by direct observation. This overall phenomenology has been reported for laponite, carbon black, micellar polycrystals, multilamellar vesicles, implying it is generic to a large class of soft materials [18].A microscopic perspective via theoretical modeling is also missing. To study this problem, one needs a model with a...

The microscopic role of deformation in the dynamics of soft colloids

“…Another way to capture dynamic heterogeneities [42] is the calculation of the mobility map, which allows us to visualize the single-particle dynamics over a given length-scale. To connect particle deformations to the dynamical heterogeneities occurring at high ζ we thus calculate the mobility function s i (∆t) = 1 − exp[δr 2 i (∆t)/2a 2 ] for all rings, where a is a length scale which can be tuned to probe different regimes and ∆t is the time interval between two configurations from which particle displacement δr 2 i is evaluated.…”

“…The fraction of these islands varies with softness and packing fraction, thereby resulting in state-point-dependent exponents for the compressed exponential relaxation and for the super-diffusive behavior.The compressed exponential relaxation is still an important open question in colloidal systems and glass-formers [33,40,41]. Simulations recently provided some clues in this process by looking into phase-separating systems [42] and by artificially altering the network dynamics [43]. Alternatively a coarse-grained elasto-plastic model has been discussed to go beyond mean-field predictions on stress-driven dynamics [44].…”

Deformation and flow of amorphous solids: Insights from elastoplastic models

“…From (Ruta et al, 2013). in supercooled liquids (Caronna et al, 2008), colloidal suspensions (Angelini et al, 2013) and even in hard amorphous materials like metallic glasses (Ruta et al, 2013(Ruta et al, , 2012. Although this anomalous relaxation was observed ubiquitously in experimental systems, it took more than a decade to reproduce dynamics with compressed exponential decay in molecular-scale simulations, until Bouzid et al (2017) and Chaudhuri and Berthier (2017) eventually reported such dynamics in microscopic models for gels. The main obstacle had been to probe the right parameter range, notably with respect to temperature and also length scales.…”

Comparing the atomic and macroscopic aging dynamics in an amorphous and partially crystalline Zr44Ti11Ni10Cu10Be25bulk metallic glass

“…26,27 In fact, theoretical and simulation studies suggest that long-range anisotropic stresses arising from dipole forces are integral to understanding aging in disordered matter. 28,29 Even in the deeply supercooled liquid state, there are indications that the relaxation dynamics can be described through an accumulation of Eshelby transformations; i.e., local structural rearrangements resulting in long-range stress fluctuations and elastic strains 30,31 . In this sense, the compressed form of the exponential relaxation function of metallic glasses probed in XPCS might also result from a series of such Eshelby transformations, each contributing a Debye term with a well-defined relaxation time.…”