Nonlinear rheology of colloidal gels with intermediate volume fraction

Abstract

SynopsisThe depletion attraction, induced upon addition of a nonadsorbing polymer to a colloidal solution, can lead to gel formation at sufficiently high polymer concentrations, which corresponds to strong attractive interactions. We have investigated the nonlinear rheological response, in particular the yielding, of colloidal gels with an intermediate volume fraction and variable interparticle attraction. Two distinct yielding processes are observed in both oscillatory experiments, namely, dynamic strain sweeps and transient experiments, here step rate, creep, and recovery tests. The first yielding process occurs at strains similar to the range of the interparticle potential and is interpreted as the breaking of bonds, which destroys the particle network and leads to individual clusters. The process of bond breaking is successfully modeled as the escape of a particle from the potential well of its nearest neighbor. The second yield point occurs at larger strains and is related to the deformation and fragmentation of clusters, consistent with the observed dependence of the yield strain on attraction. Both yield stresses exhibit a power-law dependence on attraction strength in agreement with observations of other systems and theoretical predictions. Furthermore, the observed two-step yielding reveals similarities, and also differences, to the rheology of attractive colloidal glasses.

Homogeneous percolation versus arrested phase separation in attractively-driven nanoemulsion colloidal gels

“…Here, yielding is characterized by a single yield stress, s y , where G 0 and G 00 intersect and G 00 shows a mild maximum with increasing s 0 . By contrast, gels with f > 0.23 show signicantly different yielding behaviour, where the transition between the linear and the uid regime is signicantly broader, bearing similarity to the so-called "two-step" yielding observed in a number of colloidal gels at intermediate f. 17,21,23,24,33 Specically, the maximum in G 00 is more pronounced, although it still occurs at the initial yield stress. However, at higher s 0 , both G 0 and G 00 exhibit a plateau over a signicant range of stress.…”

“…Specically, we note that the homogeneous gels formed at f < 0.23 exhibit a single, distinct yield point, whereas the heterogeneous gels with f > 0.23 exhibit so-called "two-step" yielding observed in other gelling systems at moderate volume fraction. 21,33 This suggests that the origin of two-step yielding might arise from large-scale structural heterogeneity (in this case due to arrested phase separation), since a heterogeneous distribution of cluster-cluster bonds should give rise to a broad distribution of microscopic yielding processes. This is a potential alternative to recently proposed hypotheses, 21,23,24,52 which posit that two-step yielding results from a combination of two distinct microstructural processes: bond rupture and cage breakage, which both occur at local length scales on the order of single particles and their nearest neighbours, and do not require a heterogeneous structure.…”

“…33 O to achieve samples with the desired droplet volume fraction. We note that dilution of silicone oil-in-water nanoemulsions with their own continuous phase in the absence of surfactant has a negligible effect on the droplet size distribution.…”

“…17 Whereas dilute gels typically exhibit a distinct yield stress (or strain) in nonlinear rheological measurements 18 corresponding to a single microstructural process, 19 more concentrated systems exhibit complicated yielding in which the transition from solid-like behaviour to ow is marked by a two-step or broadened transition. [20][21][22][23] Such two-step yielding has been primarily observed in systems with moderate volume fraction, and has been hypothesized to occur due to a combination of processes, including rupture of interparticle bonds and glassy dynamics of attractive clusters.…”

Self-assembly and cooperative dynamics of a model colloidal gel network

“…Various internal deformation modes, in particular the soft ones, depending on the network topology, may dominate the elastic response [7][8][9]. The nonlinear behavior is also a hallmark of the mechanics of these materials [10,11]. Nonaffine microscopic deformations of the weakly connected structure certainly play an important role, but here also the possible restructuring of the gel network has to be taken into account.…”